Therapeutic Supramolecular Polymers: Designs and Applications

Abstract

The self-assembly of low-molecular-weight building motifs into supramolecular polymers has unlocked a new realm of materials with distinct properties and tremendous potential for advancing medical practices. Leveraging the reversible and dynamic nature of non-covalent interactions, these supramolecular polymers exhibit inherent responsiveness to their microenvironment, physiological cues, and biomolecular signals, making them uniquely suited for diverse biomedical applications. In this review, we intend to explore the principles of design, synthesis methodologies, and strategic developments that underlie the creation of supramolecular polymers as carriers for therapeutics, contributing to the treatment and prevention of a spectrum of human diseases. We delve into the principles underlying monomer design, emphasizing the pivotal role of non-covalent interactions, directionality, and reversibility. Moreover, we explore the intricate balance between thermodynamics and kinetics in supramolecular polymerization, illuminating strategies for achieving controlled sizes and distributions. Categorically, we examine their exciting biomedical applications: individual polymers as discrete carriers for therapeutics, delving into their interactions with cells, and in vivo dynamics; and supramolecular polymeric hydrogels as injectable depots, with a focus on their roles in cancer immunotherapy, sustained drug release, and regenerative medicine. As the field continues to burgeon, harnessing the unique attributes of therapeutic supramolecular polymers holds the promise of transformative impacts across the biomedical landscape.

Graphical Abstract

1. Introduction

Supramolecular polymers (SPs) have garnered attention as an established branch of polymer science, largely due to the discovery that they exhibit physical, chemical and biological properties akin to those of covalent macromolecules, in addition to novel attributes exclusive to supramolecular systems [1-4]. These sophisticated systems, formed through the spontaneous association of low-molecular-weight building blocks, create shape-persistent supramolecular nanostructures with varying levels of internal order [5-7]. On the one hand, they symbolize an evolving frontier in polymer science, marked by their unique properties and potential to expand and enhance the applications of traditional polymers. On the other hand, the field of supramolecular polymers, where non-covalent bonds are utilized to join monomeric components, has established itself as a significant area within supramolecular chemistry, materials science and chemical engineering. These expanded functionalities of SPs primarily stem from two distinctive characteristics. First, the reversible and tunable nature of the interactions that hold together the monomeric units offers a wide range of enhanced material properties, including heightened toughness, as well as crack and scratch healing abilities. Such tunability in interactions facilitates the creation of dynamic, supramolecular architectures that offer novel biological and/or electronic functionalities. In addition, the ability to control the transition between polymeric and monomeric states via temperature, solution pH, or external stimuli provides the advantage of low viscosity in the monomeric state, allowing for easier handling, processing, and recycling. The second unique characteristic involves the specific arrangement of the building units within individual SPs, enabling the development of novel functions and properties solely associated with the supramolecular nanostructures and distinct from the individual building units. This characteristic makes them promising materials for both electronic and biological applications. In biomaterials, the dynamic nature of SPs allows for injectable delivery due to the low viscosity of the unassembled monomers, and natural body clearance that does not necessarily require chemical breakdown. Moreover, SPs with a high degree of internal order can be designed to mimic functional and structural features found in biology, such as the extracellular matrix, providing both specific signal presentation and mechanical support to cells of interest. Numerous examples have illustrated that supramolecular systems can mimic, and sometimes surpass, the physical, chemical and biological properties of traditional covalent macromolecules [2]. The reversibility of supramolecular polymeric systems hinges on specific non-covalent interactions introduced into the molecular building blocks [8, 9]. Hydrogen bonds, π-π interactions, host-guest interactions, and metal-ligand interactions are among the principal forces driving this self-assembly process, thereby dictating the emergence of diverse supramolecular structures. For instance, one-dimensional (1D) supramolecular polymers, the direct non-covalent counterparts of classical covalent polymers, exhibit analogous structural and functional characteristics[2, 5, 10, 11]. Their comparative robustness and adaptability have rendered supramolecular polymers a potent alternative to traditional polymer systems [12]. These innovative structures have begun to lay the foundation for a new class of non-covalent nanomaterials.

The therapeutic potential of supramolecular polymers is derived from their emergent materials properties, intrinsic adaptability, and responsiveness to external stimuli, as well as their unique interaction mechanisms with their surroundings [13-19]. When interacting within biological systems, supramolecular constructs display distinct, often enhanced traits over their monomeric analogues. This includes prolonged plasma circulation, more robust resistance to enzymatic breakdown, cellular uptake that can be tuned based on their physicochemical attributes, and improved accumulation of therapeutics in tissues. Their unique ability to form a three-dimensional network under physiological conditions enables effective delivery of therapeutics or cells, complemented by the characteristic disassembly or dissociation during body clearance, a process not contingent on chemical breakdown. A critical advantage of employing supramolecular chemistry in biomaterial design is that their attributes are rooted in their molecular-level building blocks even if the resultant supramolecular constructs possess unique and emerging properties that the individual constituents often do not have. In other words, the materials characteristics of supramolecular polymers, including reversibility, adaptability, and tunability, are directly influenced by the chemical structure of their individual elements. An added functional benefit arises from the combinatorial modularity of supramolecular interactions, wherein the robust affinity of a particular supramolecular motif facilitates significant intermolecular coordination without disrupting the primary chemical structure. Very importantly, these self-assembled materials often traverse a complex free energy landscape, leading to the emergence of nonequilibrium metastable or kinetically trapped conformations/states that could have significant practical uses [20, 21].

The continuous demands for improved medical therapies, along with the advent of nanomedicine, have expedited this field's expansion, with the creation of nanoscale carriers to enhance drug solubility, improve pharmacokinetic profiles, and increase drug accumulation in the disease sites. It is evident that harnessing the distinct attributes of supramolecular polymers holds the potential to stimulate innovation across diverse therapeutic applications. These applications span a wide range of fields, including therapeutic delivery, tissue engineering, regenerative medicine, and molecular imaging and diagnostics [22-27]. When 1D supramolecular structures or nanofibers entangle into a network, they form a shear-thinning supramolecular hydrogel. Such a hydrogel combines characteristics of both chemical and physical networks, where its mechanical properties, injectability, in situ hydrogelation for enhanced local retention and controlled payload release can be readily modulated by adjusting the crosslinking density, the composition of the polymeric material, and the use of various counterions [28]. The evolution of these supramolecular hydrogels has catalyzed the development of injectable therapeutics for drug and cell delivery, further underscoring their far-reaching potential in the healthcare industry [29].

Despite the extensive reviews and research articles on supramolecular polymers and their formation mechanisms [1, 3-7, 30-32], the application of these polymers in the biomedical field has not received commensurate attention. Addressing this gap, our review aims to elucidate the design principles, synthesis methods, and developmental strategies employed in constructing therapeutic supramolecular polymers. More specifically, we focus on their biomedical applications as therapeutic carriers, the treatment and prevention of various human diseases, and their potential to expand the toolbox of biomaterial sciences and macromolecular science. Our goal is to provide an insightful perspective on the potential and promises of therapeutic supramolecular polymers, spotlighting their burgeoning role in the fields of pharmacy, medicine, and engineering.

2. Monomer design considerations

The assembly and utility of therapeutic supramolecular polymers are fundamentally linked to their constituent monomers, which predominantly share a key feature: directionality. These polymers possess dynamic structures resulting from the interaction of molecular aggregates through noncovalent bonds [39-41]. The exploration of these monomer interactions necessitates a closer look into the ways they guide the assembly pathways into well-defined supramolecular structures (Fig. 1), affording unique functional characteristics. A large proportion of these polymers self-assemble under aqueous conditions, laying the foundation for their therapeutic applications. The dynamic nature of these polymers arises from the interplay of numerous interactions, including hydrogen bonds and π–π stacking, which enable the formation of 1D supramolecular structures when the repeating unit is designed accordingly. The work by Meijer et al. exemplifies this, where they utilized the strong and selective complexation of naphthyridines (Napy) and ureidopyrimidinone (UPy) units to prepare AA/BB-type supramolecular block copolymers [34, 42, 43]. Their strategic design involved a poly(tetrahydrofuran) macromonomer with UPy units at the chain ends and a ditopic Napy monomer, thus enabling them to tune the composition of the supramolecular polymers by controlling the stoichiometry of UPy and Napy groups.

Fig. 1.

Schematic illustration of therapeutic supramolecular polymers with the key non-covalent interaction patterns (inner circle) and representative examples of monomers. Clockwise from the top-center: peptide amphiphile [33], ureidopyrimidinone (UPy) [34], β-cyclodextrin [35], perylene bisimide (PBI) [36], β-sheet forming peptides , and 5,15-Diamide-zinc(II)porphyrin [37]. [33], Copyright 2014. Adapted with permission from Springer Nature. [34], Copyright 2005. Adapted with permission from American Chemical Society. [35], Copyright 2004. Adapted with permission from American Chemical Society. [36], Copyright 2015. Adapted with permission from Springer Nature. [38], Copyright 2023. Adapted with permission from Springer Nature. [37], Copyright 2014. Adapted with permission from Springer Nature.

The therapeutic potential of supramolecular polymers extends to peptide amphiphile nanofibers developed by the Stupp lab that incorporate a broad range of interactions [14, 44-46]. These include hydrogen bonds among peptide segments, organized secondary structures like β-sheets or α-helices, electrostatic attractions, hydrophobic collapse of the chosen alkyl tails, amino acid side chain interactions, and monomer-water interactions within the assemblies. Such rich interaction profiles enable researchers to design complex structures capable of interacting with biological systems [47]. The molecular building motif for SPs also extends to naturally occurring macromolecules like DNA and porphyrin derivatives, where perylene bisimides (PBIs) exhibit potential due to their photostability, bright fluorescence, and properties influenced by columnar stack formations in solution [36].

Host–guest interactions represent another important design strategy for supramolecular polymer construction. This class of building blocks relies on the complementary shape and size fit between the host and guest molecules, exemplified by cyclodextrins forming macrocyclic inclusion complexes [35, 48, 49]. The formation of labile metal–ligand interactions, akin to hydrogen bonding or host–guest chemistry, is also utilized to assemble supramolecular polymers, as seen in the porphyrin molecule's self-assembly through π–π stacking of the planes and hydrogen bonding of the amide groups [38]. These interactions, including hydrogen bonding and π–π-interactions in short peptide sequences linked to synthetic aromatic molecules, allow for creation of emerging properties, including the ability to form hydrogels at low concentrations due to long, entangled fibers [37]. In conclusion, the types of monomers and their complex interactions present in therapeutic supramolecular polymers lend them their intriguing properties and broad application potential in medicine and beyond. In this specific review, we confine our discussion to the therapeutic potential and applications of 1D nanostructured supramolecular polymers.

3. Thermodynamics and kinetics in supramolecular polymerization

3.1. Polymerization Mechanisms

The distinct advantages of supramolecular polymers in therapeutic applications are intrinsically tied to their unique properties as both discrete nanostructures and hydrogel materials. Central to this is the mechanism by which the building blocks are polymerized, with the two primary processes being isodesmic and cooperative polymerization [50]. The selection between these mechanisms depends on the requirement for a nucleation phase (Fig. 2A). The isodesmic model, for instance, maintains a consistent reactivity of a monomer relative to the growing polymer chain's end. This suggests that the association constant remains unchanged regardless of the polymer length, a characteristic observed in many supramolecular polymers. Such isodesmic polymerizations, mainly controlled by thermodynamics, result in polymers of varying length distributions, which may have important implications in their use as carriers for therapeutics and imaging agents. The crux of the isodesmic model is its presumption of consistent free energy changes upon the addition of a monomer to another monomer or a polymer. In contrast, cooperative polymerization follows two distinct stages: an initial nucleation step and subsequent elongation of the supramolecular polymer. The nucleation stage often demands the creation/formation of molecular clusters or a nucleus as a prerequisite, which then grows into supramolecular polymers. Unlike the isodesmic model, cooperative polymerization incorporates two association constants, denoting the different free energy requirements for the formation of the nucleus and the elongation of the supramolecular chain. The K₂/K model is a widely accepted model utilized to elucidate the process of supramolecular polymerization [51]. This model postulates that the initial nucleation step involves the formation of a dimer, resulting from the addition of two unimers (K_N), followed by the elongation phase (K_E) once a critical concentration has been attained. As depicted in Fig. 2B, the degree of cooperativity within the system can be quantified by a parameter termed σ, which represents the ratio of the association constants for nucleation and elongation (K_N/K_E). When σ is less than 1, the system exhibits cooperativity, manifesting as a non-sigmoidal curve in the context of the assembly process. Conversely, in the isodesmic polymerization mechanism, a σ value of 1 and a sigmoidal curve indicate an isodesmic system, implying that the nucleation and growth stages are indistinguishable due to their spontaneous nature.

Fig. 2.

Isodesmic and cooperative supramolecular polymerization. (A) Mechanism of an isodesmic self-assembly process, wherein each monomer species interacts with the same association constant as the previous. Mechanism of cooperative self-assembly, wherein a nucleus is formed, followed by an elongation step. Both processes are governed by separate association constants; K_N and Ké respectively. (B) Concentration-dependent assembly based on different values of σ where σ = 1 describes an isodesmic process whereas for σ < 1 the system can be described as cooperative. (C) Schematic representation of the changes in Gibbs free energy for isodesmic and cooperative growth mechanisms of self-assembly, respectively.

Under thermodynamic equilibrium, the continuous self-assembly of small molecules into 1D nanostructures can be portrayed as a series of reversible monomer addition steps, each characterized by a decrease/increase in Gibbs free energy (Fig. 2C). In an isodesmic mechanism, the Gibbs free energy of all individual steps remains the same and is thus independent of the aggregate's length. Such polymerization can be described by a single equilibrium constant that is dependent on the monomer's chemical structure, the solvent, and temperature. In contrast, cooperative polymerization is characterized by the formation of a thermodynamically unfavorable oligomer (nucleus), followed by energetically favored elongations steps. This mechanism is characterized by two equilibrium constants that describe reversible monomer addition to pre-nucleus oligomers and post-nucleus polymers. The rational design of unimers to engage in cooperative supramolecular polymerization can profoundly impact the morphology and dimensions of the resultant supramolecular polymers. For example, Lin and coworkers. delineated a two-phase polymerization mechanism for poly(l-glutamic acid)-grafted gold nanoparticles (NPs): an initial, slow nucleation stage is succeeded by an accelerated chain propagation phase [52]. Importantly, the initial nucleation stage governs the characteristics of the supramolecular assemblies, which exhibit a strong correlation with both the grafting density of the polypeptides on the NPs and the nanoparticle sizes. This sophisticated understanding of supramolecular polymerization mechanisms enhances their therapeutic potential and applications.

3.2. Kinetic Control

A key challenge in supramolecular polymerization is the consistent production of supramolecular polymers with well-defined morphologies and uniform molecular weights. This difficulty largely arises due to the dynamic nature of supramolecular systems, which often preclude precise control over the polymerization of supramolecular building blocks under thermodynamic conditions [53-55]. In addition to the thermodynamic equilibriums established between monomers and growing polymers, the presence of kinetic effects such as hysteresis and the influence of solvent composition on assembly dynamics can exert a significant impact on the resulting supramolecular structures/morphologies [20, 56]. Supramolecular polymerization can be categorized into three distinct energy states, shown in Fig. 3. Equilibrium assemblies represent global thermodynamically stable states where the system has reached a minimum in the energy landscape. Kinetic assemblies are transiently durable due to the presence of local energy minima; however, they may persist for a significantly long time under some specific conditions. Dissipative assemblies are unlike equilibrium and kinetic assemblies, in that they require a continuous supply of energy to persist and remain stable. Consequently, the conditions encountered during the preparation of these assemblies lead to intriguing behaviors and distinct assembled structures [46].

Fig. 3.

Identifying the different thermodynamic states in supramolecular self-assembly. Schematic of the Gibbs free energy landscape of a kinetically trapped state, metastable state, thermodynamic equilibrium state, and dissipative state.

In some rare cases, living supramolecular polymerization (LSP) provides tight control over polymer growth and properties, analogous to covalent polymerization [57-59]. LSP is typically initiated by a seed that triggers the polymerization process of the entire system, with monomers continually joining the growing polymer chain in a kinetically controlled manner. This ensures that the resulting supramolecular polymer remains active for further monomer addition. The synthesis of these defined structures can be complicated by factors like spontaneous disassembly and insufficient kinetic control. To actualize LSP, certain conditions must be met, including kinetic control, a cooperative nucleation-elongation mechanism, and seed preparation. Progress in LSP has been made through techniques like crystallization-driven self-assembly of polymeric unimers [60-62], pathway control[63, 64], the design of molecular templates [65, 66], and unimer activation [51].

Dissipative supramolecular polymerization provides a pathway for generating supramolecular polymers in a far-from-equilibrium state by incorporating energy-dissipating processes during supramolecular polymerization [67, 68]. Unlike traditional polymerization, which occurs at thermodynamic equilibrium, dissipative polymerization requires energy input, resulting in inherently dynamic and adaptable structures [69-71]. The process involves an external stimulus that activates the unimeric species, allowing them to self-assemble into the desired structure. Subsequently, the energy is dissipated, causing the disassembly of the supramolecular polymer. This cyclic assembly-disassembly process, powered by an energy source, endows the system with dynamic properties absent in equilibrium states. This strategy holds promise for the rapid design of materials that respond to environmental changes, with potential applications in areas like drug delivery systems, sensors, and functional materials [72, 73]. Nonetheless, achieving precise control over these dynamic systems remains a pressing challenge, warranting further research to optimize their functionality and understand the governing mechanisms.

The minimum in the energy landscapes of a supramolecular peptide amphiphile system is primarily defined by electrostatic, steric and other repulsive interactions, and the ability of the dominant attractive forces to trap molecules in thermodynamically unfavorable configurations (Fig. 4A) [21]. These competing interactions can be selectively switched on and off, with the order of doing so determining the position of the final products in the energy landscape, resulting in biomaterials of distinct biological properties. For example, within the same energy landscape, the chosen peptide amphiphile (PA) forms a thermodynamically favored product, characterized by long bundled fibers that promote biological cell adhesion and survival, and a metastable product characterized by short monodisperse fibers that interfere with adhesion and can lead to cell death [21]. This finding suggests that, in supramolecular systems, functions and energy landscapes can be closely linked, superseding the more traditional connection between molecular design and function.

Fig. 4.

Strategies to control the dimensional and structural features of supramolecular polymers. (A) Energy landscapes of peptide amphiphile self-assemblies with differing levels of molecular charge repulsions. Under low repulsion, long β-sheet fibers form, whereas high repulsion creates an energy barrier leading to a thermodynamically favored product and a kinetically trapped assembly. Various processes like dilution, annealing, and salt addition can be used to manipulate these assemblies, ultimately affecting their morphologies and internal structures [21]. Copyright 2016. Reprinted with permission from Springer Nature. (B) The use of entropic constraints to create self-limiting supramolecular polymerization. The cooperative polymerization process features a distinct nucleation kinetic rate constant (K_N) that is smaller than the elongation kinetic rate constant (K_E). The monomers with multiple oligoethylene glycol segments underwent a spontaneous self-limiting supramolecular polymerization (SPZ) process to yield supramolecular polymers of controlled lengths and narrow distribution (PDI = ~1.2). [74], Copyright 2021. Reproduced with permission from American Chemical Society. (C) The competitive catalytic transacylation and hydrolysis leads to transient production of a self-assembling aromatic peptide amphiphile from amino acid precursors. [75], Copyright 2021. Reproduced with permission from American Chemical Society.

The reversible characteristic of non-covalent interactions often leads to the formation of living SPs with high dispersity in length (Fig. 4B). Su et al. present an approach called self-limiting supramolecular polymerization (SPZ), which utilizes multiarmed amphiphiles with propagation-attenuated reactivities to automatically terminate the polymerization process [74]. By incorporating multiarmed oligoethylene-glycol (OEG) onto a quadratic aromatic segment, repulsive interactions can be built up with the increase of the degree of polymerization, eventually leading to the termination of chain growth. Therefore, the lengths of the resulting SPs can be finely tuned by using a different number of OEG segments. The researchers successfully achieved SP lengths ranging from approximately 1 μm to 130 and 50 nm, with a polydispersity index of around 1.2 for the last two SPs with more OEG arms. This control over length and polydispersity is attributed to the level of chain frustration of the multiarmed OEG segments, determined by the number of arms and the degree of polymerization, which imposes physical and entropic constraints on supramolecular propagation below a specific threshold length.

Non-equilibrium nanostructures open opportunities for mimicry of the behavior of dynamic gels found in natural systems and provide components for future adaptive artificial systems. The combination of molecular self-assembly and (bio)catalysis underlies dynamic processes in biology and provides a useful paradigm for fabrication of adaptive nanostructures [74]. A key feature of naturally occurring catalytic self-assembling systems is that they are dynamic in nature, with lengthening and breakdown tightly regulated. Synthetic mimics of these systems have been the focus of considerable research efforts in recent years. One way most synthetic systems differ from their natural counterparts is that the latter (e.g., microtubules) display dynamic instability, in that they are assembled and lengthened in a process that relies on energy input (away from equilibrium) and shortened when equilibrium is approached. Ulijn and coworkers describe the transient formation of supramolecular peptide nanofibers that display dynamic instability [75]. The systems are based on competitive catalytic transacylation and hydrolysis, producing a self-assembling aromatic peptide amphiphile from amino acid precursors that temporarily exceeds the critical gelation concentration, until the competing hydrolytic reaction takes over [75]. The process results in macroscopically observable temporary hydrogelation, which may be repeated upon refueling the system with further additions of the chemically activated amino acid precursor (Fig. 4C).

Significant effort is also invested in developing nonequilibrium molecular systems and materials which may give rise to new features that are normally not associated with synthetic systems, in that they may be reconfigurable, externally fueled, self-healing, or even self-replicating [76, 77]. Boekhoven et al. reported an exciting example of a nonequilibrium catalytic self-assembly system demonstrating temporary hydrogelation based on catalytic esterification of a diacid [67]. Von Maltzahn et al. demonstrated a nonequilibrium biocatalytic nanoparticle assembly system which could assemble upon kinase/adenosine triphosphate (ATP)-driven phosphorylation and subsequently disassemble in response to a phosphatase enzyme [78]. ATP-driven biocatalytic assembly/disassembly of supramolecular fibers has also been demonstrated using phosphatase/kinase systems [79]. However, for these systems the ATP-driven reaction results in formation of the phosphorylated, non-assembling species, and these are therefore not examples of dynamic instability or nonequilibrium self-assembly.

4. Individual supramolecular polymers as carriers for therapeutic and diagnostic agents

4.1. Interactions of Supramolecular Polymers with Biological Systems

The dynamic and responsive nature of SPs paves the way for advancements in molecular sensing and therapeutics [80-82]. The dynamic properties stemming from their inherent ability to undergo self-assembly and adaptability, affords a diverse array of architectures and interactions that can be meticulously tailored to address specific medical needs [83]. What truly sets SPs apart is their reconfigurable nature; akin to sophisticated molecular machinery, they exhibit the capability to assemble, disassemble, and reassemble in direct response to environmental changes [16, 84, 85]. Such a remarkable capacity for responsiveness and adaptability holds the potential to catalyze significant progress in the efficacy of both sensing methodologies and responsive therapeutic interventions, fostering the development of more intelligent, adaptive, and precise health-related mediations.

Equally important is their ability to present biological signals in a coordinated manner and with a tunable density, because of molecular assembly into supramolecular constructs[90-92]. A crucial aspect in their biomedical application lies in the supramolecular architecture of these polymers. Features like size, shape, and surface properties play a fundamental role in their interactions with cells [93-95]. The cellular uptake, intracellular trafficking, and ultimate fate within the cell can all be influenced by these factors [96-98]. SPs can be tailored to interact with certain cell types or cellular components by choosing suitable building blocks for supramolecular assemblies [86, 99]. This ability opens the possibility to modulate their bioactivity by tuning their supramolecular architecture. An interesting example of this lies in the challenge of using water-soluble molecular probes for intracellular sensing [86, 100]. Due to poor cellular uptake, these probes often fail in effectively sensing intracellular environments. To overcome this, Lock et al. proposed a strategy of self-assembling these molecular probes into supramolecular nanoprobes [86]. By changing their shape, their interaction mechanisms with cells can be altered, promoting or reducing their cellular uptake. In their work, two self-assembling molecular beacons were designed, both exhibiting the same chemical structures except for their terminal residues (Fig. 5A), one of which had three positively charged lysines (K), while the other had three negatively charged glutamic acids (E). Both molecules can be manipulated to assemble into either spherical or filamentous nano-beacons. The formation of filamentous nano-beacons from molecular beacons notably attenuates their efficiency in cellular internalization, with negligible uptake observed under the experimental parameters utilized. Conversely, cationic spherical nano-beacons manifest a substantially enhanced cellular uptake rate relative to their monomeric forms, positioning them as highly advantageous for intracellular sensing applications, such as the detection of lysosomal protease cathepsin B [86].

Fig. 5.

Possible mechanisms of interactions between supramolecular biomaterials and cells to improve therapeutic efficacy. (A) Enhanced or reduced cellular uptake of supramolecular constructs determined by their size and surface charge. [86], Copyright 2016. Reproduced with permission from American Chemical Society. (B) supramolecular materials can rapidly respond to multifarious external stimuli, thereby recreating aspects of the dynamics present in living systems [87], Copyright 2016. Reproduced with permission from American Chemical Society. Supramolecular biomaterials can also respond to chemical and physiological cues. For instance, glucose (C) [88] and enzyme levels (D) [89]. [88], Copyright 2021. Reproduced with permission from American Chemical Society. [89], Copyright 2015. Reproduced with permission from American Chemical Society.

Beyond their interaction with cells, SPs also exhibit a dynamic nature that allows them to rapidly respond to various external stimuli. This characteristic makes them promising tools to mimic living systems' dynamics [101, 102]. SPs can harness physical stimuli such as light, temperature, or magnetic fields to control their interactions with cells. For example, magnetic-sensitive nanostructures can be designed to react to the unique microenvironments found within cells or specific tissues, enabling on-demand release of therapeutics or modulation of cellular responses [103]. Similarly, photo-responsive supramolecular structures can change their conformation, size, or disassembly in response to light, altering their cellular interactions. A multicomponent coordination self-assembly strategy was employed to design and engineer light-responsive metallo-nanodrugs for antitumor therapy (Fig. 5B) [87]. These metallo-nanodrugs were constructed based on the coordination, hydrophobic, and electrostatic interactions among short peptides, photosensitizers, and metal ions. The resulting metallo-nanodrugs exhibited uniform sizes, well-defined nanosphere structures, and high loading capacities.

The chemical environment, such as the ionic strength, pH, and presence of redox agents, significantly influences the stability and behavior of SPs [ 104-109]. By understanding these effects, it is possible to design supramolecular assemblies that are stable in the extracellular environment but rapidly disassemble in response to specific intracellular conditions to release their payload [110-113]. Moreover, by exploiting the unique chemical environments within different cellular compartments, it is possible to design nanostructures that target specific organelles, such as extracellular matrices or nuclei [114]. An approach to glucose-responsive materials was proposed. Unlike traditional strategies that engineer materials to degrade and release insulin under high-glucose conditions, this study employed transient nanofibrillar hydrogel materials that were stabilized in the presence of glucose (Fig. 5C) [88]. The hydrogels were destabilized under conditions of limited glucose to release encapsulated glucagon, which is a key antagonist to insulin in responding to hypoglycemia by signaling the release of glucose stored in tissues to restore normal blood glucose levels. The efficacy of this approach was tested in diabetic mice, showing a successful release of glucagon in response to a sudden drop in blood glucose, brought on by an overdose of insulin. This new paradigm in glucose-responsive materials offers a rare functional example using a disease-relevant fuel to drive the deployment of a therapeutic agent.

Supramolecular polymers can also be designed to interact with biological molecules such as enzymes or nucleic acids, thereby influencing cellular signaling pathways [115-120]. For example, supramolecular assemblies can be designed to inhibit or enhance protein-protein interactions (Fig. 5D), which consequently affect signal transduction pathways and ultimately influence cellular behavior and microenvironments [121-123]. Notably, therapeutic supramolecular polymers with responsiveness to tumors have attracted considerable attention. A diverse array of strategies has been meticulously developed to enhance their therapeutic efficacy [124, 125]. These strategies encompass precise targeting of mitochondria and fibrin, transmembrane receptor interactions, reactive oxygen species (ROS) regulation, and tumor microenvironment modulation [89, 126-128]. Not only is treatment effectiveness improved, but drug resistance can be addressed and metastasis is inhibited, presenting a comprehensive and compelling avenue for advancing cancer therapy [129-131].

Enzyme-instructed self-assembly (EISA), a concept pioneered by the Xu Lab [132-141], signifies a compelling frontier in cancer treatment, promising innovative techniques and broad potential. The dynamic and adaptable nature of EISA harnesses the power of supramolecular nanostructures to selectively inhibit cancer cells by simultaneously targeting multiple cancer hallmarks [142]. Despite these promising capabilities, the design of small molecules for EISA from the vast molecular space remains a significant challenge. EISA involves a complex process where the self-assembling capacity of small molecules crucially influences its anticancer activity. For EISA precursors, composed of an N-capped d-tetrapeptide, a phosphotyrosine residue, and a diester or a diamide group, it has been observed that the anticancer potential largely corresponds to their self-assembling abilities [143, 144]. Irrespective of the stereochemistry and the regiochemistry of the tetrapeptidic backbones, the effectiveness of these precursors against cancer cells is inextricably linked to their capacity to self-assemble (Fig. 6A) [145]. The assemblies of these small peptide derivatives trigger cell death and instigate a significant rearrangement of cytoskeletal proteins and plasma membranes. The diester or diamide derivatives of the D-tetrapeptides self-assemble both pericellularly and intracellularly to initiate cell death. This study associates thermodynamic properties, such as the self-assembling ability of small molecules, with the efficacy of a molecular process against cancer cells, a crucial insight for developing potential cancer therapies.

Fig. 6.

Enzyme-instructed peptide assemblies. (A) Molecular structures of the precursors and the correlation between the ability for self-assembly of small molecules and anticancer activity. The self-assembling ability of EISA determine the potency of EISA against cancer cells. [142], Copyright 2023. Reproduced with permission from American Chemical Society. (B) Schematic depictions of the membrane engineering outcomes using peptide–protein (top panel) and peptide–peptide (bottom panel) co-assembling strategies, capable of responding to ALP and interacting with EGFR, for cancer cell membrane modifications. [146], Copyright 2023. Reproduced with permission from American Chemical Society.

A fascinating aspect of EISA is the capability of an in-situ approach, which manipulates cell-cell interactions to boost cell-based cancer therapeutics [147-149]. Yang and coworkers devised a method that employs EISA to selectively modify cancer cell membranes [146]. This work utilized phosphopeptides targeting the membrane-bound epidermal growth factor receptor (EGFR) and illustrated that site-specific phosphorylation patterns in these peptides command their preorganization levels, self-assembling kinetics, and the spatial distribution of the resultant peptide assemblies in cells (Fig. 6B). Intracellular sensing of pathologically relevant biomolecules, a vital aspect of disease evaluation and progression, is another potential use of EISA [150]. The self-assembly of molecular probes into supramolecular nanoprobes presents a strategy to alter their interaction mechanisms with cells, to either promote or reduce cellular uptake. This method provides valuable guiding principles for the design of supramolecular nanoprobes with adjustable cellular uptake characteristics, thereby opening the door for more versatile and effective cancer treatments.

4.2. Supramolecular Polymers as Long-Circulating Drug Carriers

The use of therapeutic supramolecular polymers as long-circulating drug carriers represents a significant and promising development in the field of drug delivery [151-154]. It has become increasingly clear that self-assembled prodrugs can be engineered into supramolecular materials to increase drug loading efficiencies and control release kinetics [15, 155-159]. Owing to their customizability and dynamic properties, these nano-assemblies surpass many traditional carriers in delivering drugs efficiently, with high specificity [160-162]. An impressive feature of these high-aspect-ratio nanostructures is their prolonged circulation time in the bloodstream, which results from the evasion of rapid immune system clearance [93, 163]. This attribute increases the bioavailability of the encapsulated drug, significantly improving its pharmacokinetic profile [164]. Furthermore, the adaptable physical and chemical characteristics of these polymers allow for the creation of responsive drug delivery systems that can selectively release their payloads in response to specific triggers. Significantly, the exploration of employing therapeutic agents as fundamental molecular components has introduced a novel avenue for constructing therapeutic supramolecular polymers with a wide range of medical applications[15, 74, 155, 165-173]. In this section, we explore the potential of supramolecular polymers as long-circulating drug carriers, discussing their design strategies, interaction with biological systems, drug encapsulation and release mechanisms, as well as recent progress in effective disease management.

Filamentous supramolecular nanostructures, namely filomicelles, engineered for the delivery of paclitaxel have demonstrated prolonged circulation times and efficacious targeting of tumor sites for chemotherapy [93, 174]. Inspired by the early work by the Discher Lab, Cui and coworkers performed a study that evaluated four self-assembling prodrugs carrying camptothecin (CPT) as the drug payload, and each exhibiting a different critical micellization concentration (CMC) value [155]. Their findings illustrate that a lower CMC yields more stable SPs, with SAPD-1 possessing the lowest CMC (2.7 μM) and forming the most stable filamentous SPs [155, 170]. This stability led to enhanced tumor growth suppression efficacy in mouse models. However, this improved efficacy was countered with increased toxicity. Pharmacokinetic investigations demonstrated that lower CMC values slowed plasma clearance and increased drug accumulation in both tumor and healthy tissues, as compared to those with higher CMCs, which dissociated rapidly into monomers (Fig. 7A). The connection between CMC, SP stability, therapeutic efficacy, toxicity, and pharmacokinetics illustrates the intricate balance required when engineering supramolecular drug delivery systems. Hence, adjusting the CMC emerges as a vital parameter in designing SP drug delivery systems to modulate the therapeutic index, offering an avenue to optimize the efficacy-toxicity balance. It indicates how the careful engineering of supramolecular polymers' CMC might provide a direction to maximize therapeutic outcomes while minimizing potential side effects. Zwitterionic micelles with extraordinarily low CMCs, specifically below 10⁻⁶ mM, could have substantial implications for enhancing drug targeting and improving therapeutic outcomes [175]. This low CMC aids in preventing premature micelle disassembly and the consequent loss of the encapsulated drug. These micelles, owing to their significantly low CMC, exhibit remarkable stability against dissociation and payload release when diluted, which hold substantial promise for fine-tuning the stability and effectiveness of micelle-based drug delivery systems.

Fig. 7.

Supramolecular polymers for long-circulation drug delivery. (A) The pharmacokinetic profiles of the four SAPD SPs in Sprague–Dawley (SD) rats demonstrated CMCs represent an important characteristic to determine the morphological and structural integrity of supramolecular assemblies during circulation. SAPD 1-4 represent self-assembling prodrugs comprising two CPT moieties and OEG decorated peptides of various OEG numbers (2, 4, 6, 8). [155], Copyright 2020. Reproduced with permission from National Academy of Science. (B) The development of supramolecular polymer prodrugs using a small-molecule, prodegenerative drug and a low-molecular-weight, hydrophilic polymer shows that subtle changes in prodrug molecular architecture have important effects on self-assembly morphology, drug release profiles, and cellular internalization. [176], Copyright 2023. Reproduced with permission from National Academy of Science.

Messersmith and coworkers examined the structure-property relationship of supramolecular polymer prodrugs derived from poly(ethylene glycol) (PEG) and 1,4-dihydrophenonthrolin-4-one-3-carboxylic acid (DPCA), a hydrophobic, proregenerative small-molecule drug (Fig. 7B) [176]. They synthesized three PEG-DPCA prodrugs, each displaying a different ratio of DPCA molecules to PEG chain. Higher DPCA content led to a transformation in the morphology of supramolecular structures from spherical micelles to worm-like micelles and nanofibers. The prodrug with a single DPCA molecule per PEG chain (P7D1) exhibited the fastest hydrolysis kinetics below the CMC. Above the CMC, hydrolysis was slowest for P7D1, likely due to the protective nature of the hydrophobic core. P7D1 showed limited bioactivity in stabilizing HIF-1α in cells compared to the prodrug with three DPCA molecules per PEG chain (P7D3), which was attributed to the differences in cellular uptake driven by the molecular architecture. Modeling clarified how DPCA content could guide self-assembly, with higher DPCA content leading to fewer, larger clusters and the formation of high aspect ratio fibers. The design of prodrug molecules dramatically influences supramolecular morphology, drug release kinetics, cellular delivery, and bioactivity. Thus, understanding fundamental structure-property relationships is instrumental for the advanced molecular and therapeutic engineering of supramolecular polymers based on hydrophobic small molecules [177].

4.3. Supramolecular Polymers for Imaging and Diagnostics

The performance of in vivo imaging methodologies is heavily contingent upon the employed molecular imaging agent. The translation of theoretical understanding into tangible clinical benefits necessitates the real-time visualization of biological phenomena at the cellular and molecular levels. The burgeoning demand for precise diagnostic and therapeutic tools has catalyzed advancements in the realm of molecular imaging, a discipline that leverages the versatile capabilities of nanoparticles for target identification and drug conjugation [178]. For effective in vivo molecular imaging, nanoparticle carriers navigate through a complex landscape of biological barriers prior to establishing specific interactions with target biomolecules[179]. Such interactions necessitate that the nanoparticles achieve selective molecular recognition within the specialized microenvironments that harbor their intended targets [180-182]. The intricacy of this process mandates the employment of more refined strategies to ensure spatial control over therapeutic delivery. The triumph of these nanoparticle-centric systems hinges on the creation of stimuli-responsive systems to specific biological or chemical triggers. Moreover, exploration into biocatalytic control of assembly methodologies promises to instill greater precision in target delivery. A crucial factor meriting attention is the physical characteristics of these nanoprobes [183, 184]. The geometry and aspect ratio of a nanoscale delivery vehicle significantly influence its interaction with the circulatory system, inclusive of its margination and adhesion to the vascular wall [185, 186]. Consequently, investigation into optimal configurations for these characteristics may yield significant dividends in the application of supramolecular polymers in therapeutic interventions.

Most magnetic resonance (MR) agents capitalize on the paramagnetic metal ion gadolinium (Gd(III)), chosen for its seven unpaired electrons and extended electronic relaxation time [187-190]. The established reliance on Gd-based materials in magnetic resonance imaging (MRI) contrast agents has prompted substantial safety concerns, given their inherent toxicities when ionized [191, 192]. This apprehension underscores the urgent need to pioneer safer, metal-free alternatives. A promising avenue explores the utility of supramolecular polymer hydrogels, derived from FDA-approved drugs, as novel MRI contrast agents that double as drug delivery platforms [193-195]. A supramolecular polymer from an anticancer drug, Pemetrexed (Pem), was used to engineer a distinctive type of MRI contrast agent (Fig. 8A) [196]. Through strategic peptide conjugation, Pem was transformed into a molecular hydrogelator, emitting innate chemical exchange saturation transfer (CEST) MRI signals [196]. This drug-peptide compound self-assembles into filamentous structures under physiological conditions, yielding theranostic supramolecular hydrogels that are apt for injectable delivery. The unique attributes of Pem, namely the exchangeable amine protons, engender an intrinsic CEST MRI signal at 5.2 ppm. When conjugated to a short peptide to create an amphiphilic molecule dubbed PemFE, it demonstrates a propensity to self-organize into filamentous nanostructures. At increased concentrations, PemFE aggregates into a physical hydrogel consisting of intertwined nanofibers approximately 9 nm in diameter. Of interest, the hydrogel structure of PemFE shows a slower hydrolytic drug release in comparison to free Pem, potentially attributable to the hydrophobic protection afforded by the assembled structure. Utilizing an animal model, a local injection of the PemFE hydrogel into mouse brain tumors remained detectable by CEST MRI for a duration of up to four days. Furthermore, CEST MRI enabled the visualization of the spatial and temporal diffusion of the PemFE hydrogel within the tumor. The integrating of theranostic functionality within a supramolecular polymer represents an innovative approach in the development of next-generation, drug-based theranostic supramolecular materials, enabling non-invasive tracking of their in vivo distribution and drug release.

Fig. 8.

Supramolecular Polymers for Imaging and Diagnostics. (A) Supramolecular strategy to convert an FDA-approved anticancer drug, Pemetrexed (Pem), to a molecular hydrogelator with inherent chemical exchange saturation transfer (CEST) MRI signals. [196], Copyright 2017. Reproduced with permission from American Chemical Society. (B) C-SNAF in vivo is dependent on the tumor response to chemotherapy (Rx). After intravenous administration, C-SNAF extravasates into tumor tissue because of its small size. In live tumor tissue that does not respond to applied chemotherapy, the pro-caspase-3 is inactive and the DEVD capping peptide remains intact. Caspase-3/7 and GSH reduction control the conversion of C-SNAF into C-SNAF by cycling through the bio-orthogonal intramolecular cyclization reaction, followed by self-assembly into nanoaggregates in-situ. [197], Copyright 2014. Reproduced with permission from Springer Nature. (C) Porphyrin derivatives that can emit near-infrared persistent luminescence (PL) over 60 min are reported and a mechanism for the phenomenon are used to synthesize supramolecular probes with peroxynitrite-activated PL and light-triggered imaging-modality transformation characteristics that permit improved biological uses. [198], Copyright 2022. Reproduced with permission from John Wiley & Sons, Inc.

In an innovative approach to enhance the therapeutic efficacy of Bortezomib (BTZ), a boronate proteasome inhibitor, natural polyphenols have been repurposed from their conventional role as inhibitors into facilitators. This paradigm shift facilitates the creation of a novel category of supramolecular nanomedicines, which navigate around the conventional impediments posed by dietary polyphenols, primarily due to boronate-catechol complexation [199]. By strategically conjugating BTZ with catechol-rich natural polyphenols using a boronate ester bond, the researchers created a dynamic series of drug amphiphiles. Characterized by pH-responsive assembly and disassembly behavior, these amphiphiles align with various physiological conditions. Through the incorporation of ferric ions into the supramolecular system via metal-phenolic coordination interactions, the nanomedicines achieved bioimaging functionality, while also enhancing their stability. The researchers particularly utilized natural polyphenols such as tannic acid and catechin, which were conjugated to BTZ through dynamic covalent catechol-boronate bonds to generate amphiphilic drug conjugates. These conjugates were further stabilized via the addition of iron(III) ions, culminating in the formation of 100 nm nanoparticles via metal-phenolic coordination interactions. One of the commendable features of these nanoparticles is their pH-responsive drug release. In acidic tumor microenvironments, the BTZ release rate was found to be accelerated, making these nanoparticles highly suitable for targeted anticancer activity. This attribute was validated in vitro, where the nanoparticles exhibited low toxicity at a neutral pH (7.4), while toxicity increased significantly at pH levels of 5-6.5. In preclinical models, a nanoparticle comprising tannic acid, BTZ, and iron was found to be more effective in inhibiting tumor growth and bone resorption than free BTZ. Additionally, this nanoparticle facilitated T1-weighted MRI contrast, enabling effective imaging of tumor accumulation.

The in-situ self-assembly of synthetic small molecules into nanoaggregates within living murine models has been exploited for visualizing tumor responses to chemotherapy. Notably, the fluorescence imaging probe, C-SNAF, has elucidated the potential of tracking chemotherapy-induced tumor responses in vivo [197]. The distinctive modus operandi of C-SNAF hinges upon an intramolecular cyclization reaction initiated by caspase-3/7 cleavage in tandem with disulfide reduction. In consequence of this reaction, the petite molecular probe transforms into a hydrophobic macrocycle, instigating the formation of self-assembled fluorescent nanoparticles (Fig. 8B). These nanoparticles have exhibited exceptional proficiency in labeling apoptotic cancer cells over their healthy counterparts, attributed to the fluorescence emitted upon caspase-3/7 activation. The in vivo deployment of C-SNAF delivered striking results. Intravenous injection of C-SNAF triggered a noticeable surge in tumor fluorescence in chemotherapy-treated murine models relative to untreated ones. This finding underscores the potency of this tool in illuminating chemotherapy-induced caspase-3/7 activity and ensuing apoptosis. To further corroborate this effect, the self-assembled nanoparticles were visualized directly within tumor tissues using sophisticated 3D microscopy. The fluorescence originating from C-SNAF within tumors was discovered to be strongly correlated with caspase-3 activation, as well as the magnitude of tumor response to chemotherapy. Such observations underscore the enormous potential of C-SNAF in affording real-time, non-invasive insights into the therapeutic efficacy of chemotherapy.

The amalgamation of porphyrin derivatives, with near-infrared emission capabilities upon light excitation or peroxynitrite oxidation, instigate novel avenues in imaging and diagnostic applications, specifically in persistent luminescence imaging [200-203]. Owing to its immunity against autofluorescence interference, this imaging modality harbors potential for biological applications, which are often marred by compromised clarity and precision due to such interferences in other imaging techniques. Nonetheless, the development of materials capable of long-wavelength emission suitable for clinical use presents considerable challenges, particularly concerning biosafety considerations, material instability, and the propensity for short-wavelength emission. [204, 205]. The fusion of Protoporphyrin IX (Ppa) with a self-assembling peptide motif, FFGYSA, has been demonstrated to amplify both photoacoustic and persistent luminescence signals. This supramolecular self-assembly intensifies imaging signals and allows for modality transformation, constituting a remarkable stride in imaging and diagnostic technology [198]. In vivo trials have evidenced the light-induced transformation from photoacoustic to persistent luminescence imaging via the supramolecular probe Ppa-FFGYSA, facilitating enhanced tumor excision (Fig. 8C). This progress is pivotal in oncological surgery as it supports more accurate and effective tumor tissue removal. The research reveals a new facet of porphyrins—the capacity to emit persistent luminescence via oxidation of vinylene bonds. Furthermore, the peroxynitrite-activated persistent luminescence of Ppa-FFGYSA expedites the screening of immunogenic cell death drugs, thereby offering potential for accelerated development and implementation of novel cancer therapies.

4.4. Supramolecular Polymers as Inhalable Carriers

Supramolecular polymers have garnered significant attention as promising materials for the development of inhalable carriers in the realm of nanomedicine [206]. These supramolecular polymers offer distinct advantages over traditional covalent polymers due to their reversible noncovalent interactions, which enable precise control over their physicochemical properties and functionalities. Simultaneously, the field of inhalable materials has witnessed remarkable progress in the design of carriers capable of delivering therapeutic agents directly to the lungs [207-209]. Inhalable materials, often in the form of nanoparticles or microparticles, possess unique characteristics that enable efficient pulmonary drug delivery, including high surface area, favorable aerodynamic properties, and the ability to bypass barriers in the respiratory system [210-212]. By integrating the principles of supramolecular polymers and inhalable materials, researchers have opened new avenues for the development of inhalable nanomedicine systems. The self-assembly behavior of supramolecular polymers allows for the creation of carriers with tailored properties, such as controlled release kinetics, enhanced stability, and improved biocompatibility. These characteristics are crucial for ensuring the efficient and targeted delivery of therapeutic agents to the lungs, making supramolecular polymers an attractive choice for inhalable carriers.

Anderson et al. designed several peptide amphiphiles with varying charges that are capable of self-assembling into filamentous structures upon their introduction into aqueous solutions [213]. They present a proof of concept for utilizing peptide-based supramolecular filaments as inhalable drug carriers within liquid aerosol droplets generated by a jet nebulizer. Notably, the resulting filaments displayed the unique capability to encapsulate drugs and dyes relevant to lung diseases. As shown in Fig. 9A, peptide amphiphile monomers, comprised of a hydrophobic alkyl chain (green) and a hydrophilic peptide segment (blue), encapsulated with hydrophobic drugs and dyes (red), undergo spontaneous self-assembly, leading to the formation of supramolecular filaments, with the drug/dye being loaded within the hydrophobic core of the filaments (left). Subsequently, the solutions containing the supramolecular filaments are loaded into a jet nebulizer, which is shown in the left of Fig. 9A, facilitating the generation of liquid aerosol droplets carrying the filaments.

Fig. 9.

(A) Inhalable supramolecular polymers formed by self-assembly of peptide amphiphile composed of a hydrophobic alkyl chain (green) and a hydrophilic peptide segment (blue) in aqueous solutions. Solutions of these filaments are then loaded into a jet nebulizer to produce filament-bearing liquid aerosol droplets. [213], Copyright 2019. Reproduced with permission from American Chemical Society. (B) Peptide nanofibers constituted of an epitope from influenza acid polymerase and Q11 domain were internalized by dendritic cells in lung-draining mediastinal lymph nodes after intranasal immunization, leading to a notable augmentation in the number of enduring antigen-specific tissue-resident memory CD8+ T cells within the pulmonary tissues. [214], Copyright 2018. Reproduced with permission from Elsevier Science Ltd..

Transmission electron microscopy (TEM) comparative analyses of filament-containing solutions pre- and post-nebulization reveal a reduction in filament length following the nebulization process. This alteration in filament dimensions could potentially amplify therapeutic efficacy and may confer advantages for pulmonary delivery. Similarly, peptide nanofibers delivered intranasally were found to markedly enhance the population of persisting antigen-specific tissue-resident memory CD8+ T cells in the lung compared to those delivered subcutaneously (Fig. 9B). This resulted in a more expedited response to infection six weeks post-vaccination. The findings underscore the immunogenicity of intranasally delivered self-assembled peptide nanofibers, even when they deliver CD8+ epitopes without adjuvant or CD4+ epitopes. These nanofibers are non-inflammatory and foster a larger population of lung-resident memory CD8+ T cells compared to subcutaneous immunization. This platform presents the potential for facilitating needle-free vaccination via the intranasal route, provided it is designed with multiple pathogen epitopes [214].

Recent studies also show that supramolecular polymers could serve as a platform for coronavirus capture and infection prevention. The entry of SARS coronavirus 2 (SARS-CoV-2) variants into host cells is primarily mediated through the formation of complexes with angiotensin-converting enzyme 2 (ACE2) [215, 216]. Anderson et al. developed supramolecular filaments, denoted as fACE2, with the purpose of suppressing ACE2’s enzymatic function and anchoring ACE2 to their surface through the formation of enzyme-substrate complexes [217]. The fACE2 solution is formulated using a ligand, which is a well-established peptide inhibitor with potent activity against ACE2 enzymatic function. This ligand is strategically incorporated into the design of an ACE2-bingding peptide amphiphile and combined with a filler component that possesses self-assembling properties, forming the filaments (Fig. 10A).

Fig. 10.

Supramolecular polymers dock ACE2 to enhance and extend inhibition of viral entry of SARS coronaviruses. (A) The two PAs can be dissolved together in aqueous solution to spontaneously associate and co-assemble into supramolecular filaments that display the ACE2-binding ligand on their surfaces. Subsequently, soluble ACE2 can be added to filament solutions to allow binding via inhibition of the ACE2 proteolytic active site to yield decoy ACE2-docking filaments, called fACE2. (B) Chemical structure of the investigated ACE2-binding ligand (top) and filler (bottom) PA molecules. (C) Inhibitory activity of fACE2 and Fil after set incubation times (continuing from F) against SARS-CoV-2 PsV viral entry. [217], Copyright 2023. Adapted with permission from Elsevier Science Ltd..

Upon achieving a 20:1 molar ratio of filler to ligand in an aqueous environment, the fACE2 solution demonstrates the ability to self-assemble into supramolecular filaments, as evidenced by the TEM images presented in Fig. 10B. Subsequently, soluble ACE2 can be introduced to these filament solutions, facilitating its binding to the surface-bound ligand. As shown in Fig. 10C, the fACE2 solutions can be administered to the nasal passages and lung tissue, enabling the decoy ACE2 to interact with the spike proteins of SARS-CoV and effectively impede viral entry into host cells. This novel docking approach facilitated the efficient delivery of ACE2 via inhalable aerosols, leading to improved structural and functional integrity of ACE2. Their results indicate the fACE2 demonstrates superior and sustained inhibition of viral entry compared to ACE2 in isolation as shown in Fig. 10C, while concurrently mitigating pulmonary damage in vivo.

5. Supramolecular polymeric hydrogels

One exciting feature of supramolecular polymers is their ability to form hydrogels through reversible and non-covalent connections [28, 218]. The effective entanglement of supramolecular polymers in response to an increase in concentration, addition of counterions or other molecules, or a variation in solution temperature, could lead to the formation of supramolecular polymeric hydrogels (SPHs) [219-223]. Rational molecular design of biofunctional supramolecular materials offer many advantages, including responsiveness, reversibility, tunability, biomimicry, modularity, predictability, and most importantly, adaptiveness [224-227]. Given their unique advantages, supramolecular polymeric hydrogels have been explored for localized and systemic therapy because of their tunable mechanical properties, stimuli-responsive drug release, and injectability [218, 228-232].

5.1. Local and Topical Delivery

Supramolecular hydrogels with shear-thinning and self-healing properties are positioned in a particularly interesting class of injectable platforms for drug delivery and enhanced localization [233, 234]. In the case of injectable or sprayable materials, large changes in viscosity upon application of shear (shear-thinning behavior) allow the materials to be prepared as a solid-like gel that can flow in a liquid-like state upon applied stresses [235]. Compared to covalently bonded hydrogels, supramolecular polymers that employ reversible, non-covalent interactions show superior recovery of mechanical and biomaterial function and can be administered in a minimally invasive manner at the target site [236-238]. Supramolecular hydrogels that can be sprayed or syringe injected onto large anatomically complex, uneven surfaces [239, 240] and deliver therapy could potentially be used as a surgical adjuvant if they could be effectively delivered to debulked tissue surfaces [241]. When appropriately designed, supramolecular polymeric hydrogels allow for controlled release either in response to specific enzymes [231, 242] or through effective monomer designs[243].

One important contribution to the field of one-component supramolecular hydrogels for therapeutic delivery in regenerative medicine was made by the Meijer laboratory [244] . The lab developed a transient network based on supramolecular polymers consisting of PEG end-functionalized with four-fold hydrogen-bonding ureidopyrimidinone (UPy) moieties that are shielded in a hydrophobic alkyl pocket decorated with a urea motif primed for lateral hydrogen-bonding (Fig. 11A) [244]. By tuning the hydrophobic alkyl spacer and hydrophilic PEG chain length, the gelation dynamics and erosion kinetics can be optimized. Due to its nonlinear structural formation, this system allows active proteins to be added to the hydrogel during formation. As a proof-of-concept, bone morphogenetic protein 7 was efficiently encapsulated in the gels using a simple heating/cooling process. Once implanted in vivo it releases the protein via dissolution as the gels gradually eroded over 7 days.

Fig. 11.

Supramolecular polymers for local and topical therapy. (A) Hierarchical formation of supramolecular transient networks in water as a modular injectable delivery system. [244], Copyright 2012. Reproduced with permission from John Wiley & Sons, Inc. (B) Spraying collagen-binding supramolecular hydrogels into resection cavities covering different surface areas. [240], Copyright 2022. Reproduced with permission from American Chemical Society. (C) A supramolecular polymer nanogel that utilizes host–guest interactions between the groups pillar [5] arene and alkyl chains on hyperbranched polyglycerol backbone as crosslinking agents for a new dermal drug delivery system. [245], Copyright 2012. Reproduced with permission from John Wiley & Sons, Inc.

Spraying serves as an attractive, minimally invasive means of administering hydrogels for localized delivery, particularly due to high-throughput deposition of therapeutic depots over an entire target site of non-uniform surfaces [246]. However, it remains a great challenge to design systems capable of rapid gelation after shear-thinning during spraying and adhering to coated tissues in wet, physiological environments. To address the limitations associated with previous sprayable gels that suffered from inadequate adhesion in wet environments, supramolecular filaments endowed with collagen-binding epitopes have been developed [240]. This approach employs a specially designed self-assembling peptide amphiphile (CBPA) that is enhanced with a unique collagen-binding sequence, thus empowering it to form robust, sprayable supramolecular hydrogels (Fig. 11B). Despite the increased post-spray degradation rate, these hydrogels can maintain their structural integrity and exhibit rapid gelation while upholding a low toxicity profile. In ex vivo experiments, CBPA gels showcased potent adherence to the surfaces of a variety of organs, including the lungs, spleen, and liver. The adhesive properties of CBPA have been strategically harnessed to modulate drug release kinetics through conjugation with the chemotherapeutic agent, paclitaxel. It was observed that as the coated surface area expanded, the rate of drug release into the surrounding fluid decreased, as confirmed by the slow diffusion of CBPA-dye conjugates into the collagen gel.

Nanogels that are assembled by supramolecular interactions as compared to covalent crosslinked nanogels, exhibit new functionalities with potential for eased processability, recycling, and self-healing due to the nature of dynamic and reversible non-covalent interactions. The supramolecular polymer nanogel that utilizes host-guest interactions between the groups pillar [5] arene and alkyl chains on a hyperbranched polyglycerol backbone serve as crosslinking agents for a new dermal drug delivery system [245]. The anti-inflammatory drug Dexamethasone (Dexa) can be efficiently loaded into the nanogels and released from the assemblies. Additionally, the supramolecular polymer nanogels exhibit better drug loading capacity and skin penetration enhancement than the individual host polymer and guest polymer (Fig. 11C). In vitro skin permeation studies show that supramolecular polymer nanogels can improve Nile red penetration through the skin by up to 9-fold, compared to the individual polymers or a conventional cream formulation on a barrier deficient skin model.

Localized, sustained delivery is needed to address the important clinical challenges posed by cancers formed on complicated biological surfaces, such as pleural mesothelioma and peritoneal mesothelioma. The development of a sprayable microRNA surface-fill hydrogel (SFH) is presented as a creative approach to counter the complexities of treating surface cancers such as mesothelioma [239]. This two-component hydrogel ingeniously combines tissue-adhesive peptides with microRNA nanoparticles, formed by electrostatic interactions, all within a self-assembling peptide network (Fig. 12). Notably, the SFH demonstrated the ability to conform to surface coatings, penetrate tissue recesses, and maintain shape stability due to its shear-thinning behavior. The hydrogel offered a sustained in vivo release of miRNA nanoparticles for several weeks, contributing to its therapeutic potential. SFH, incorporating miRNA-215 and miRNA-206, showed a ability to silence target genes and inhibit cancer cell proliferation. This innovative approach yielded significant therapeutic effects: a single SFH application slowed tumor progression in a subcutaneous model, demonstrated effectiveness against aggressive orthotopic peritoneal and pleural mesothelioma, and prevented cancer recurrence post-surgical resection. SFH demonstrated considerable promise as a localized miRNA delivery platform, particularly pertinent for the management of intractable surface neoplasms such as mesothelioma.

Fig. 12.

Application of surface-fill supramolecular hydrogel to treat complex surface cancer. (A) SFH can be sprayed (or syringe injected) into the pleural cavity as a primary treatment or adjuvant after surgical debulking of mesothelioma. (B) SFH uniformly covers and fills complex tissue surfaces to effectively deliver miRNA nanoparticles to tumor cell foci. Cell entry and endosomal escape allows miRNA-mediated attenuation of genes important to oncogenesis. (C) SFH is sprayed onto the surface of a porcine lung. Peritumorally injected SFH delivers miRNA locally to cells in a subcutaneous xenograft. [239], Copyright 2021. Adapted with permission from Springer Nature.

5.2. Cancer Immunotherapy

Cancer immunotherapy, which boosts the patients' own immune system to attack cancer cells, has shown great promise in treating patients with advanced or metastatic tumors [247-249]. However, systemic administration of cancer immunotherapy is often associated with potentially life-threatening side effects as immune cells exert different functions at multiples sites in the body [250]. Supramolecular hydrogels can significantly improve immune therapy efficacy while limiting systemic exposure. Biocompatible macroscale scaffolds can be used for controlled, targeted and local delivery of immunomodulatory factors, including proteins (e.g., cytokines and antibodies), oligonucleotides (e.g., silencing RNA and plasmid DNA) and scavengers that remove or redirect unfavorable factors from injured or diseased tissue sites [119, 251-254].

Immunotherapy aiming to harness the exquisite power of the immune system has emerged as a crucial part of clinical cancer management [255]. Supramolecular hydrogel systems, when combined with immune-modulating agents, have been shown to enable chemoimmunotherapy in tumors that typically resist treatment due to an immunosuppressive microenvironment [230, 256-260]. For example, prior research has highlighted that interferon genes (STING) pathway activation can robustly stimulate innate and adaptive immune responses in the tumor microenvironment, potentially augmenting tumor immunogenicity. However, the associated high toxicity and premature degradation of stimulator of STING agonists have impeded their efficacy. Despite the promising potential of cancer immunotherapy in treating advanced or metastatic tumors, only tumors that incite immune responses have witnessed effective clinical outcomes[261]. Although STING agonists have shown promise in creating an immune-stimulating tumor microenvironment and boosting antitumor immune responses, their use as a monotherapy has yielded limited therapeutic benefit in preclinical trials due to high toxicity, the need for multiple doses, and the necessity of combination with immune checkpoint blockades. Moreover, systemic delivery of cyclic dinucleotides (CDNs) can trigger off-target inflammation or autoimmunity since these molecules are unlikely to selectively localize to tumor tissues. Thus, innovative strategies are required to sensitize tumors to CDNs while minimizing immune-related adverse events.

Hartgerink and coworkers showed that the extended intratumoral release of the STING agonist cyclic di-AMP transforms the tumor microenvironment from immunosuppressive to immunostimulatory, increasing the efficacy of antitumor therapies (Fig. 13A). The STING agonist was electrostatically complexed with nanotubes comprising a peptide–drug conjugate (a peptide that binds to the protein neuropilin-1, which is highly expressed in tumors, and the chemotherapeutic agent camptothecin) that self-assemble in-situ into a supramolecular hydrogel [262]. In multiple mouse models of murine tumors, a single low dose of the STING agonist led to tumor regression and increased animal survival, and to long-term immunological memory and systemic immune surveillance, which protected the mice against tumor recurrence and the formation of metastases. Locally delivered STING agonists could help to reduce tumor immunosuppression and enhance the efficacy of a wide range of cancer therapies.

Fig. 13.

Supramolecular polymers for cancer immunotherapy. (A) MultiDomain Peptide (MDP) which self-assembles to form a STINGel provides controlled release of CDN delivery. [262], Copyright 2018. Reproduced with permission from Elsevier Inc. (B) Localized CPT and CDA delivery using a bioresponsive CPT-based nanotube hydrogel for TME regulation and chemoimmunotherapy [263], Copyright 2020. Reproduced with permission from Springer Nature. (C) Localized PTX and antiCD47 delivery using a PTX-based hydrogel for combined chemoimmunotherapy [172], Copyright 2023. Reproduced with permission from National Academy of Science. (D) Complexing β-cyclodextrin-grafted hyaluronic acid (HA-CD) with a reduction-activatable heterodimer of NLG919 and pyropheophorbide A to perform combination immunotherapy by simultaneous immunogenic cell death (ICD) induction and inhibitor of indoleamine 2,3-dioxygenase 1 (IDO-1) inhibition [264], Copyright 2020. Reproduced with permission from John Wiley & Sons, Inc.

Localized chemotherapy employing supramolecular hydrogel has the potential to work synergistically with immunotherapy, fostering an amplification of immune activation [265, 266]. A drug-bearing supramolecular hydrogel system involved chemically linking a hydrophilic peptide moiety iRGD, a tumor-penetrating peptide capable of binding to neuropilin-1 and triggering tumor tissue penetration, to the hydrophobic anticancer drug CPT to yield a self-assembling and self-formulating peptide–drug conjugate (diCPT–iRGD) [263]. In an aqueous environment, the resultant drug amphiphile spontaneously forms supramolecular nanotubes (Fig. 13B). The negatively charged STING agonist (cyclic di-AMP (CDA)) can bind to the surface of these positively charged nanotubes through electrostatic interactions. Once injected into the tumor site, the CDA-nanotube solution immediately forms a hydrogel, serving as a localized reservoir for extended, localized release of both CDA and CPT, thereby awakening both the innate and adaptive immune systems. The hydrogel facilitated sustained, localized STING agonist release within the tumor over several weeks, compared to the rapid clearance of an injected free STING agonist solution. Upon hydrogel treatment, innate immune cells like natural killer cells and dendritic cells were activated, and there was an increase in tumor-infiltrating CD4⁺ and CD8⁺ T cells. In mouse models of brain, colon, and breast cancer, a single intratumoral injection of the hydrogel containing the STING agonist and camptothecin led to established tumor regression and improved survival. This resulted in a shift in the tumor microenvironment from an immunosuppressive to an immunostimulatory state. The anti-tumor efficacy was dependent on STING signaling and adaptive immune responses, as STING-deficient mice demonstrated a lower response rate. The localized delivery method mitigated the systemic toxicity associated with systemic STING agonist administration, with no adverse effects observed, thereby augmenting anti-tumor immune responses and improving cancer immunotherapy outcomes.

A similar approach to treating glioblastoma multiforme (GBM) introduces a self-assembling paclitaxel filament hydrogel that carries an anti-CD47 therapy [172]. GBM is a challenging form of brain cancer, notorious for its unique immunosuppression properties and lack of T cell infiltration, which often results in poor outcomes for T cell-based immunotherapy (Fig. 13C). In this approach, the paclitaxel filament hydrogel generates an immune-stimulating tumor microenvironment. This strategy sensitizes the tumor to the aCD47-mediated blockade of the "don't eat me" signal, an instruction to the immune system to ignore the cancer cells. By disrupting this signal, the treatment promotes tumor cell phagocytosis by macrophages and initiates an antitumor T cell response [172]. The aqueous solutions containing the paclitaxel filaments and aCD47 are directly applied to the tumor resection cavity, allowing for a seamless hydrogel filling and a long-term release of both therapeutics. This adjuvant therapy has been effective in suppressing primary brain tumor recurrence and prolonging survival with minimal off-target side effects. In a mouse model of glioblastoma, this therapy has shown promising results, including an increased presence of tumor-associated macrophages and greater infiltration of dendritic cells and CD8+ T cells. When combined with anti-CD47, the therapy induced regression of orthotopic glioblastoma tumors and even prevented tumor recurrence after surgical resection. Impressively, the treatment also generated an anti-tumor immunological memory, providing mice with protection from future tumor challenges. This research marked a critical advancement in co-delivery drug systems, bringing together the distinct material properties of the two therapeutic agents for long-acting local release, and stimulating tumor-associated macrophages with a concurrent T cell-mediated immune response for improved tumor treatment. The utilization of supramolecular hydrogels for the delivery of anti-PD1 antibodies offers a promising avenue for manipulating the tumor microenvironment and potentiating anti-tumor immunity, a significance further amplified when employed in synergy with immune checkpoint blockade therapies [260].

Host-guest prodrug nano-vectors have been reported for active tumor targeting and combating immune tolerance in tumors [267-269]. The prodrug nano-vectors are designed by integrating hyaluronic acid (HA) and a reduction-labile heterodimer of Pheophorbide A (PPa) and NLG919 into the supramolecular nanocomplexes (Fig. 13D), where PPa and NLG919 act as a photosensitizer and potent inhibitor of indoleamine 2,3-dioxygenase 1 (IDO-1), respectively [264]. Meanwhile, HA is employed to achieve active tumor targeting by recognizing CD44 overexpressed on the surface of tumor cell membranes. Near infrared (NIR) laser irradiation triggers the release of reactive oxygen species to provoke antitumor immunogenicity and intratumoral infiltration of cytotoxic T lymphocytes (CTLs). Meanwhile, the immunosuppressive tumor microenvironment (ITM) is reversed by NLG919-mediated IDO-1 inhibition. Combination of photodynamic immunotherapy and IDO-1 blockade efficiently eradicates CT26 colorectal tumors in the immunocompetent mice.

The limitations of current immunotherapies present a compelling case for the utilization of nanomaterials, laden with both cytotoxic agents and immune stimulants, as an in-situ cancer vaccine, exploiting the tumor itself as the antigen source [270]. However, there exist inherent contradictions in employing a solitary nanoparticle to address all the prerequisites in vivo for immune activation [271]. The development of vaccines and additional immunotherapies has been rendered intricate due to the heterogeneous antigen display and the incorporation of incompletely characterized immune adjuvants, bearing intricate mechanisms of action. Supramolecular polymers achieved through self-assembling peptides can be specifically designed to either stimulate or prevent immune responses, thereby functioning as either an adjuvant or a carrier for adjuvants [272]. Assembling T and B cell epitope peptides into nanofibers using a non-covalent method and a short C-terminal peptide extension represents a promising strategy for eliciting robust antibody responses in murine models, independent of any additional adjuvant co-administration [273]. This approach underscores the potential of precision engineering in optimizing and potentially revolutionizing immunotherapy paradigms. Considerable effort has recently been focused on engineering platforms comprising highly defined combinations of epitopes, antigens, immunomodulatory compounds, or minimal adjuvants [274].

Collier lab demonstrated that self-assembling peptide-based materials, themselves, may also be useful as chemically defined adjuvants [275]. Peptide-based vaccines and immunotherapies have traditionally posed considerable challenges due to the limited inherent immunogenicity of peptides. This has necessitated the use of adjuvants, compounds designed to augment the immune response to an antigen. Yet the complexity and the often poorly-defined composition of most adjuvants have hampered their broad utilization. The constructed self-assembling epitope peptide (O-Q11) incorporates both B and T cell epitopes from Ovalbumin (OVA) in conjunction with a self-assembling domain (Q11). This integration leads to the formation of nanofibers enriched with β-sheet structures (Fig. 14A). Importantly, the OVA epitope is prominently exhibited on the surface, enhancing its accessibility and interaction potential. The O-Q11 construct elicited significant IgG titers in a murine model, an effect achieved without the need for an auxiliary adjuvant. This study's findings offer a paradigm shift, illustrating that self-assembling peptide nanofibers can effectively act as their own adjuvants to augment antibody responses to displayed epitopes.

Fig. 14.

Supramolecular polymers as vaccines. (A) Epitope-bearing self-assembling peptides indicate a surprisingly robust antibody response generated against a self-assembled peptide containing antigenic determinants from ovalbumin known to be recognized by both T cells and B cells. [275], Copyright 2010. Reproduced with permission from National Academy of Science. (B) A model cytotoxic T-cell epitope is linked to a synthetic lipid tail, forming a peptide amphiphile that self-assembles into cylindrical micelles. [276], Copyright 2012. Reproduced with permission from John Wiley & Sons, Inc. (C) Programmable Immune Activation Nanomedicine (PIAN), designed for immune activation and tumor inhibition, represents a new stride in the field. This innovative nanomedicine is constructed through a singular step of supramolecular assembly process, relying on the host-guest interactions between beta cyclodextrin (β-CD) and adamantine (Ad) across various components. [277], Copyright 2021. Reproduced with permission from John Wiley & Sons, Inc. (D) Sortase-mediated conjugation of gp120 to Q11 nanofibers preserved bnAb epitopes. [278], Copyright 2022. Reproduced with permission from American Association for the Advancement of Science.

Many peptide amphiphiles have also proven to be effective synthetic, “self-adjuvanting” vaccines that act by stimulating toll-like receptors (TLRs) on DCs, specifically TLR2 (Fig. 14B) [276]. While TLR2 activation by the lipid portion of the PA can be a benefit in creating an immune response, TLR2 activation had to be ruled out to evaluate how PA self-assembly specifically contributes to in vivo efficacy. To this end, a PA was designed with a synthetic lipid tail that does not stimulate TLRs. A dialkyl tail with two palmitic (C16) chains was conjugated to a peptide containing the known cytotoxic T-cell epitope from the model tumor antigen ovalbumin. The cytotoxic T-cell epitope sequence, residues 257-264 represented by the single letter amino acid code SIINFEKL, was elongated to include residues 253-266. This served to increase the hydrophilic character of the peptide for self-assembly purposes and to increase the length of the peptide to aid in processing by DCs, the immune cells that are responsible for presenting peptide antigens to T-cells.

The necessity for distinct cellular targeting of cytotoxic agents and immune agonists when encapsulated in nanoparticles presents a formidable challenge in the field of nanomedicine. Complicating matters further, anionic nanoparticles often exhibit compromised antigen-capturing capabilities during systemic circulation. Moreover, these larger nanoparticles are predisposed to tumoral accumulation, while manifesting suboptimal retention within tumor-draining lymph nodes (TdLNs) [251]. Given these inherent complexities, the development of programmable nanomedical platforms capable of autonomous environmental adaptation holds significant promise in addressing the multifaceted in vivo requirements. For example, a new programmable immune activation nanomedicine (PIAN) has been developed and elicits robust antitumor immunity in-situ [277]. The programmable nanomedicine (Fig. 14C) is constructed by supramolecular assembly via host–guest interactions between poly-[(N-2-hydroxyethyl)-aspartamide]-Pt(IV)/β-cyclodextrin (PPCD), CpG/polyamidoamine-thioketal-adamantane (CpG/PAMAM-TK-Ad), and methoxy poly(ethylene glycol)-thioketal-adamantane (mPEG-TK-Ad). After intravenous injection and accumulation at the tumor site, the high level of reactive oxygen species in the tumor microenvironment promotes PIAN dissociation and the release of PPCD (mediating tumor cell killing and antigen release) and CpG/PAMAM (mediating antigen capturing and transferring to the tumor-draining lymph nodes). This results in antigen-presenting cell activation, antigen presentation, and robust antitumor immune responses. In combination with anti-PD-L1 antibody therapy, PIAN cures 40% of mice in a colorectal cancer model. This platform furnishes a new framework for designing programmable nanomedicine as an in-situ immunotherapeutic cancer vaccine.

To develop vaccines for prominent global pathogens, such as HIV, it is crucial to elicit both neutralizing and non-neutralizing Fc-mediated effector antibody functions. Clinical evidence indicates that non-neutralizing antibody functions including antibody-dependent cellular cytotoxicity (ADCC) and antibody-dependent cellular phagocytosis (ADCP) contribute to protection against several pathogens [279]. Fig. 14D depicts the conjugation of HIV Envelope (Env) antigen gp120 to a self-assembling nanofiber material deemed Q11, which induced antibodies with higher breadth and functionality when compared to soluble gp120 [278]. Immunization with the Q11-conjugated gp120 vaccine (gp120-Q11) demonstrated higher tier-1 neutralization, ADCC, and ADCP as compared to soluble gp120. Moreover, Q11 conjugation altered the Fc N-glycosylation profile of antigen-specific antibodies, leading to a phenotype associated with increased ADCC in animals immunized with gp120-Q11. Thus, this nanomaterial vaccine strategy has the potential to enhance non-neutralizing antibody functions through the modulation of IgG Fc N-glycosylation.

5.3. Regenerative Medicine

Numerous therapeutic modalities have been pursued that utilize material scaffolds to treat diverse tissues and organs, deliver soluble growth factors or pharmaceuticals, emulate potent signaling proteins, convey therapeutic cellular populations, and reconstitute functional tissue [280-282] [283]. The challenge of regenerating the central nervous system is especially daunting, given its known limited intrinsic regenerative capacity [284, 285]. Strategies for neural regeneration have encompassed the use of injectable supramolecular peptides to facilitate neural reconnection post-injury. The transformative potential of supramolecular polymers in creating new types of functional soft matter is particularly notable, as they offer high degrees of internal order and tunable dynamics, courtesy of the non-covalent binding among monomers [286-288]. Biological evolution has favored supramolecular polymerization for critical functions, as evidenced by the highly dynamic cellular cytoskeleton and the extracellular matrix (ECM) fibrils [91, 289-292]. Experimentation with aligned peptide scaffolds in conjunction with encapsulated mitogenic proteins has facilitated peripheral nervous tissue regeneration, even restoring erectile function in a rat model of a crush injury to the cavernous nerve. Diseases such as Alzheimer's and multiple sclerosis represent urgent areas where supramolecular biomaterials could greatly influence therapeutic outcomes within the nervous system [293, 294].

In one example, the Stupp group has designed bioactive PA monomers that consist of: 1) an alkyl tail to drive hydrophobic collapse, 2) a tetrapeptide motion control domain where the amino acid sequence was rationally mutated to tune the supramolecular motion, 3) four glutamic acid residues for solubility, and 4.) a terminal bioactive peptide sequence for signaling [289, 295] (Fig. 15A). They evaluated the correlation of the dynamics and biological function in supramolecular polymers displaying either one or two signals. In the single-monomer assemblies, PA monomers contained one of two different peptide sequences, VVAA (PA1) or AAGG (PA2), used to control supramolecular motion and display the laminin-derived IKVAV epitope to cells, which interacts with the β1 integrin receptor. Motion in these supramolecular polymers was evaluated computationally through CG-MD simulations, spectroscopically via fluorescence anisotropy (FA), and using solution NMR transverse relaxation time measurements. CG-MD simulations revealed that the average displacement (translational dynamics) of PA monomers within supramolecular polymers can vary by a few nanometers when peptide sequences are mutated in the structure of monomers. The sequence with greater mobility in simulations (PA2) and lower internal order by WAXS (wide angle X-ray scattering) gave significantly lower anisotropy values, suggesting lower local viscosity and greater molecular motion. In in vitro bioactivity assays, the more dynamic IKVAV-PA2 had a higher rate of β1 integrin receptor activation in human neural progenitor cells (hNPCs) compared with the PAs containing IKVAV-PA1 monomers and less supramolecular motion, which led to greater numbers of human neurons after differentiation of neural stem cells. As an application in neural cell differentiations, three artificial ECM scaffolds were designed based on PA nanofibers to enhance the maturation of human stem cell-derived neurons [290]. All nanofibers display the laminin-derived IKVAV signal on their surface, but differ in the nature of their non-bioactive domains (Fig. 15B). Nanofibers with greater intensity of internal supramolecular motion have enhanced bioactivity toward hiPSC-derived motor and cortical neurons. Proteomic, biochemical, and functional assays reveal that highly mobile PA scaffolds caused enhanced β1-integrin pathway activation, reduced aggregation, increased arborization, and matured electrophysiological activity of neurons. This work highlights the importance of designing biomimetic ECMs to study the development, function, and dysfunction of human neurons.

Fig. 15.

The role of dynamics in bioactive synthetic supramolecular polymers. (A) Chemical structures of two different bioactive monomers containing the neuronal signal IKVAV with strong (IKVAV-PA1) or weak (IKVAV-PA2) intermolecular cohesion and schematic of the nanofibers. Color-coded computationally determined molecular mobility. Confocal micrographs of human neural progenitor cells (hNPCs) (labeled by green and red markers) treated with IKVAV PA1 and PA2 and showing different populations of cells differentiated into neurons (white). [289], Copyright 2021. Adapted with permission from American Association for the Advancement of Science. (B) highly mobile PA scaffolds caused enhanced β1-integrin pathway activation, reduced aggregation, increased arborization, and matured electrophysiological activity of neurons. [290], Copyright 2023. Adapted with permission from Elsevier Science Ltd..

6. Conclusion and future perspectives

Therapeutic supramolecular polymers, with their unique properties and multifunctionality, have distinguished themselves in the fields of biomaterials, pharmacy, and medicine. This prominence stems from the strategic molecular design of their monomeric units, which governs the self-assembled architecture, structural stability, and materials functionality of the resultant supramolecular polymers. By harnessing the power of directional non-covalent interactions, such as hydrogen bonding and π-π stacking, along with the incorporation and integration of bioactive motifs, researchers have been able to modulate supramolecular polymerization mechanisms and pathways, thereby creating supramolecular polymers of well-defined size, morphology, stability, and bioactivity. These modulating strategies, encompassing pathway control, seeded growth, and dissipative supramolecular polymerization, hold promise for constructing supramolecular biomaterials that enhance therapeutic efficacy. The unique properties of these supramolecular polymers—stimuli-responsiveness, reversibility, tunability, and time-dependent materials properties—elevate their potential for advanced biomedical applications. Their dynamic nature facilitates unique interactions with biological systems, an advantage not typically observed with traditional polymers. Discrete supramolecular polymers have been explored as long-circulating carriers, serving a role in enhancing pharmacokinetics and drug delivery efficacy. Their effectiveness is further amplified by their distinctive surface properties, stealth effects, and innate ability to incorporate/recognize a variety of ligands.

Supramolecular polymeric hydrogels present equally, if not more, captivating prospects. The interlocking of supramolecular polymers within the hydrogel network enables localized and sustained drug delivery, while the hydrogels' shear-thinning and rapid recovery properties allow for minimally invasive administration. The reversible breakup of supramolecular polymers into individual units offers a crucial avenue to control the rate of drug release and serves as an efficient mechanism for hydrogel elimination once their tasks are fulfilled. These materials hold significant potential, extending beyond drug delivery to encompass various fields such as cancer immunotherapy, regenerative medicine, wound healing, biosensing, and infectious diseases[179, 296, 297]. For example, considerable patient interest exists in long-acting injectable (LAI) formulations, particularly when it comes to managing chronic infections caused by the hepatitis B virus (HBV) and HIV [298] [299].

As we glance into the future, the path forward for supramolecular polymers is paved with both challenges and exhilaration. While considerable advancements have been made in the creation of diverse supramolecular polymers and polymeric hydrogels, the field remains in its nascent phase. Notably, their potential applications in the realms of medicine and biology have yet to be extensively investigated. Due to their multifaceted/multivalent characteristics, intrinsic capacity for reversible polymer-monomer transitions, and responsiveness to biomolecular and physical cues, there is a pressing requirement to enhance our understanding of their behavior and function within intricate biological contexts. This understanding is pivotal in advancing the creation of adaptive and biodegradable iterations that offer optimized therapeutic effectiveness to address dire medical needs. Clearly, accelerating their clinical translation necessitates rigorous safety and efficacy testing. On the verge of numerous anticipated advancements, the realm of supramolecular polymers is positioned to tackle unaddressed requirements in biomedicine. Collaborative endeavors spanning chemistry, materials science, engineering, pharmacy, medicine, and computation/machine learning will drive this research ahead, harnessing the adjustability, dynamic characteristics, and multifaceted capabilities of supramolecular polymers to transform the domain of polymeric biomaterials. The forthcoming years hold immense promise for these materials, and we eagerly anticipate the innovations that will emerge as time goes on.

Abbreviations

SP

Supramolecular Polymers

ACE2

Angiotensin-Converting Enzyme 2

ADCC

Antibody-Dependent Cellular Cytotoxicity

ADCP

Antibody-Dependent Cellular Phagocytosis

ALP

Alkaline Phosphatase

AMP

Adenosine Monophosphate

ATP

Adenosine Triphosphate

BTZ

Bortezomib

CDN

Cyclic Dinucleotide

CEST

Chemical Exchange Saturation Transfer

CMC

Critical Micellization Concentration

CPT

Camptothecin

CTL

Cytotoxic T Lymphocyte

DPCA

1,4-Dihydrophenonthrolin-4-One-3-Carboxylic Acid

ECM

Extracellular Matrix

EGFR

Epidermal Growth Factor Receptor

EISA

Enzyme-Instructed Self-Assembly

Env

HIV Envelope Protein

GBM

Glioblastoma Multiforme

HIV

Human Immunodeficiency Virus

HNPC

Human Neural Progenitor Cell

IDO-1

Indoleamine 2,3-Dioxygenase 1

ITM

Immunosuppressive Tumor Microenvironment

K_E

Elongation Rate Constant

K_N

Nucleation Rate Constant

σ

Association Constants for Nucleation and Elongation

LSP

Living Supramolecular Polymerization

Mirna

Microrna

MRI

Magnetic Resonance Imaging

Napy

Naphthyridines

NIR

Near Infrared

NPs

Nanoparticles

OEG

Oligoethylene-Glycol

PA

Peptide Amphiphile

PBI

Perylene Bisimide

PD-1

Programmed Cell Death Protein 1

PD-L1

Programmed Cell Death Ligand 1

PEG

Poly(Ethylene Glycol)

PIAN

Programmable Immune Activation Nanomedicine

Ppa

Pheophorbide A

ROS

Reactive Oxygen Species

SAPD

Self-Assembling Prodrug

SARS-Cov-2

Severe Acute Respiratory Syndrome Coronavirus 2

SFH

Surface-Fill Hydrogel

SPH

Supramolecular Polymeric Hydrogel

SPZ

Supramolecular Polymerization

STING

Stimulator Of Interferon Genes

TdLNs

Tumor-Draining Lymph Nodes

TEM

Transmission Electron Microscopy

TLR

Toll-Like Receptor

Upy

Ureidopyrimidinone

β-CD

β-Cyclodextrin