Nanoparticle-Hydrogel: A Hybrid Biomaterial System for Localized Drug Delivery

Abstract

Nanoparticles have offered a unique set of properties for drug delivery including high drug loading capacity, combinatorial delivery, controlled and sustained drug release, prolonged stability and lifetime, and targeted delivery. To further enhance therapeutic index, especially for localized application, nanoparticles have been increasingly combined with hydrogels to form a hybrid biomaterial system for controlled drug delivery. Herein, we review recent progresses in engineering such nanoparticle-hydrogel hybrid system (namely ‘NP-gel’) with a particular focus on its application for localized drug delivery. Specifically, we highlight four research areas where NP-gel has shown great promises, including (1) passively controlled drug release, (2) stimuli-responsive drug delivery, (3) site-specific drug delivery, and (4) detoxification. Overall, integrating therapeutic nanoparticles with hydrogel technologies creates a unique and robust hybrid biomaterial system that enables effective localized drug delivery.

1. Introduction

Nanoparticle technology offers a series of advantages for drug delivery, including high loading yield, combination therapy, controlled release, prolonged circulation, and targeted delivery.^{6, 14, 70, 96} As a result, a myriad of nanoparticle-based drug delivery systems have been developed to improve therapeutic index of drugs by altering their pharmacokinetics and biodistribution profiles, resulting in nanomedicines for clinical treatment of various diseases.^{59, 89, 90} Advances in biotechnology and biomedicine continue to accelerate the development of novel therapeutic nanoparticles aimed at further enhancing therapeutic efficacy.^{22, 95} For example, recent progress in genomics and proteomics has led to new nanoparticle formulations accommodating rational drug combinations for personalized treatment.^{2, 50, 60} Biological discovery on the network of interconnected pathways within a cell has also resulted in nanoparticle formulations capable of precisely modulating the tempo-spatial distribution of drug molecules to target the internal state of malignant cells for bioactivity.^{19, 64} Further understandings on disease pathogenesis, particularly that of cancer and infections, have triggered the development of highly responsive nanoparticles that conduct physicochemical changes, when exposed to external stimuli, to enable preferential drug release at the target site.^{26, 27} Moreover, biomimetic nanoparticles made of natural biomaterials have demonstrated superior biointerfacing capabilities, leading to innovative therapeutics especially for drug delivery, detoxification, and vaccination.^40–43

Meanwhile, on the front of nanotechnology development, therapeutic nanoparticles have been increasingly combined with other biomaterials to form hybrid systems for novel applications.^{17, 44, 71} In this perspective, loading nanoparticles into hydrogels has received much attention.^{61, 78, 83} Hydrogels are hydrophilic 3D polymer networks with extensive uses in tissue engineering and drug delivery.⁶⁹ Nanoparticles can be embedded into hydrogel network by mixing with monomer solution, followed by gelation (i.e., nanocomposite hydrogel, Figure 1A).²⁸ Alternatively, they can be incorporated into gel matrix after gel formation by allowing the gel network to swell and ‘breath in’ nanoparticles for entrapment, a method especially useful when nanoparticles interfere with the gelation process.⁹⁴ Furthermore, inorganic nanoparticles are often grown in situ within gel matrix by loading nanoparticle precursors into a gel first, followed by reduction reactions for nanoparticle formation.⁶³ With appropriate compositions, hydrogels not only preserve the structural integrity and the functionalities of the contained nanoparticles, but also offer additional engineering flexibility to improve the overall therapeutic efficacy. Besides hydrogel entrapment, directly using nanoparticles as cross-linkers to construct 3D hydrogel network offers another approach for nanoparticle assembly to acquire hydrogel-like properties (i.e., nanoparticle colloidal hydrogel, Figure 1B).^{33, 54, 110} In this approach, nanoparticles can be linked together through strong hydrophobic interactions or by mixing nanoparticles carry opposite surface charges.⁹⁹ Alternatively, nanoparticles can be covalently linked, acting as nodes to form a hydrogel network.²⁴

Figure 1.

Schematic illustrations of two types of nanoparticle-hydrogel (NP-gel) systems: (A) nanocomposite hydrogel, where nanoparticles are embedded into hydrogel network, and (B) nanoparticle colloidal hydrogel, where nanoparticles are directly used as cross-linkers to construct 3D hydrogel network.

Overall, nanoparticle–hydrogel hybrid systems (denoted as ‘NP-gels’) judiciously integrate two distinct materials into one formulation with unique physicochemical and biological properties that neither one of the two building blocks can achieve independently.⁶¹ NP-gels have demonstrated superior biointerfacing properties and attracted increasing attention to address various biological and medical challenges. In this article, we provide a review on recent advances in engineering NP-gels, with a particular emphasis on their novel applications in drug delivery. Specifically, we highlight four areas where their development and use have received the most attention, including (1) passively controlled drug release, (2) stimuli-responsive drug delivery, (3) site-specific drug delivery, and (4) detoxification. Although this review focuses on nanoparticles in combination with hydrogels, the development principles in above areas can be applied to hydrogels combined with other delivery platforms featuring different sizes, geometries, or dimensions such as microparticles¹⁰⁸ and nanofibers.²¹ Overall, NP-gels have emerged as a versatile class of biomaterials, unleashing unique synergistic properties with strong application potential to improve drug delivery.

2. Passively controlled drug release

Both nanoparticles and hydrogels are widely used for controlled drug release. Drug molecules encapsulated within the hydrogel network are released through diffusion, swelling, and chemically controlled mechanisms.⁵⁶ Meanwhile, nanoparticles control release kinetics through their tailored polymer structure, particle size, and fabrication conditions.⁸⁸ By integrating the two platforms, NP-gels create hierarchical matrices with remarkable versatility in modulating drug release kinetics for various delivery purposes, which are difficult for each platform to achieve alone.

Hydrogels can offer tissue-like properties but may suffer from burst release and rapid diffusion of drug molecules out of the polymer matrix.¹³ In contrast, NP-gels that use nanoparticles as drug depot can overcome this drawback and significantly prolong drug release duration, which in turn can enhance drug bioavailability and patient compliance. For example, anti-inflammatory drug methylprednisolone(MP), effective in treating spinal cord injury was loaded into poly(lactic-co-glycolic acid) (PLGA) nanoparticles and then embedded within agarose hydrogels at a concentration of 2 mg/mL (based on PLGA mass); the continuous release of the fluorescence labeled MP was observed for 6 consecutive days.^{4, 10} These formulations extended effective drug release from a few hours up to a few days. Prolonged drug release by NP-gel formulation is also popular in ocular drug delivery. For example, drug-loading liposomes,³⁵ micelles,⁴⁷ and polymeric nanoparticles⁴⁶ have all been incorporated into various hydrogels for the slow release of glaucoma drugs. Another area of interest is localized antibiotic release. In this perspective, metallic nanoparticles made of silver, gold, or copper, which slowly release ions to cause strong bactericidal effects, have been widely used to formulate NP-gels and shown enhanced efficacy in treating bacterial infections including those caused by Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa bacteria.^{11, 76}

NP-gel formulation has also been used to directly modulate nanoparticle release with varying matrix porosity (i.e. the fraction of void space in the polymer network).^{7, 91} One example is the controlled release of nanoparticle-stabilized liposomes, an emerging class of antimicrobial delivery platform, for topical administration to treat bacterial infections (Figure 2).²⁸ In this formulation, carboxyl-modified gold nanoparticles were used as stabilizers for cationic liposomes consisting of L-α-phosphatidylcholine (EggPC, a zwitterionic phospholipid) and 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP, a cationic phospholipid) and the stabilized liposomes were loaded into a chemically cross-linked polyacrylamide hydrogel. The hydrogel viscoeleasticity could be precisely tailored by varying the cross-linker poly(ethylene glycol) dimethacrylates (PEGDMA) concentration, which subsequently resulted in tunable release kinetics of the incorporated liposomes. Liposome release percentage correlated linearly with the square root of release time, consistent with a diffusion dominant Higuchi model characterized by weak interactions between liposomes and the gel matrix.^{37, 92} The hydrogel formulation preserved the structural integrity of the nanoparticle-stabilized liposomes and the released liposomes could fuse with bacterial membranes in response to acidic environment (i.e., pH = 4.5) relevant to various skin infections.^{82, 85} As another example, cisplatin self-assembled nanoparticles were loaded into hydrogel composed of poly(ethylene glycol)-b-poly-(acrylic acid) (PEG-b-PAA).¹¹⁷ Drug release from this NP-gel system was determined by two steps: first, nanoparticles were released from the hydrogel, which was controlled by network dissolution; second, cisplatin was released from the nanoparticles, which was controlled by ligand substitution with chloride ions in the surroundings. The two distinct release kinetics determined the overall cisplatin release profile and thus the drug’s therapeutic efficacy.

Figure 2.

(A) Schematic illustration of hydrogel containing nanoparticle-stabilized liposomes for topical antimicrobial delivery. Carboxyl-modified gold nanoparticles (AuC) were adsorbed onto the outer surfaces of cationic liposomes to stabilize them against fusion. The AuC-liposomes were subsequently embedded into an acrylamide-based hydrogel. At neutral pH, AuC-liposomes were released from the hydrogel in their entirety. When the pH drops below the pKa value of the carboxylic group (pKa~5), the AuC stabilizers detached from the liposomes, resulting in the formation of bare liposomes with resumed fusion activity. (B) Accumulative liposome release profile from the hydrogel made with three different cross-linker concentrations of 0.6, 0.7, and 0.8 vol %, respectively. (C–E) Fluorescence study of the fusion interaction between hydrogel-released liposomes and S. aureus bacteria. Liposomes were labeled with fluorescent dye RhB (red) and the bacteria were stained with DAPI (blue). (C) Control bacteria without any treatment, (D) bacteria incubated with AuC-liposome-loaded hydrogel at pH = 7.4, (E) bacteria incubated with AuC-liposome-loaded hydrogel at pH = 4.5. Sscale bar, 1 μm. Reproduced with permission from Ref. 28.

Besides single drug delivery, NP-gels also combine fast diffusion-controlled release of drug molecules directly dispersed in hydrogel network with slow drug release from nanoparticle reservoirs, a unique feature useful for concurrent delivery of multiple drugs while achieving independent control of their release kinetics. This controlled release strategy is particularly suitable for the combinatorial delivery of neuroprotective and neuroregenerative agents for spinal cord injury repair. The former aims to enhance cell survival during the trauma of secondary injury and thus requires a delivery on the hours to days following the primary injury, while the latter aims to enhance axonal outgrowth and thus needs to be delivered for an extended period ranging from 7 to 28 days.^{74, 77} For example, an NP-gel was formulated with hyaluronan and methyl cellulose blend loaded with PLGA nanoparticles, which achieved independent delivery of neuroprotective agents such as 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F) quinoxaline (NBQX) and fibroblast growth factor-2 in a time span ranging from 1 to 4 days and neuroregenerative agents such as neurotrophin-3 and anti-NogoA up to 28 days.³ In addition to loading one drug into the hydrogel matrix and one drug into embedded nanoparticles, two or more different types of drugs can be loaded separately to different types of nanoparticles, which are then mixed as a cocktail to prepare an NP-gel. In this case, sequential drug release can be achieved by controlling the size and surface properties of the different nanoparticles. For example, cholesterol-bearing pullulan nanogel nanoparticles with a diameter of approximately 30 nm were mixed with 136 ± 4 nm liposomes (dimyristoylphosphatidylcholine:cholesterol = 3:1 mol:mol) and then loaded in a PEG hydrogel.⁸⁷ The study showed that 6% of the liposome-encapsulated dye had leaked from the nanogel-coated liposome complexes by day 40, whereas 100% of the dye had leaked by day 25 from the uncoated liposomes, implying that coating the nanogel stabilizes the lipid bilayer and strengthens the permeability barrier as an artificial cytoskeleton.

3. Stimuli-responsive drug delivery

Smart hydrogels that drastically change their volume in response to environmental stimuli such as temperature, pH, and chemical signals are attractive biomaterials for drug delivery.^{36, 72, 84} Such responsiveness can be coupled with additional cues such as light and magnetic field via nanoparticles embedded within the matrix, creating unique NP-gels capable of remotely controlling drug release.¹¹⁴ For example, nanoparticles made from melanin with an average diameter of approximately 250 nm showed UV-induced photothermal heating.⁶⁶ These nanoparticles were dispersed into heat degradable hydrogel networks physically cross-linked by amphiphilic PLGA-PEG-PLGA copolymer. Under UV irradiation, the hydrogel was able to disintegrate. Compared to UV light, near-infrared (NIR) light is relatively safer and able to penetrate deeper into the soft tissues.²⁹ For example, gold nanorods doped into a thermally responsive hydrogel made with methoxylpoly(ethylene glycol)-poly(ε-caprolactone)-acryloyl chloride, glycidylmethacrylated chitooligosaccharide, N-isopropylacrylamide, and acrylamide, were able to induce the contraction of the thermo-responsive hydrogels and trigger the release of loaded doxorubicin to inhibit breast cancer under NIR irradiation (Figure 3).⁷³ Other NIR-absorbing nanoparticles such as carbon nanotubes¹¹³ and graphene oxide nanoparticles¹¹⁶ were also incorporated into thermo-responsive polymers to harness NIR for remotely controlled drug delivery. Recently, upconversion core-shell nanoparticles made from NaYF₄:TmYb (core = NaYF₄:0.5 mol % Tm³⁺:30 mol % Yb³⁺; shell = NaYF₄, in a uniform hexagonal prism shape with an average length of 36.0 ± 1.1 nm and width of 32.0 ± 1.5 nm) capable of converting NIR light into UV light were also integrated with a photo-responsive hydrogel consisting of acrylamide monomer and the photocleavable PEG cross-linker.¹⁰⁷ The platform could “hide” large biomacromolecules (such as proteins) inside a polymer hydrogel, effectively “shutting down” their bioactivity, and then release them “on demand” using NIR light to enable their bioactivity.

Figure 3.

(A) Schematic illustration of a hybrid hydrogel system in which gold nanorods (GNRs) were doped into a thermally responsive hydrogel. A near-infrared (NIR) laser was used to trigger the release of loaded Doxorubicin (DOX) by utilizing the photothermal effect of GNRs to induce the contraction of the thermo-responsive hydrogels. SEM images of hydrogels at (B) 25°C and (C) 45°C (C). (D) DOX-release profiles in the presence and absence of NIR laser at pH 5.0 and 7.4. (E) In vivo survival rate of mice bearing breast cancer treated by different samples. Reproduced with permission from Ref. 73.

Additionally, superparamagnetic particles that dissipate local heat upon the application of alternating magnetic field (AMF) have also been used as a trigger for remotely controlled “on-demand” drug release.^{49, 57} For example, superparamagnetic iron oxide (Fe₃O₄) nanoparticles (20–30 nm diameter) were incorporated into N-isopropylacrylamide (NIPAAm)-based matrix. Drug release was modulated by applying external AMF: continuous AMF accelerated drug release from the NP-gel, while pulse application of AMF caused pulsatile release in addition to continuous Fickian release profile.⁸⁰ Using hydrogel with a concentric-layered structure instead of amorphous structure further improved the responsiveness and drug release efficiency.¹⁰³ The synergy between nanoparticles and the hydrogel network has also led to NP-gels highly responsive to external pH changes. For example, silver nanoparticles were synthesized within a hydrogel containing 2-hydroxyethyl methacrylate), poly(ethylene glycol) methyl ether methacrylate, and methacrylic acid in situ.¹⁰⁶ Although the hydrogel itself had pH-responsive deswelling property due to the free carboxylic groups, the incorporation of silver nanoparticles significantly increased the deswelling rate. Such synergy was attributed to the presence of a large inter-phase region between the silver nanoparticles and the polymer matrix, which likely reduced the characteristic diffusion length of water molecules and accelerated the deswelling process.

To further enhance the responsiveness, a new type of hydrogel has been made by covalently linking nanoparticles onto hydrogel matrix.^{45, 105} Compared to physically embedding, covalent link allows for using various hydrogel responsiveness to modulate inter-particle distances, hence offering precise control of nanoscale interactions through bulk templates.¹¹⁵ Meanwhile, when nanoparticles are covalently linked, they form the nodes of the polymer network and reduce the relaxation length of hydrogel scaffolds, leading to significantly enhanced bulk response to environment stimuli.⁹ Permanent linkage to the network also prevents nanoparticles from diffusing out of the gel, thereby limiting the loss of responsive materials and the potential toxic effects in biomedical applications.⁶² These covalent NP-gels provide a system with better-defined coupling between nanoparticles and polymer matrix and thus open a way for better understanding of material structure-property relationship and subsequent modification for improved drug delivery properties.

Using nanoparticles as sole building blocks to form cohesive gel-like network represents an emerging class of NP-gel.⁵ Combining oppositely charged nanoparticles, especially those that have already been established as drug nanocarriers such as PLGA,^{99, 101} gelatin,⁹⁸ and chitosan nanoparticles,¹⁰⁰ is a common approach to preparing this type of NP-gel. For example, positively charged chitosan-coated dextran nanoparticles (diameter: 340 nm, surface zeta potential: 10.6 ± 1.9 mV) and negatively alginate-coated dextran nanoparticles (diameter: 293 nm, surface zeta potential: −11.5 ± 1.7 mV) were mixed at 20% (w/v) to form colloidal hydrogel for glucose-responsive and self-regulated insulin delivery (Figure 4).³⁴ At a macroscopic level, the construct was made with nanoparticles of small sizes and uniform distributions, which ensured sufficient cohesive strength for gelation. Such cohesive force decreased at high shear rate, resulting in low viscosities suitable for injection. At a microscopic level, the nanoparticles allowed for simultaneous loading of insulin and two glucose responsive enzymes (glucose oxidase and catalase). In a hyperglycemic state glucose was catalytically converted to gluconic acid, which caused dissociation of the gel-like network and subsequent release of insulin. This NP-gel system allowed for fast and pulsatile release of insulin in response to glucose concentrations and thus provided better glucose control in a type 1 diabetic mouse model. An alternative approach to forming colloidal gel is to use nanoparticles with responsive hydrophobic interactions. In this approach, nanoparticles are typically consisted of a core made of a hydrophobic polymer and a shell made of a thermo-responsive polymer that undergoes hydrophilic-to-hydrophobic transition in response to temperature change.^{1, 23} When temperature is below a critical temperature, the polymeric shell is water-soluble and stabilizes the nanoparticle; however, when temperature is above a critical temperature, the polymeric shell becomes hydrophobic, which flocculates the dispersed nanoparticles into a hydrogel network.

Figure 4.

(A) Nanoparticles encapsulating insulin and glucosespecific enzymes (GOx, glucose oxidase; CAT, catalase) are made of acidic sensitive acetal-modified dextran (B) and coated with chitosan and alginate, respectively. (C) NP-gel is formed by mixing oppositely charged nanoparticles together and efficiently degrades to release insulin upon the catalytic generation of gluconic acid under hyperglycemic conditions. (D) Schematic of glucose-mediated insulin delivery for type 1 diabetes treatment using the STZ-induced diabetic mice model. (E) SEM images of nanoparticles coated with chitosan and alginate and formation of a NP-gel. (F) In vitro accumulated insulin release of the NP-gel in different glucose concentrations at 37°C. (G) Self-regulated profile of the NP-gel presents the rate of insulin release as a function of glucose concentration. Reproduced with permission from Ref. 34.

4. Site-specific drug delivery

NP-gels detain nanoparticles within 3D polymer matrices that allows for better local drug delivery. Various NP-gel formulations have been developed for targeting drugs to disease sites at the spinal cord,^{4, 10, 74, 77} the eye,^{35, 46, 47} and the skin.^{28, 76} In these formulations, drug localization primarily relies on the properties of the hydrogel network, which allows nanoparticle design to be solely focused on modulating drug release. Such hybrid strategy offers an opportunity to decouple the spatial and temporal aspects in designing materials for drug delivery, resulting in a modular approach for highly functional drug targeting applications.

One such application is to use an anti-inflammatory tripeptide Lys-Pro-Val (KPV) loaded NP-gel for drug targeting to treat inflammatory bowel disease in the gastrointestinal (GI) tract.⁵³ In the study, a “double gavage” method was employed based on ion-induced polysaccharide gelation. Specifically, the first gavage was aimed to deliver 100 μL of the polysaccharides solution (alginate, 7 g/L; chitosan, 3 g/L) with homogenous suspended NPs (2 mg/mL). The polysaccharides biomaterials were in liquid phase at the time of gavage, which allows for easy administration. Then a second gavage was performed with a 50 μL solution containing 70 mmol/L calcium chloride and 30 mmol/L sodium sulfate. Once the ions and the polysaccharides solution were mixed, a hydrogel was formed. Since the cross-linking was through electrostatic interactions and sensitive to pH levels, the gel degradation could be finely controlled at different sites within the GI tract by simply varying the gel composition. Using this approach, polymeric nanoparticles loaded with KPV peptides were encapsulated within hydrogel, forming a NP-gel formulation that was stable in gastric solutions of pH 1, 2, and 3 for 24 hours. However, when exposed to the intestinal solution with a higher pH level of 5, the hydrogel structure rapidly decomposed and the nanoparticles were released to the colonic lumen. Herein, the NP-gel formulation protected drug molecules during transit through the gastric gland and the small intestine, thereby maximizing the effective drug dosage to the site of action.

Site-specific drug targeting can also be achieved by modifying hydrogels with targeting ligands capable of specific binding to disease sites. For example, a silver-releasing antibacterial NP-gel consisting of silver nanoparticles and tissue-adhesive hydrogel has been developed.²⁵ In this design, the hydrogel building material PEG was modified with reactive catechol moieties to mimic the function of mussel adhesive proteins, in which the amino acid 3,4-dihydroxyphenylalanine (DOPA) that contains catechol played a critical role in enabling the mussel to adhere to various surfaces in an aqueous environment.⁵⁵ In the study, silver nitrate (AgNO₃, 212 mM) was used to oxidize catechol gel (Ag:catechol = 2:1 mol/mol), leading to covalent cross-linking and hydrogel formation with simultaneous reduction of Ag(I). Silver release was sustained for a period of at least two weeks in biological buffer solutions. The NP-gel was found to inhibit bacterial growth without significant cytotoxicity. Recently, gelatin hydrogel modified with DOPA showed similar bioadhesive properties and was able to withstand the shear force no less than 12.5 Pa inside the blood vessels (Figure 5).⁴⁸ In a mouse model of atherosclerosis, inflamed plaques treated with dexamethasone-eluting adhesive gels had reduced macrophage content and developed protective fibrous caps covering the plaque core. Treatment also lowered plasma cytokine levels and biomarkers of inflammation in the plaque. Meanwhile, the gel was shown to encapsulate both drug-loaded microparticles with diameters of 2~25 μm and fluorescently labeled nanoparticles with a diameter of 200 nm, and was able to durably adhere to the inside of a carotid artery in a living mouse. These glued particles released drugs directly into the vessel wall. Currently, novel bioadhesive hydrogels with remarkable performance under various physiological conditions are rapidly emerging.^{51, 67, 68} Combining drug-loaded nanoparticles with these hydrogels are expected to generate novel NP-gels with enhanced drug delivery efficiency and efficacy.

Figure 5.

(A) Chemical structure of alginate-catechol, synthesized from alginate. (B) Schematic of a microfluidic system used to compare the adhesion of alginate and alginate-catechol gels. (C) Schematic of the gel (green) being deposited on endothelial cells (blue) grown inside the device. (D) Fluorescence images of the gels containing fluorescent particles (green and purple) coated on endothelial cells in microfluidic channels (dashed lines). The unmodified alginate does not remain adhered at physiological shear stress, whereas the alginate-catechol remains adhered above physiological shear stress. (E) Quantifying the percent of gel that remained covered on the microfluidic channels at various shear stresses. (F) Quantifying the shear strength of the gels using a lap-shear tensile strain test. (G) Intravital fluorescence microscopy image of a carotid artery in a living mouse coated with the adhesive gel containing 200-nm particles (green). A fluorescent dye (red) is seen in the artery after it was injected intravenously, indicating that blood flowed through it. (H) Histological section of one wall of a carotid artery (red, elastin) coated for 28 d with the adhesive gel (green fluorescent particles). (I) Intravital microscopy images of the gel adhered to a carotid artery in a living mouse on days 1 and 5 after deposition. Reproduced with permission from Ref. 48.

5. Detoxification

At first sight, it may appear a bit contradictory to dedicate a section to reviewing NP-gels for removing toxic compounds from the body, rather than delivering drugs. Upon closer inspection, these two opposing fields share many commonalities, as many new approaches in the field of detoxification rely on established drug delivery systems including nanoparticle technologies.²⁰ In fact, toxin-binding nanoparticles have recently attracted significant attention for detoxification applications aimed at removing both endogenous and exogenous poisonous from the body.¹¹² Among various detoxification platforms, red blood cell (RBC) membrane–coated nanoparticle system, where intact RBC membrane was wrapped onto polymeric nanoparticles (denoted as “nanosponges”), has been demonstrated as a robust toxin decoys for broad detoxification applications.⁴⁰ The ability of the nanosponges to neutralize pore-forming toxins indicated their application potential for the treatment of methicillin-resistant Staphylococcus aureus (MRSA) infection.^{41, 42} Notably, MRSA infection commonly localizes to skin and soft tissues.³⁰ In the infection, a critical element of virulence results from a diverse arsenal of pore-forming toxins secreted by the bacteria, which attack the host cells.^{31, 93} These distinctive features of MRSA infection recently motivated the development of a nanosponge–hydrogel hybrid formulation capable of localizing nanosponges at the infection site for localized anti-virulence treatment of MRSA infection (Figure 6).⁹⁷

Figure 6.

(A) Schematic illustration of a hydrogel retaining toxin-absorbing nanosponges (NS-gel) for local treatment of MRSA infection. The nanosponge was constructed with a polymeric core wrapped in natural RBC bilayer membrane and was subsequently embedded into an acrylamide-based hydrogel. (B) A scanning electron microscopy (SEM) image of the NS-gel. Scale bar, 1 μm. (C) A transmission electron microscopy (TEM) image showing the spherical core–shell structure of the nanosponges under negative staining with uranyl acetate. Scale bar, 50 nm. (D) In vivo nanosponge retention by hydrogel. Nanosponges labeled with DiD fluorescent dye was used to formulate NS-gel, which was then injected subcutaneously under the loose skin over the left flank of the mice. Free nanosponges (without hydrogel) were injected as a control group at the right flank of the same mice. Fluorescence images taken at different time points show the retention of the nanosponges under mouse skin. (E) In vivo treatment of MRSA infection. 1×10⁹ CFU of MRSA 252 was mixed with 0.2 mL of 2 mg/mL NS-gel or empty gels, followed by subcutaneous injection to the backs of the mice (n = 9). Skin lesions were monitored and photographed on day 1–4 after the injections and the lesion sizes were measured. Bars represent median values. * P < 0.05, n.s.: not significant. Reproduced with permission from Ref. 97.

In this NP-gel system, a 0.6% (w/w) PEGDMA crosslinker concentration was determined as it effectively retained nanosponges (with a diameter of 88.4 ± 0.3 nm, a surface zeta potential of −13.9 ± 0.9 mV, and a concentration of 2 mg/mL) within its matrix without compromising toxin transport for neutralization. Following subcutaneous injection to mice, nanosponges were effectively retained at the injection sites. In an MRSA subcutaneous mouse model, mice treated with the nanosponge–hydrogel hybrid showed markedly reduced MRSA skin lesion development. These results collectively indicate that the nanosponge–hydrogel hybrid formulation represents a new and effective detoxification strategy for the treatment of localized bacterial infection. Given the critical roles played by pore-forming toxins in pathogenesis of a wide range of bacteria, the nanosponge–hydrogel formulation holds significant potential to treat infectious diseases caused by various types of bacteria. More importantly, no antibiotics are involved in this new treatment, which is therefore unlikely to be affected by the existing bacterial antibiotic resistance mechanisms and will not exert selective pressure to bacteria for developing new resistance.

NP-gels offer additional advantages for detoxification applications. For example, confining detoxification nanoparticles within hydrogel network can potentially improve treatment safety. Particularly, for systemic detoxification, the conventional intravenous administration of nanoparticles for detoxification often leads to nanoparticle accumulation in the liver, posing a risk of secondary poisoning especially in liver-failure patients.^{39, 65} NP-gel formulation becomes attractive as the particle entrapment prevents their accumulation in liver. In addition, NP-gels can be easily removed from the body, a feature useful to limit the interaction of toxin-bound nanoparticles with the body to further enhance detoxification efficiency and safety. Furthermore, NP-gels can be molded to various shapes to recapitulate organ-specific detoxification mechanisms. For example, by using an advanced 3D printing technology, PEG hydrogels loaded with detoxification nanoparticles were molded to liver-mimetic structures with modified liver lobule configuration aimed to recapitulating the fast substances’ exchange between blood stream and hepatic cells.³² The resulting NP-gel system had higher surface area than the original liver lobule and allowed toxins to enter the PEG matrix more effectively for neutralization.

For enzyme-based detoxification, hydrogel can encapsulate enzyme nanocomplexes and provide a confined environment similar to that of subcellular compartments for effective chemical transformation, molecule transport, and elimination of toxic metabolic wastes.^{12, 86} For example, invertase, glucose oxidase, and horseradish peroxidase with complementary functions were assembled and encapsulated within a thin layer of acrylamide gel to form enzyme nanocomplexes with a diameter approximately 30 ± 7 nm.⁵⁸ These nanocomplexes exhibited improved catalytic efficiency and enhanced stability when compared with free enzymes. Furthermore, the co-localized enzymes displayed complementary functions, whereby toxic intermediates generated by one enzyme could be promptly eliminated by another enzyme. In the study, nanocomplexes containing alcohol oxidase and catalase could reduce blood alcohol levels in intoxicated mice, offering an alternative antidote and prophylactic for alcohol intoxication.⁵⁸

6. Summary

The innovative combination of nanoparticles and hydrogels, two entirely different types of biomaterials, has generated novel NP-gels with increased structural diversity and enhanced drug delivery properties. These hybrid biomaterials have shown superior capabilities in modulating drug release kinetics, releasing drugs in a remotely controlled and “on-demand” fashion, assisting site-specific drug targeting, and facilitating nanoparticle-based detoxification. These applications, collectively, have demonstrated NP-gels as a new and robust class of biomaterials with significant potential to improve drug delivery efficiency. Together with the development of NP-gels, nanoparticles have also been combined with other biomaterial platforms and devices, such as nanofibers,^{8, 79, 102} microneedle patches,^{15, 16} and nanomotors,^{81, 104} leading to more sophisticated drug delivery systems.

While significant progress has been made in developing and optimizing NP-gel formulations, challenges remain to improve their clinical applicability for drug delivery.³⁸ Especially for in vivo applications, one challenge is the foreign-body reactions, which frequently cause collagenous capsule formation and can potentially limit the performance of implantable NP-gels.^{52, 75} To address this challenge, novel material designs are emerging with superior capability of preventing capsule formation. Especially, the ultra-low-fouling zwitterionic hydrogels have been recently developed to resist the formation of a capsule for at least 3 months after subcutaneous implantation in mice.¹¹¹ These hydrogels also promote angiogenesis in surrounding tissue, with anti-inflammatory and pro-healing functions. Another set of major challenges lies in improving the ease of clinical usage of NP-gel formulation. In this respect, mechanisms that promote gelation at lower polymer concentrations and narrower gelation temperatures would reduce the risk of premature gelation inside the needle upon injection.¹⁰⁹ Similarly, for covalently cross-linked hydrogels, triggered gelation in situ may reduce the risk of syringe clogging. Novel cross-linkers or initiators can reduce potential in vivo toxicity and enable the use of single syringe as opposed to double-barreled syringes.¹⁸ With continual efforts, we expect NP-gel hybrid approaches to generate new designs of biomaterials and to facilitate the development of novel therapeutic and diagnostic tools in medicine.