Recent advances in cell membrane-based biomimetic delivery systems for Parkinson’s disease: Perspectives and challenges

Abstract

Neuroinflammation, α-synuclein pathology and dopaminergic cell loss are the hallmarks of Parkinson’s disease (PD), an incurable movement disorder. The presence of the blood-brain barrier (BBB) impedes the delivery of therapeutics and makes the design of drug-targeting delivery vehicles challenging. Nanomedicine is designed and has significantly impacted the scientific community. Over the last few decades, to address the shortcomings of synthetic nanoparticles, a new approach has emerged that mimic the physiological environment. Cell membrane-coated nanoparticles have been developed to interact with the physiological environment, enhance central nervous system drug delivery and mask toxic effects. Cell membranes are multifunctional, biocompatible platforms with the potential for surface modification and targeted delivery design. A synchronous design of cell membrane and nanoparticles is required for the cell membrane-based biomimetics, which can improve the BBB recognition and transport. This review summarizes the challenges in drug delivery and how cell membrane-coated nanoparticles can overcome them. Moreover, major cell membranes used in biomedical applications are discussed with a focus on PD.

Graphical abstract

1. Introduction

Parkinson’s disease (PD) is an incurable movement disorder characterized by selective loss of dopaminergic neurons and abnormal aggregation of α-synuclein protein [1]. As the second most common neurodegenerative disorder worldwide [2], PD currently has treatments that are focused solely on symptom alleviation. Many investigations have been carried out to explore therapeutic interventions targeting multiple pathways; however, the major difficulty lies in the precise delivery of therapeutics to the brain. The central nervous system (CNS) and peripheral nervous system (PNS) are separated by the blood-brain barrier (BBB) serving a protective function by maintaining brain homeostasis and blocking harmful substances [[3], [4], [5], [6]]. However, the BBB's high selectivity also restricts therapeutic efficacy, resulting in nonspecific drug distribution and poor penetration. Disruption of the BBB may increase the risk of harmful substance accumulation in the CNS, potentially causing irreversible damage [7,8].

The emergence of nanoformulations and their various modifications has been observed as a feasible solution with suitable carriers [9,10]. But they also have certain drawbacks, including immune response, rapid systemic clearance and potential toxicity [11,12]. Even certain nanoparticles (NPs) have been reported to induce inflammatory responses and oxidative stress after reaching systemic circulation [13]. New interventions are required to overcome these effects and exert the full potency of targeted therapies. Therefore, recently scientific attention has shifted toward cell membrane-coated nanoparticles (CMNPs). These core-shell structures feature a NP core enveloped by a natural cell membrane layer, often further modified with targeting motifs to enhance biological specificity. CMNPs not only retain the benefits of NPs but also enhance the biocompatibility, prolonged drug circulation, reduce clearance, and enhance BBB penetration [14]. The major issue of drug delivery is addressed via the movement of cells by biological membranes via these biomimetics [15]. Moreover, the chemotactic abilities of leukocyte membranes are explored to target inflammation in CNS [16]. As they also retain membrane receptors and tightly adhere to the brain endothelial cells, they migrate toward the lesion site via chemokine receptor pathways [16]. These are drug-delivery cargoes with desired targeting abilities [17]. CMNPs have the advantage of combining the multifunctionality of biological membranes with synthetic NPs. This bestows their homologous targeting ability on the extraneous NPs to interact within the physiological environment.

In this review, we attempted to highlight the challenges in CNS targeting in the current scenario, and how different CNMPs can be unique with a special focus on PD. Numerous types of cell membranes derived from platelets, red blood cells (RBCs), cancer cells, neural cells, macrophages, white blood cells and numerous others have been discussed, along with their utilization in the fabrication of CMNPs. Moreover, the latest investigations carried out to address the clinical manifestations of PD and the effects of these biomimetics have been updated in this review.

2. Cell membrane-coated nanoplatforms

Drug delivery system (DDS) utilizes the principle of delivering therapeutic molecules to specific tissue or cells. Traditional systems were based on the administration of “naked drugs” via parenteral, topical, oral, inhalational and intravascular routes. However, each route has certain limitation. For instance, inhalational routes require specialized devices for administration and may cause respiratory tract irritation. This route is also limited to specific drugs only and results in variable absorption. Similarly, transdermal systems are limited to drugs with specific molecular size and lipophilicity, and may cause skin irritation or allergic reactions upon application. The rectal route is inconvenient, not suitable for all patient types, and can have variable absorption rates influenced by rectal contents. Similar issues frequently occur with the sublingual route, which limits dose capacity and affects compliance due to the taste [18,19]. Therefore, smart delivery systems became popular for delivering the maximum amount of drugs to the target site, and targeted drug delivery systems (TDDS) came into the limelight. When compared to other routes, TDDS offer significant advantages which mainly focus on enhancing therapeutic efficacy while minimizing the side effects. TDDS delivers drugs directly to the target site while sparing healthy tissue, leading to overall improved outcomes. By incorporating nanocarriers and other advanced delivery systems, TDDS has improved the solubility and stability of poorly water-soluble drugs, enhancing overall bioavailability. This is specifically beneficial for drugs that undergo significant first-pass metabolism. In addition to this, TDDS provides controlled and sustained release of therapeutic agents over extended periods. Moreover, the precision of TDDS enhances drug effectiveness by ensuring higher concentrations reach the intended site of action. A versatile range of therapeutics can be delivered by TDDS such as small molecules, nucleic acids, peptides and proteins [20,21].

The major focus has been on nanotechnological advancement that aim at sustained and controlled drug release. Commonly used nanoplatforms include biodegradable dendrimers, polymers, lipids and metal-based NPs, each with its own merits and demerits [22]. Encapsulation or conjugation of therapeutic drugs, miRNA, small molecule inhibitors and siRNA can improve solubility and bio-distribution properties. Thus, the development of therapeutic NPs has been encouraged primarily by their potential to advance treatment, prognosis and diagnosis [23]. Recently, cell membrane-coated nanoplatforms have gained much attention which are designed by wrapping cell-derived membranes around nanosized cores for therapeutically beneficial applications [24]. These natural systems have the benefit of effectively evading immune clearance and serving as longer-circulating drug carriers [25,26]. These membrane-based platforms can also detoxify harmful pathogens and molecules without prior danger signals and have better antigen-presenting properties [27]. These are highly encouraged for complex biological systems. To enhance efficiency and target specificity, the addition of function groups, improves the performance of these nanoplatforms [28]. Various methods are employed for the conjugation of functional ligands to the cell membrane, but damage to the cell membranes can occur due to aggregation of cell membrane proteins and disruption of immune integrity. Although method selection is important for minimizing membrane damage, it may limit the number of ligands and compatibility [29,30].

2.1. Basis for cell membrane coating nanosystems

Cell membrane coating technology-based nanoplatforms came into existence in 2011 in an investigation based on leveraging the entire CMNPs. To develop the desirable properties of a cell, its outermost layer is transferred to encapsulate the NPs in a while complex setting preserving the bio-constituents of the membrane [31]. Cell membrane coating systems adopt advanced top-down approaches for intricate functionalities that have the potential to integrate with natural cell membranes [32]. The inherent feature of CMNPs to mimic the surface characteristics of target sites makes them unique by enhancing their biocompatibility, reducing macrophage cells-mediated uptake, improving tissue penetration, and extending circulation times [33,34]. A schematic representation of the process diagram is shown in Fig. 1.

Fig. 1.

Schematic illustration of the formation of core-shell and cell membrane coating on the NP core-shell.

Core-shell structures in cell membrane-based biomimetics are engineered to mimic natural cellular structures, enhancing their function for drug delivery and tissue engineering. Core fabrication, membrane extraction and membrane coating are the primary three steps of developing these biomimetics. The inner core is used to encapsulate drugs and other biological vectors and can be composed of various materials such as PLGA, inorganic substances and liposomes [21]. The core serves as a scaffold for the subsequent membrane coating, influencing the overall features of the final structure. Another crucial step is isolating the cell membrane from the parent cell, which needs to be performed delicately, as it is responsible for maintaining the integrity and functionality of the membrane. Common methods for membrane isolation include ultrasonic waves, hypotonic lysis, freeze-thaw cycles and homogenization. Different techniques are used to delicately monitor the yield and intactness of the membrane, and effectively replicate their natural functions when coated with the core. Further, when the membranes are effectively isolated, these are coated onto the inner core using methods such as extrusion, sonication or microfluidic electroporation [35,36]. In general, when fabricating these NPs, the surfaces are entirely and uniformly covered by the cell membranes, creating a structure like the integrated core-shell. However, to isolate the cell membranes, first the cell integrity must be disintegrated, and then, by applying external forces such as sonication or extrusion, the membranes fuse with the core of NPs [37]. The resulting core-shell structures exhibit characteristics such as improved drug delivery, reduced immune recognition and better cellular uptake. By utilizing various types of cell membranes- from RBCs to cancer cells-biomimetics can be tailored for specific therapeutic applications [38]. This was first demonstrated using the RBC membrane, as RBCs are responsible for oxygen delivery in the body and have a viability of 4 months. The specific markers of RBCs and their ability to circulate throughout the body make them desirable candidates for developing biomimetic systems [39]. Initially, membranes were lysed by incubating them in the hypotonic solution, and with the application of mechanical extrusion and sonication, vesicles were generated. These vesicles were then co-extruded with PLGA, resulting in the development of core-shell structured NPs surrounded by the membrane. Interestingly, these NPs were able to circulate in the mouse model for extended periods, having an effective half-life of 40 h, which is better than the PEGylated platforms [39]. When the cell membranes are lysed to develop cell-membrane-based biomimetics, nano- or micrometre-sized lipid vesicles are formed, retaining the membrane-bound components from the original cell. These vesicles retain the signaling networks and intrinsic functionalities of their parent cells. This specific property makes them more biocompatible and low in immunogenicity [40].

This biomimetic application of the vesicles can also be harnessed by using the outer membrane vesicles (OMVs) to deliver therapeutic proteins or vaccines directly to the target cells. Moreover, to improve stability, the surface properties of the vesicles can be modified, which can also help with the targeting capabilities and immune detection-related issues crucial for overall therapeutic efficacy [41]. Cell membranes thus are a more viable option than polymers as high targeting efficiency can be achieved due to their ability to recognize self-antigens [42]. Different cell membrane-based biomimetics specifically employed in neurodegenerative disorders have been schematically illustrated in Fig. 2. This platform has an outstanding ability to mimic intricate cellular functions for the development of newer therapeutic modalities. For instance, various CMNPs beneficially evade immune clearance by acquiring “markers of self” from the source cells, developing into better long-circulating drug carriers [31]. Various native affinity ligands on the parent cell have the exquisite capability of targeting the disease sites [43]. Others can work as cell decoys to detoxify pathogens and harmful molecules, imparting protection to the cells. Additionally, these biomimetics also have the ability for more relevant antigen presentation [44], while others can scavenge bacterial toxins and maintain their cell integrity. Given the need for multitasking and multifunctionality in complex biological systems, the development of CMNPs has become increasingly important. Having multifunctional ligands is beneficial to improve the performance of NPs. While cell membrane coatings offer better immune evasion and stealth, off-target side effects are limited due to target selectivity, ultimately enhancing therapeutic efficacy [45]. These NPs have better potential for immune uptake by presenting better antigenic information, and better control over immune activation for modulatory immunity [46]. Furthermore, responsiveness to the environment and a more dynamic bio-interface are characteristics of membrane-coated nanoformulations [47]. To introduce better functionalities, conjugation can be employed for groups such as biotin-, amine- and sulfhydryl-based reactions. These methods are employed for decorating functional ligands on the cell membrane for better targeting.

Fig. 2.

Schematic illustration of cell membrane based biomimetics in neurodegenerative disorders, which shows different organic and inorganic NPs encapsulated in bio membrane having surface proteins act on specific site in different neurogenerative diseases.

2.2. Types of cell membranes used for CMNPs

2.2.1. Neural cell membranes

Drug delivery application of cell membranes is a unique approach involving the transfer of the outermost layer of a cell onto the surface of drugs or drug-encapsulated nanosystems. However, this system remains unexplored for the nervous system. Specialized cell-to-cell contacts are often observed in the nervous system, such as those for oligodendrocytes, the myelinating cells that have close interactions with the neuronal cells [48]. In turn, neurons are in close contact with the surrounding glial and neuronal cells, which impacts neurite outgrowth [49]. Thus, cloaking drug delivery vehicles with suitable neural membranes can develop better neuronal biomimetics and positively enhance brain drug delivery while reducing the rapid clearance [50]. Exploiting this concept, in a recent study, rat cortex-derived four cell membranes—astrocytes, microglial, cortical neurons and oligodendrocyte progenitor cells (OPCs)—were used to coat di(thiophene-2-yl)-diketopyrrolopyrrole (DPP) conjugated poly(caprolactone) (PCL) NPs, which were fluorescently tagged [51]. When comparing CMNPs with non-coated ones, it was observed that the microglial activation was reduced in all three membrane-coated systems except for the cortical neuron cell membrane. This resulted in a more favorable system for neuronal cells to regenerate. In addition to this, efficient uptake and neural cell response to the membrane-coated nanosystem were observed. Overall, microglial cell coating was the most efficient among all [50]. This cell membrane has the potential for efficient targeted delivery as it biomimics the natural make-up of neuronal cells, can establish better communication signals with the target site, and can overcome the BBB limitations. However, the significant considerations before developing the neural cell membrane-based biomimetics include choosing the appropriate neural cell type, optimizing the size, surface charge and drug loading capacity, and extensive pre-clinical evaluation to assess the potential toxicity. Different sources of cell membranes used for the development of biomimetics and areas where improvement is needed is briefly discussed in Table 1.

Table 1.

Different sources of cell membranes, their emerging role in developing biomimetics, development status, and areas requiring focus.

Cell membrane source	Emerging role in biomimetic system	Improvement areas	Development status	Ref.
Erythrocytes (RBCs)	Long circulation time; immune evasion due to self-markers	Enhance targeting capabilities for certain tissues	Widely used; ongoing research for hybridization with other membranes	[52,53]
Cancer cells	Targeting efficiency due to unique surface proteins	Optimization of extraction methods to maintain membrane integrity	Successful applications with targeted therapy	[52]
Platelets	Targeting efficiency due to adhesion properties	Stability and circulation time	Actively researched, hybrid membranes with RBCs show promising results	[53]
Macrophages	Immune modulation and targeting of inflammatory sites	Enhance functionalization for better targeting and uptake	Under development for macrophage-based NP efficacy	[53,54]
Leukocytes (WBCs)	Modulates immune responses	Need improvement in extraction technique to preserve cell functionality	Emerging for its therapeutic applications	[53,54]
Hybrid membrane	Combine features of different cell types for enhanced functionality	Needs standardization for fusion techniques and assessment in long-term stability.	Innovative approaches are being explored with promising results in biocompatibility and targeting efficiency	[52,53]
Neural cells	Biomimics the CNS components; potential for neuroprotection	Needs improvement for penetration through BBB	Developing rapidly specifically for neurodegenerative therapies	[54]
Mesenchymal stem cells (MSCs)	Tissue regeneration and repair; immunomodulation properties	Needs enhancement in homing capabilities to target specific tissues effectively	Actively researched for regenerative medicine applications	[53]
Bacterial cell	Mimics pathogen interactions	Needs improvement in stability and functionality in diverse environments	Emerging interests for DDSs	[52,53]
Endothelial cells	Targets vascular systems; promotes tissue repair	Needs enhancement for membrane integrity and functionalization to target	On-going research focusing on DDSs	[53,54]

2.2.2. Cancer cell membranes

Due to high target specificity and similar mechanism for adhesion to other cancer cells, membranes isolated from cancer cells are considered excellent sources to encapsulate nanomedicines [44,55]. Their unique properties, such as self-targeting, make them suitable for replicative immortality, angiogenesis, drug delivery, and activating metastasis and invasion [56]. These biomimetics are stable and can efficiently transfer antigens to the target sites from tumor membranes, overcoming the inefficient ability of therapeutic drugs to target tumors [44]. Beyond oncology, cancer cell membrane-based nanoplatforms have opened new avenues for neurodegenerative disorders such as PD. The transfer of the cell membrane to the NP surface has various advantages, as the membrane is the most basic component of the cell that participates in cell-environment interactions and multiple cellular functions, including signal transduction and self-recognition. Direct transfer of the membrane to the cell surface retains all the biological components and functions of the membrane. In comparison, cancer membranes are more notorious than other membranes. The foremost role of cancer membranes is the expression of CD47 protein, a ligand for signal-regulated protein alpha (SIRPα) that assists in sending the “don’t eat me” signals to macrophages and bypasses the engulfment process, hence achieving immune escape [57]. Immune escape is significant as it can reduce the inflammatory responses that can exacerbate neuronal damage in PD. For NP-based DDS, it can enhance the their efficacy by prolonging the circulation time without being recognized and cleared by the immune system. The increased half-life assists in the effective delivery of therapeutic agents to the brain, which is crucial for managing PD symptoms and halting disease progression. Particularly, this NP design is significant to cross the BBB. Furthermore, cancer cell membranes are rich in adhesion molecules, including N-cadherin which also plays a crucial role in the pathology of PD. N-cadherin is essential for the differentiation and maturation of dopaminergic neurons [58]. Various reports suggest that particularly N-cadherin expression could promote the apoptotic signals in the neuronal cells, thereby protecting the dopaminergic neuronal population [59]. Thus, cancer CMNPs can be utilized to effectively deliver the neuroprotective agents that may target the PD pathology at the molecular level. Cancer cell membrane-camouflaged NPs represent a promising strategy for addressing some of the challenges associated with delivering therapies for PD. Recent investigations have demonstrated that these nanoplatforms improved outcomes in various biomedical applications. For instance, paclitaxel-loaded PCL NPs for targeting 4T1 tumor [60] and doxorubicin loaded Fe₂O₃ NPs for improved internalization and retained membrane markers [61]. Another important application of this nanoplatforms is vaccines. In a recent investigation, researchers developed cancer CMNPs with an adjuvant layer of monophosphorylipid A (MLPA), a TLR4agonist, used as a vaccine. These NPs were used to investigate the release of antigens specific for tumors and dendritic cell maturation. Furthermore, dendritic cells were combined with the tumor antigen-specific pmel-1 transgenic murine splenocytes, which assisted in confirming the accumulation of T cells and secretion of interferon-gamma around the dendritic cells and cancer cell membrane having MLPA-coated NPs [62].

Altogether, these studies confirm the induced tumor-specific immune responses by using cancer-cell-derived membranes that stimulate the tumor-associated antigens and adjuvant coating that can be effective for cancer immunotherapies.

2.2.3. Mesenchymal stem cell (MSC) membranes

MSC membrane camouflages NPs can potentially help in PD by allowing targeted delivery of therapeutic moieties directly to brain regions, evading the immune system due to the cloaking effect of the cell membrane, and modulating inflammatory responses within the brain, all while promoting better drug penetration across the BBB due to the cell membrane’s natural ability to interact with brain tissue. Certain benefits of MSCs include multi-lineage differentiation, immunomodulation and homing, and high-speed proliferation [63], which are crucial for PD. A very recent investigation clarified how MSCs-based cell membrane-derived nanocarriers can help in PD. The main benefits include the MSCs and MSCs-derived neuron-like cell membrane-coated therapeutics that can specifically target the disease site and are biocompatible. In this study, microfluidic electroporation chips prepared MSC-derived neuron-like cell membrane-coated curcumin PLGA NPs (MM-Cur-NPs), which were investigated in cellular and animal PD models. As per the results, cellular mitochondrial membrane potential and oxidative stress were restored to normal, and improved accumulation and distribution were observed in PD mice, restoring damaged dopaminergic neurons. Moreover, fluctuated neurotransmitter metabolites were restored to normal levels; genes expressing anti-inflammatory factors increased, and genes expressing pro-inflammatory factors were reduced significantly. In addition, microglial expression of IBA-1, and neuronal apoptosis were inhibited. This study in conclusion demonstrated the synergistic effect of cell membrane-derived nanoplatforms for neuronal protection and alleviation of movement disorder [64].

Stromal stem cells, better known as MSCs, have the potential for self-renewal and are also considered as multipotent progenitor cells. They greatly influence the field of regenerative medicine, since they can decrease the side effects induced by chemotherapeutic drugs and are utilized to treat autoimmune disorders. Membranes extracted from these cells can be used to develop nanomedicines by coating NPs, as these cell membranes have the advantage of easy isolation and store abundant information for large-scale synthesis [65,66]. In a DDS using PLGA NPs loaded with doxorubicin exhibited effective tumor localization and antitumor effect [67]. MSC membranes have also been utilized to mask NPs for enhancing deep tissue tumor treatment by photodynamic therapy [90]. Interestingly, using these cell membranes in regenerative medicine does not have any impact on the interactions between diseased cells and stem cells [68]. Similarly, magnetic resonance imaging (MRI) has also been carried out by coating iron oxide NPs with adipose-derived MSC membranes. Doxorubicin co-encapsulated with superparamagnetic iron oxide NPs and coated with MSC membranes showed effective therapy of colon cancer. Efficiently, these NPs improved the anti-tumor effect, and cellular uptake, attenuated the immune response and better targeting efficiency was confirmed [69]. Another research team developed stem cell-coated nanocarriers with C-X-C chemokine receptor type 4 (CXCR4) expression that reduced uptake by murine and human macrophages while enhancing the endothelial barrier penetration [70]. Thus, this coating system has proven efficient for several therapeutic applications. In a recent investigation, glyburide (a protective agent) has been used to improve ischemic stroke outcomes such as hemorrhagic transformation, edema and infarct volume. To overcome glyburide's limited penetration, neural stem cells have been employed for targeted delivery, with CXCR4 overexpression used to specifically target stroke area. The results demonstrated a better accumulation of drugs in the infarct zone [71,72].

Overall, this coating system thus has proven efficient for several therapeutic applications. Current literatures suggest that the use of MSC membrane-camouflaged NPs holds promise as a strategy to enhance targeted drug delivery by leveraging the unique homing and immunomodulatory features of MSCs. However, applying the membrane of stem cells only means utilizing its functionality and not its distinct differentiation capacity. Future researchers should focus on optimizing membrane extraction methods, improving NP coating techniques, and evaluating potential toxicity concerns.

2.2.4. Bacterial cell membranes

Lack of cell organelles and nuclei is a specific trait of bacterial cells [73]. These pathogens have also been exploited for their cell membrane isolation to develop biomimetics for treating and curing diseases. For instance, E. Coli membrane was extracted and used to coat drugs delivered to neutrophils, which are the predominant immune cells near inflamed areas. This coating efficiently maintained biocompatibility by facilitating neutrophil-mediated endocytosis in contrast to non-functionalized NPs [74]. Similarly, PLGA-encapsulated NPs were coated by the extracellular vesicles obtained from S. aureus strains. These NPs demonstrated improved targeting of infection sites and infected macrophages, showing higher accumulation at the sites with greater bacterial load compared to non-infected regions. These were also explored by loading the antibiotics in the NPs further decreasing the bacterial loads [75]. So far, no study has demonstrated the effect of bacterial cell membrane-covered NPs in PD. However, their potential in other disorders suggests promising applications Given the ability of these membranes to cross the BBB, they could be exploited for drug delivery in PD. NPs coated with the bacterial membrane can evade the immune system and allow targeted delivery of therapeutic agents directly to the affected brain regions.

2.2.5. Red blood cells membranes (RBCMs)

RBCMs are the most promising cell type among all for developing and treating numerous disorders as they are efficient transporters for drugs, nucleic acids, proteins and active therapeutic molecules. They are abundant in blood and have a life span of ∼120 d in humans [76]. Their lack of organelles simplifies membrane extraction and purification, making them a pioneer choice for biomimetic applications. RBCM-based systems are widely used due to their rapid manufacturability and biocompatibility. The highly hydrophilic polysaccharides on the surface of RBCs enhance the stability of coated NPs. RBCMs are advantageous for brain targeting because the brain has a rich blood supply, facilitating unobstructed NP delivery [77]. However, there are certain drawbacks that limits its use for brain delivery including limited targeting ability and potential damage to integrity, stability and functional proteins during chemical modification [77]. Despite these limitations, its widespread use for various biomedical applications has been observed in recent times.

These membranes were initially isolated by Hu et al., through hypotonic treatment of RBCs [31]. Subsequent studies led to the development of RBCs with prolonged circulation time and enhanced oxygen-carrying capacity [78]. With the TEM demonstrations, it was found that RBCMs can efficiently coat NPs (65–340 nm in diameter) while preserving surface proteins, glucans and lipid membrane stability [79]. Recently, reactive oxygen species (ROS) responsive NPs modified by stroke homing peptide (SHp) encapsulated with RBCM designated as SHP-RBC—NP/NR2B9C were designed for ischemic stroke where NR2B9C (a therapeutic peptide) prevents the neurotoxicity produced by the N-methyl-d-aspartate receptor (NMDAR) without interfering in its primary activity. However, its high molecular weight and hydrophilic nature limits its BBB penetration. To overcome this, the scaffold was coated with RBCM, significantly improving BBB crossing efficiency—1.71% for RBC-coated drugs, 1.44% for drug-loaded NPs, and only 0.53% for the free drug [80]. Furthermore, the researchers inserted SHp in the RBCM for better targeted delivery to the ischemic region [81,82].

Conversely, studies have reported electrostatic repulsions between negatively charged NP core and membrane extracellular regions, which confirms that the surface proteins reside on the outer region of the membrane when RBCMs coat the NPs. And conventionally this NP offers a better half-life than uncoated NPs [31,79]. These coatings have been widely exploited for MRI applications through the incorporation of magnetic iron oxide groups with the NPs. They serve as better contrasting agents while demonstrating enhanced capacities for heat conversion [83,84]. Essentially, hypotonic method of isolation can better preserve the biophysical and immune properties of the cell [85,86]. In cancer biology also, doxorubicin-loaded NP cores coated with RBCMs have demonstrated efficient drug loading capacity and sustained release kinetics [87]. In another investigation, inhibition of phagocytosis by macrophages was achieved by specific RBCM marker CD47, which protected gold particles from various interactions and resulted in better half-life of particles in the biological system [88,89]. Similarly, various studies have been carried out to address inflammatory diseases, atherosclerosis, bacterial infections and leukemia [43,[90], [91], [92]].

Current literature suggests the beneficial effects of RBCM-based platforms for neurological disorders, particularly PD, due to their unique properties. RBCMs significantly enhance the circulation time of NPs in the bloodstream and minimize the immunogenicity, ensuring therapeutic agents reach their target site in brain without being rapidly cleared from circulation. Another important factor that makes the RBCMs suitable as drug delivery vehicles for PD is their capability to inhibit inflammation and apoptosis, which are significant contributors to neuronal damage in the PD. In addition, these nanoplatforms have natural biomimic characteristics. Hence, they can more effectively navigate biological barriers and are particularly suitable for enhancing BBB penetration, making them more suitable for PD DDSs. This has been demonstrated in a recent investigation where a RBCM/UCMG DDS was designed, which wrapping lanthanide ion (Ln³⁺)-doped upconverting NPs (UCNPs) with RBCMs. These NPs can excite nitric oxide (NO) donors after converting near-infrared (NIR) light to higher energy green light. While this scaffold was initially employed for cancer therapy, the investigators evaluated it for PD-related neuroinflammation. The scaffold effectively crossed the BBB and post-irradiation with an NIR beam of 980 nm, NO was released in the brain. This significantly decreased the pro-inflammatory mediators, protected dopaminergic neurons, and ultimately improved PD outcomes [2].

2.2.6. White blood cells (WBCs) membranes

Based on their functionality, WBCs (or leukocytes) are divided into different subtypes including eosinophils, mast cells, lymphocytes, basophils, macrophages, granulocytes, monocytes and neutrophils. These cells mainly carry out immune functions and are abundantly recruited to different tissues in response to stress. Since they contain intricate intracellular components and nuclei, the membrane isolation process is very difficult. Their unique infection-fighting and chemotactic properties make them suitable for coating purposes [93]. WBCs based membrane coatings are mainly used for the delivery of chemotherapeutic drugs, as these cells can effectively locate inflammatory sites, which are useful targets for tumor vasculature remodeling [94]. In a study, WBC membranes were coated onto nanoporous silicon using high-density sucrose for purification. To enhance stability, positive (3-aminopropyl) triethoxysilane (APTES) was subsequently applied [95].

Monocytes, derived from hematopoietic stem cells, are the precursors of macrophages that tend to accumulate at the tumor site, hence playing an essential role in tumor progression and metastasis [96]. To exploit this property, researchers coated doxorubicin-loaded PLGA-NPs with monocyte membranes. As a result, it was found that these NPs target the tumor cells and maintain the integrity of membrane-based heteroproteins much more effectively than uncoated NPs. Another investigation established the advantages of the monocyte coating system, showing enhanced stability against the invasion of mononuclear phagocytes due to chemokine receptor-mediated tumor tropism [97]. Another crucial membrane of this class is macrophages derived from monocytes [98]. They play a key role in cancer immunology by recognizing and engulfing foreign substances or tumor cells lacking surface biomarkers [99]. In a report, WBC-like vectors were designed by coating the nanoporous silicon NPs, which influenced the circulation time and accumulation at the tumor site in contrast to uncoated nanoparticles, along with better endothelial penetration and transport capabilities [100]. A recent study developed neutrophil-like cell-membrane-coated mesoporous Prussian blue nanozyme (MPBzyme@NCM) for targeted therapy in ischemic stroke. This scaffold effectively crosses the BBB by actively binding to the inflamed microvascular endothelial cells of the brain. Effective accumulation of nano-scaffold was observed in the damaged brain and was efficiently phagocytized by the brain microglial cells to produce long-term benefits. In addition to this, the developed nanozyme also induced microglial polarization, neutrophil recruitment, decreased neuronal apoptosis, and neural stem cell, precursor cells and neuronal cells proliferation [101]. This application of neutrophil cell membrane to cross the BBB also opens avenues for PD. These cells can migrate to injured and inflammatory sites, making them desirable for targeting the neuroinflammatory processes associated with PD. By utilizing the WBC membranes, their homing abilities can be inherently exploited to deliver the therapeutic agents directly to the brain regions. These membranes can also evade the immune system and decrease the likelihood of rapid clearance from the system. In addition to this, WBCs also secrete various neuroprotective factors which mitigates oxidative stress and inflammation. As such, these nanoplatforms represent a promising strategy for enhancing drug delivery and providing neuroprotective effects.

One of the WBC membranes includes bone-marrow-derived macrophage (BMM) membrane, which was explored in PD about a decade ago and has shown versatile results. These cellular membranes are innovative in drug delivery research particularly due to their affinity towards the endothelial cells and biological properties. In an earlier investigation, a group of researchers found that BMM were used to load a significant amount of catalase (an antioxidant enzyme) for sustained release over 5–7 d in the MPTP-intoxicated murine model of PD. To effectively eliminate the issue of early degradation, the nanoscale block ionomer complex was used to package the catalase enzyme which further decomposed microglial hydrogen peroxide upon activation by tumor necrosis factor-alpha (TNF-α) or nitrated alpha-synuclein (N-α-syn). Interestingly, substantial amounts of catalase were released and found to accumulate in the mice's brains after the implantation of the nanozyme-laden BMMs [16]. This study provided essential insights into the loading capacity of the carrier, the neuroprotective effects of the nano-scaffold, and the safety cellular study. This laid the groundwork for a cell-mediated delivery system for PD therapeutics. Further, to improve the given delivery system, researchers developed polyion complexes obtained through the coupling of different block copolymers. This resulted in the formation of an insoluble polyion core, which positively modulated the uptake, viability, kinetics and release studies, contributing to development of optimal carrier for PD [102]. All these reports suggest the biomedical advantages of using WBCs derived membranes presents a wide variety of applications with effective abilities of immune evasion, inflammation resolution, elimination of toxins and tumor targeting.

2.2.7. Platelets cell membranes

Thrombocytes or platelets originate in the bone marrow and lack a nucleus. They help in maintaining homeostasis and accumulate at the site of injury, initiating the healing process by forming clots. The membranes isolated from these cells are novel carriers for drugs, treat immune thrombocytopenia and various types of cancers [100,103,104]. In an initial study, platelet-derived membranes were used to coat the PLGA-NPs, which maintained the surface marker integrity and functionality of immunomodulatory and binding proteins. With the assistance of membrane glycoproteins, these NPs were found to bind with human-type IV collagen. In contrast to uncoated NPs, macrophage uptake was decreased, forming stable and biocompatible material. These NPs were loaded with docetaxel for the intervention of restenosis following angioplasty in vivo settings. They were specifically guided to the damaged vasculature and remained stable for 5 d [105]. In another study, verteporfin loaded into PLGA-NPs and encapsulated by platelet-derived membrane was used to establish a link between ROS and tumor cells, fighting the melanoma without damaging the skin [104]. In a different study, doxorubicin and melanin-loaded NPs coated with platelet membrane were developed and functionalized with arginyl-glycyl-aspartic-peptide for better immunological evasion property and recruitment of the peptide towards the αvβ3 integrin, thereby improving the targeting of tumor vasculature [106].

Platelet-based nanoplatforms have not been extensively explored but represent an emerging and promising strategy for targeted therapy in various disorders. They could potentially be applied in PD due to their multifunctional features, including natural ability to target sites of injury and inflammation. Since neuroinflammation plays a key role in the progression of PD, their ability to cross the BBB under stress conditions also makes them a suitable candidate to be explored in PD. In a recent investigation on intracerebral hemorrhage (ICH), platelet-membrane-modified-polydopamine NPs (Menp@PLT) were developed, demonstrating high targeting efficiency toward the hemorrhage site. These nano-scaffolds have ROS-scavenging properties, alleviate the neuroinflammation microenvironment of ICH, reduce the hemorrhage volume, and repair the blood vessel injury [107]. This demonstrates that the platelet-based nanoplatforms have a high tendency to reach the different brain regions, evade the immune responses, and potentially modulate the neuroinflammation, opening broader possibilities for effective drug delivery designs in PD treatment.

2.2.9. Endothelial cell membranes

These are the major cells for maintaining the integrity of healthy vasculature. For instance, Fe₃O₄-NPs have been employed, encapsulated with HUVECs derived cell membrane, and were well internalized and directed with the help of magnetic field [108]. As a result, a variety of drug delivery nanosystem designs can be developed using this method. BBB is a complex, dynamic, multicellular barrier mostly composed of endothelial cells [109]. Vascular endothelial cells are the component of BBB, making them an excellent source for the production of CMNPs, which is also attributed to their self-targeting property [110]. Few investigations have been carried out for CNS targeted delivery using vascular endothelial cells membranes to fabricate NPs. For instance, dihydroartemisinin (DHA) was formulated as DMSN-DHA@BMECM, encapsulated with vascular endothelial cells membranes, against the experimental cerebral malaria (CM), and for specific targeting, plasmodium falciparum RBCM protein 1 (PfEMP1) was used which is highly expressed on the infected RBCs [111,112]. As a suitable candidate capable of crossing the BBB, endothelial cells can be further explored for the application in PD, as these membranes can mimic the natural cellular interactions and directly deliver therapeutic agents to neuronal cells potentially enhancing the therapeutic efficacy.

2.2.10. Hybrid cell membranes

In recent times, multifunctional hybrid cell membranes have been developed to fabricate NPs that utilize multiple cell membranes, and their hybridization is carried out for unique biological functions. Various hybrid cell membranes have been used such as dendritic and cancer cell [113], platelets, and cancer cells [114], RBCs and cancer cells [115], and neutrophils and macrophages hybridized membranes. These scaffolds inherit the advantages of multiple membranes and overcome the disadvantages of individual cell membranes. For instance, platelet cell membranes having a better immune evasion activity when combined with cancer cell membranes improves the homologous targeting and reduces the drug clearance by the immune system [114]. More importantly, these cell membrane-based nanoplatforms mimic the surface functions and features of the source cells, having a major impact on the therapeutic efficacy. In a very recent study, a novel cell membrane-based design was developed by hybridizing the chemokine (C—C motif) receptor 2 (CCR2) and platelet membranes to overcome the BBB and target neuroinflammatory lesions. This bio-engineered scaffold was explored in the 5xFAD mice. This bio-engineered scaffold was loaded with rapamycin and 1‐trifluoromethoxyphenyl‐3‐(1‐propionylpiperidin‐4‐yl) urea (TPPU). Administration of these drug-loaded bioengineered scaffolds has demonstrated a significant reduction in cognitive impairment, neuroinflammation, and amyloid plaque deposition [116]. These membranes can be versatile platforms for PD. They can effectively target the specific brain regions where dopamine neurons are degenerating and enhance the overall drug delivery efficiency. The hybrid cell membranes can be developed in a way that they may mimic the natural properties of the brain cells and neurons, which can selectively cross the BBB and these membranes can be developed in a way that can help mask the NPs from the immune system maintaining the sustained release of the nano-formulation. By combining the membranes from different cell sources, researchers can also develop the formulation with multiple functionalities, such as delivering neuroprotective agents alongside dopamine precursors to address various aspects of PD pathology. The only things to be considered are the targeted delivery, high loading capacity, and thorough safety testing.

2.3. Basic layout of synthesis and characterization of membrane-coated nanoplatforms

The main process for well-synthesized and characterized membrane-coated NPs includes extraction of membrane fragments or vesicles followed by fusion of these membranes to the NPs and finally characterization of the resulting scaffold. For instance, RBCs are devoid of nuclei, making their isolation process straightforward. During processing, RBCs are cleared out of PBS, serum, buffy coat and hemoglobin through high-speed centrifugation, followed by sonication and extrusion through a 100 nm polycarbonate membrane to get definite vesicles [121,122]. However, in the case of other eukaryotic cells such as cancer cells, WBCs and stem cells, the membrane extraction processes are quite complex due to their intricate composition, which mainly involves the culture of isolated cells, further treating them to hypotoniclysis, mechanical rupture and discontinuous sucrose gradient centrifugation to remove cytoplasm and nuclei for pure membrane isolation [62,123]. Subsequent centrifugation, sonication, and membrane extrusion remain consistent [124]. Once the membranes are isolated, core NPs are synthesized for the fusion process [125]. The major fusion methods include the co-extrusion process, sonication, microfluidic electroporation and cell membrane-templated polymerization. In the co-extrusion method, vesicles along with a NP solution are co-extruded, and the mixture is sonicated for a few minutes. Extrusion is responsible for generating the force that disrupts the integrity of the membrane and enabling it to coat the NPs [126]. The major challenge with this method is scaling up production. On the other hand, sonication utilizes ultrasonic energy at certain frequencies for nanovesicles. The amplitude of the ultrasound plays a critical role in the fusion of the co-incubated vesicles and NP core [127]. Microfluidic electroporation is carried out in a microfluidic chip, where pores are created in the cell membrane by the generated electromagnetic energy. This method is particularly advantageous for preserving NP stability [128]. A relatively newer technique, cell membrane-templated polymerization, exploits the interfacial interactions between the cell membrane and the polymeric NP core. Here polymeric cores are used to manipulate the NP size and carry out the efficient coating [129]. Although sonication and extrusion are commonly used, recent studies have revealed limitations in these conventional methods. For instance, investigation has demonstrated that the final fusion product often contains partially coated NPs, and source cells internalize only 40% of these partially covered NPs, reducing targeting and therapeutic efficacy. To address these drawbacks, recently an investigation reported ways to ratiometrically enhance the full cell membrane coating for better targeting efficiency. They found that limited membrane fluidity results in the formation of cell membrane patches and fails to fuse properly. Thus, understanding the crucial role of cell membrane fluidity for the final fusion product, the investigators utilized the external phospholipid to tune the membrane fluidity. This led to better internalization and targeting of the biomimetic NPs [130].

To characterize cell membrane-coated NPs, standard physicochemical methods are carried out, such as dynamic light scattering (DLS) and transmission electron microscopy (TEM). For instance, RBCs coated NPs illustrate ∼8 nm increase in size due to the thickness of the RBCMs. Additionally, surface charge is also an essential indicator of these NPs [131]. Usually, zeta potential depicts the surface charge of the respective vesicle membrane [125]. Antibody tagging for cell membrane-coated NPs is often analysed with the help of flow-cytometric analysis [131]. Other confirmatory analysis methods are also performed, including western blotting and SDS-PAGE, depending on the type of membrane used [132]. Most of the time, universal procedures for characterization and preparation are adapted which might sometimes require minor modifications.

2.4. Significance of cell membrane coated biomimetics in targeted therapies

The brain serves as the body's central control system, regulating essential functions including respiration, heartbeat, cognitive functions, and emotional tasks. As one of the most versatile organs, the brain's intricate architecture and diverse functionalities make its protection critically important. The BBB plays a pivotal role in this protective mechanism. This physiological marvel defends the neural environment from environmental toxins and pathogens. But the characteristics that make BBB a safeguard also create significant therapeutic challenges [133]. BBB is formed by tightly packed endothelial cells with tight junctions, which restrict paracellular transport and prevent the passage of large or hydrophilic molecules into the brain. While only small and lipophilic molecules pass freely, essential nutrients like amino acids and glucose are transported via specialized carrier proteins. The BBB exhibits plasticity, adjusting its permeability in response to physiological changes or pathological conditions. This dynamic nature influences drug delivery efficacy [134]. Over 98% of small-molecule drugs and nearly all large-molecule therapeutics struggle to cross BBB effectively, necessitating innovative strategies to enhance drug delivery [134].

The intricacies of BBB restrict the entry of anti-PD medications. As a result, despite many drugs, such as matrine and curcumin, showing promising anti-PD effects in pre-clinical settings, they have not demonstrated clinical efficacy at clinical levels. Significant efforts are underway to develop strategies that facilitate drug passage across the BBB [135]. However, in chronic conditions like PD, alterations in BBB integrity occur [136], posing opportunities in drug delivery. Cellular components, including stem cells and immune cells, are activated and recruited in the region of post-injury lesion. Inside the lesion vicinity, a chemokine concentration gradient was developed, and the deformation of the recruited cells enables them to cross the endothelial cell slit and enter the brain region [137]. Various receptor-ligand interactions occur for the arrest step, to attain high affinity conformation for the deformation process. Furthermore, matrix metalloproteinases are secreted by cells to degrade extracellular matrix and basement membrane for invasion and BBB crossing through TJs fenestrations [138]. When BBB integrity is compromised, there is a notable increase in pinocytotic vesicles within brain microvascular endothelial cells (BMECs). On average, injured BMECs contain 25 pinocytotic vesicles, which range in size from 70 nm to 200 nm [139]. Additionally, the capacity for phagocytosis and transcytosis of damaged BMECs rather than through the compromised tight junctions [140]. Recent studies indicate that endocytosis and transport mechanisms in injured BMECs may be the primary pathways for cell membrane-based biomimetic vehicles to traverse the BBB. However, it has been observed that these vehicles struggle to cross the BBB even when it is temporarily opened using hypertonic solutions. Therefore, the processes of recruitment, cell recognition, intracellular transport, and BBB crossing may operate through different mechanisms. Currently, research on targeting mechanisms of cell membrane-based biological vehicles focuses primarily on their recognition by BMECs, transport processes, and targeting interventions for diseased cells [141].

Acute encephalopathies associated with disorders such as glioblastoma (GBM) and ischemic stroke are responsible for the disruption of dynamic BBB integrity [142]. However, chronic encephalopathies associated with disorders such as PD, Huntington’s disease (HD) and Alzheimer’s disease (AD) degrade the BBB integrity very slowly and do not present targets with major variations, which generates hurdles in designing target-specific delivery systems [143]. Different mechanisms adopted by these biomimetics to enter the biological interfaces have been illustrated in Fig. 3. Cell-membrane-based biomimetics cannot sense changes in chemokine concentrations and may arrive at the lesion site through passive transport after systemic administration. Major cellular events for CMNPs include recruitment, cell recognition, intracellular transport and BBB crossing. Currently, brain targeting mainly focuses on cell memory-based bio-vehicles. Although RBCs are commonly used biomimetic vehicles, they lack intrinsic brain-targeting capability, so their applications are dependent on the modification by targeting moieties [144]. In contrast, the targeting properties of stem cells and immune cells depend on their systemic homing ability, which follows a cascade of tethering, rolling, activation, arrest and transendothelial migration. Certain factors play an essential role in the trafficking and recruitment of WBCs and stem cells, such as CXCR4/stromal cell-derived factor. Recently, a study demonstrated that upregulated CXCR4 expression on MSC membranes was converted to cell-membrane derived biomimetic, which showed enhanced brain-targeting efficiency in the mice model of MCAO [145,146]. However, protein translation and intracellular signal transduction are necessary for cell migration following the interaction of CXCR4 and SDF1 molecules on the cell membrane. The intracellular process is not necessarily complete even if the cell membrane-based biomimetic over expresses the CXCR4. Thus, more investigations are required to explore its targeting mechanism [147]. Cell membrane-based biomimetics are at the nano-meter size level and mostly transferred via endothelial cell transport. Some investigations suggest that this process might depend on the injured BMECs by recognition, internalization, and finally transportation [148]. Adhesion molecules including vascular cell adhesion molecule-1 (VCAM-1) and intercellular adhesion molecule-1 (ICAM-1) are majorly expressed in the injured BMECs which provides a suitable target for inflamed BBB [149]. Very late antigen-4 (VLA-4) is the naturally expressed ligand for VCAM-1. Therefore, a target delivery vehicle is grafted after the fusion of liposomes to the stem cell membrane. These biomimetics are mainly taken up via clathrin-mediated endocytosis and micropinocytosis, which is dominant for the uptake of BMECs post-inflammatory insult. However, the fate of cell membrane-based biomimetics remains unclear, and further research is needed to validate their brain-targeting capacity. Most existing studies have employed the synthetic membrane or in vitro evidence, but physiological evidence is still limited. A recent investigation demonstrated a dual-targeting strategy for tuberculosis by encapsulating polymeric cores with mycobacterium-stimulated macrophage membranes with a photothermal agent excitable with a 1064 nm laser. This specific biomimetic expressed mycobacterium tuberculosis-related receptors enabling them to target granuloma, which demonstrated alleviation in pathological damage and inflamed lungs resulting in improved therapeutic efficacy [150].

Fig. 3.

Brain targeting mechanism of cell membrane-based biomimetics which includes lipophilic transcellular pathway, receptor-mediated transcytosis, transporter-mediated transcytosis, adsorptive-mediated transcytosis, functionalized-carrier mediated pathway. Recent advancement and biomedical application of bioinspired nanoplatforms.

Renowned physicist Richard Feynman first introduced the concept of nanotechnology in 1959 during his presentation [151]. NPs have emerged as crucial components in modern medicine, finding applications as targeted delivery in neurodegenerative diseases, contrast agents in imaging, and carriers for drug and gene delivery into tumors. Their utilization enables novel analyses and therapeutic interventions that were previously unfeasible [152]. The NPs are designed to control surface properties and particle size, and to target specific delivery to achieve the desired therapeutic response [13]. The disadvantage of NPs in targeted drug delivery is the potential for unintended toxicity while the goal of targeted delivery is to enhance the specificity of drug action, the unique properties of NPs, such as their small size and increased surface area, may lead to unexpected interactions with biological systems. These interactions could trigger immune responses or cause adverse effects on healthy tissues, compromising the overall safety of the delivery system. The challenge lies in carefully engineering NPs to achieve effective targeting without inducing harmful side effects, emphasizing the need for thorough preclinical and clinical evaluations to ensure the safety and efficacy of NP-based DDSs [153].

Due to the various disadvantages associated with synthetic NPs, biomembrane NPs can be a better approach for targeted delivery. Biomembrane NPs, also known as biomimetic NPs, represent a novel approach in nanotechnology that leverages natural cell membranes' structural and functional attributes. Recently, different cell sources such as blood-derived cells, immune cells, bacterial cells and even cancer cells have been explored for developing biohybrids having versatile functionalities. These NPs are designed to mimic the composition and properties of biological membranes, offering unique advantages in various applications. One notable application is in drug delivery, where biomembrane NPs can enhance biocompatibility and reduce immunogenicity, improving their interactions with biological systems. Additionally, biomembrane NPs may facilitate targeted drug delivery by incorporating specific targeting molecules that recognize and bind to receptors on the surfaces of target cells. This biomimetic approach holds promise in overcoming some of the challenges associated with traditional NPs, such as potential toxicity and immune responses [154]. By utilizing the specific features of cell membranes such as tissue binding affinity, immune escape, prolonged circulation time and anti bioadhesion, the effects of synthetic materials can be masked [155].

CMNPs are expected to offer advantages in drug delivery tools, bioimaging, and cancer therapeutics. However, always using the conventional systems of cell membrane coating may limit functional diversity. To address this, the incorporation of mixed types of cell membranes or new membrane sources, such as different cell or bacterial membranes, is suggested [156].

3. Cell-membrane camouflaged biomimetics for PD

PD is an age-related neurodegenerative disorder attributed to the selective loss of dopaminergic neurons in substantia nigra causing significant brain inflammation, secretion of neurotoxins and microglial activation [154]. Abnormal functioning of dopaminergic neurons in the brain results in hallmark symptoms of PD including tremor, postural imbalance, bradykinesia and rigidity, primarily neuronal death in the substantia nigra. Dopamine, a crucial neurotransmitter for movement, transmits messages within the brain for smooth functioning of muscles and purposeful movements. Another fundamental factor believed to play a crucial role in the development of PD is alpha-synuclein located in the presynaptic terminals and is important for the normal functioning of the brain. The build-up of abnormally folded protein aggregates results in the development of PD [157]. Neuroinflammation associated with PD is characterized by an inflammatory response within the brain. Pro-inflammatory cytokines are released by the hyperactivated microglia causing neuronal death and enhancing neurodegeneration within the CNS [158]. Clinically, controlling the disease mainly includes drugs such as dopaminergic agonists, glutamate antagonists and anticholinergics [[159], [160], [161], [162], [163]]. Levodopa is the first-line treatment for PD used to restore dopamine levels. To prevent peripheral metabolism, it is usually given along with carbidopa. However, resistance to levodopa has also been reported frequently. Other commonly used therapies include dopamine agonists such as ropinirole and pramipexole, which are taken alone or in conjunction with levodopa. However, their long-term use also comes with several side effects. In conjunction with MAO inhibitors (e.g. selegiline, rasagiline) are also commonly used in the early stages of PD, which are often along with the levodopa. But limited efficacy is reported in their case as well. Recently, surgical interventions have been employed involving implanting electrodes in specific regions of the brain which are known to modulate the neural activity and often used in advanced PD stages where the medications become less effective. Reversible therapy often requires careful patient selection and management [164]. Magnetic resonance-guided focused ultrasound ablation has surfaced as a newer approach for PD where tremors are the predominant symptom. This is a non-invasive technique for more precise targeting of lesions in the thalamus [165]. However, current treatments only halt the progression of the disease; none therapy primarily act on the disease modification strategy or addresses the underlying disease mechanism. The major obstacle is the BBB whose compact physiological structure restricts drug delivery (Fig. 4). To overcome this, various strategies have been laid down in the last few years. Intrathecal and intracerebroventricular administration of drugs and maintaining concentrations in the brain are adopted, but limited due to their invasive nature [166].

Fig. 4.

The existence of the blood-brain-barrier poses challenges in treating CNS disorders, as the majority of chemical drugs and biopharmaceuticals encounter obstacles in crossing this protective barrier to enter the brain.

Nanocarriers have commonly been employed to reduce peripheral degradation and improve the specific targeting of the brain. Recently, extracellular vesicles have emerged as an effective drug carrier. These are the lipid-membrane-enclosed nanometer-sized vesicles secreted by most cells composed of various nucleic acids, proteins and lipids of the parent cells, which are heterogeneous and mainly classified as microvesicles, exosomes and apoptotic antibodies [167]. Certain lipids such as sphingomyelins, glycosphingolipids, phosphatidylserines, phosphatidylethanolamines, cholesterol and phosphatidylcholines are more abundantly present in the extracellular vesicles than the plasma membrane [168]. Furthermore, various proteins like annexin II, integrins, heat shock proteins (HSPs) and tetraspanins are also more enriched in the extracellular vesicles [169]. These cell-specific proteins may vary depending on the source of the parent cell. Therefore, by mimicking the natural cellular processes, these vesicles can be engineered to deliver therapeutic agents directly to the affected neurons, enhancing DDSs and improving treatment outcomes. Table 2 summarizes the different cell membrane-based biomimetics for drug delivery in PD. The major drawback in drug delivery is the lack of specificity and rapid clearance by the immune system. A broad range of strategies for systemic delivery of therapeutic moieties have been explored, such as targeting sequence conjugation including aptamers, peptides and antibodies [32]. These targeting motifs are based on the structure of unique components of cellular compartments of interest. But another issue arises during large scale-ups especially when more than one targeting sequence is employed, in which case specificity can be hindered [32]. Additionally, it is critical to identify the targeting molecules for various cell types, making drug delivery not a straight-forward process.

Table 2.

Summary of cell membrane-based biomimetics used for drug delivery in PD.

System	Cell type of membrane	Brain penetration and targeting efficiency	Drug loading	PD model Brain targeting strategy		Ref.
Curcumin nanocrystals	RVG-modified RBC	Cyanine 5 (Cy5) tagged Cy5@RVG 29-RBCm/ CurN CS primarily accumulated in brain in comparison to liver& kidneys -Long circulation time (15.236±1.340 h); sustained release of curcumin levels (7.500 ± 1.000 h); high peak curcumin levels (0.125±0.015 µg/g); high AUC0-t values (3.550±0.332µg·h/g); high mean residence duration (21.841±1.777 h)	5.13% ± 0.31%	Mice MPTP; SH-SY5Y MPP+ model	RVG-29	[117]
Curcumin loaded liposome	Natural killer cell	Signal in the brain after administration remained in the brain until 36 h higher relative to liposomal curcumin	12.6%±0.3%	Mice MPTP, SH-SY5Y MPP+ model	MLV	[118]
Mesoporous silicon-loaded S-Nitro glutathione NPs	RBC	Compared to the UCMG group, RBCM/UCMG group efficiently crossed BBB within 2 h and was able to concentrate in brain	∼12.50%	Mice MPTP model	CD47 protein	[2]
Quercetin loaded-PVP NPs (CSPQ)	Neuronal cell	Staining and focused ultrasound demonstrated the accumulation of NPs in the substantia nigra and striatum of the brain	17.6%	Mice MPTP, SH-SY5Y MPP+ model	Ultrasound	[119]
Curcumin loaded-PVP NPs (CSCCT)	Macrophage	Efficiently accumulated in neuronal mitochondria with the assistance of focused ultrasound	16.5%	Mice MPTP, SH-SY5Y MPP+ model	DSPE-PEG2000-TPP	[120]
SC79 loaded Cu2−xSe-PVP NPs (CSS@CM NPs)	RAW 264.7 cell	Staining and focused ultrasound demonstrated accumulation of NPs in substantia nigra of the mice brain	7.1%	Mice MPTP model	Cu2−xSe NPs	[141]

Recent investigations have focused on the coating drug-delivery scaffolds with the physiological cell membranes in which the whole scaffold corresponds to the lipids, carbohydrates and protein composition of the cell membrane. These membrane-cloaked drug delivery materials exhibit properties of the source cell [32,44,[170], [171], [172]], and have exhibited better and prolonged residence time and disease-relevant specificity when conjugated with the desired targets [67]. It is interesting and holds significant potential to coat NPs with mammalian cell membranes. These mini physiological moieties can selectively recognize and target specific cell compartments, certain cell types, and even tissues. These specialized NPs can reduce toxicity, allergic reactions, and immune reactions for PD associated drug delivery [173]. In a recently developed targeted therapy for PD, to by-pass the limitation of BBB, a group of researchers proposed the delivery of curcumin liposomes encapsulated in a membrane of natural killer cells, forming a biomimetic nano-complex named BLIPO—CUR. This complex was delivered by meningeal lymphatic vessel (MLV) route via subcutaneous injection in the neck region of MPTP mice. Through this route, the biomimetic liposomes escape the macrophage-based phagocytosis and effectively recognize the damaged neurons. Moreover, BLIPO—CUR efficiently cleared α-synuclein and ROS and enhanced the expression of toll-like receptor 4 (TLR4) on the natural killer cell surface. Moreover, in vivo, images illustrated the delivery of BLIPO—CUR to the mice's brain and compared subcutaneous and intravenous routes. This highlights the importance of the natural killer cell membrane for effective amelioration of PD signs, with better effects observed when the meningeal lymphatic route was adopted for the delivery [118]. In another investigation, researchers modified the UCNPs surface with the RBCM for effective brain targeting. UCNPs were used to coat the mesoporous silicon and S-nitrosoglutathione as a source of NO donor. NO at appropriate concentrations can promote neuronal growth and decrease cell apoptosis and inflammation [174]. Thus, in this study, it was hypothesized that Ln³⁺-doped UCNPs could be used for the efficient conversion of NIR light to higher-energy green light, causing excitation at 540 nm of NO donor as shown in Fig. 4. This resulted in the light-mediated anti-inflammatory response and a decrease in the levels of pro-inflammatory factors in brain [2].

In a more recent report, investigators used a 29-amino acid-based rabies virus polypeptide-modified RBCM. These nanodecoys were designed to encapsulate curcumin nanocrystals. The RVG29-RBCm/Cur-NCs nanodecoys efficiently bypassed the reticuloendothelial system (RES) uptake, effectively improving the BBB penetration and residence time, with improved dopamine levels, decreased aggregated α-synuclein and mitigation of mitochondrial dysfunction associated with PD in mice model. The illustrated overview is shown in Fig. 5 [117].

Fig. 5.

(a) Schematic illustration of the synthesis of RBCM/UCMG; (b) By utilizing the surface CD47 protein that modifies the RBCM to exert immune evasion, UCNPs could rapidly pass through the BBB and target brain lesions. Adapted with permission from [2], Copyright© 2023 American Chemical Society.

In another study, novel biomimetic NPs (Cu-_xSe-PVP-Qe) were designed containing quercetin loaded poly (vinylpyrrolidone) (PVP) NPs cloaked with neuronal cell membrane denoted by CSPQ@CM NPs for ameliorating PD by effectively targeting and modulating microglia. This investigation revealed that biomimetic CSPQ@CM NPs were directed toward the microglia via precise interactions between α4β1 integrin and vascular cells present on the surface of the membranes adhering to microglia-expressed molecule-1. MES23.5 substantia nigra dopaminergic neuron cell membrane was used to camouflage NPs. The mean size as per the TEM image was found to be 24.7 nm, while the hydrodynamic size was found to be 78.8 nm as shown in Fig. 6b. In addition, the protein profile showed efficient retention of proteins. As demonstrated by BV2 cells, coating of NPs not only improves biocompatibility but also enhances endocytosis of the formulation. Fig. 6d–6e demonstrates that FITC-labelled NPs were phagocytosed better by BV2 cells than NIH-3T3 cells. Additionally, enhanced expression of VCAM-1 and α4β1 integrin shows their strong binding affinity, high accumulation and better targeting efficiency in BV2 cells. Quantification of the amount of NPs in the cells further proved the targeting capacity and retention in cells as shown in Fig. 6i. This highlights the advantage of biomimetic NPs for brain-targeting therapy in PD [119].

Fig. 6.

Modification of CSPQ NPs with MES23.5 cell membrane (i.e., CSPQ@CM) for targeting microglia. (a) Schematic illustration of the preparation of CSPQ@CM NPs. (b) TEM image of CSPQ@CM NPs. (c) Proteins determined by SDS-PAGE electrophoresis assay of (1) MES23.5 cell lysates, (2) MES23.5 cell membrane and (3) CSPQ@CM NPs. (d) CLSM images and (e) fluorescence distribution and intensity in NIH-3T3 and BV2 cells after incubation with different concentrations of CSPQ@CM NPs. The cell membrane was labeled as FITC. (f, g) Integrin α4 expressed by BV2 cells and (h) VCAM-1 expressed by MES23.5 cells detected by immunofluorescence staining and observed under a laser scanning confocal microscope. (i) Cu concentration in the same number of cells after culturing with 0.2 mM CSPQ or CSPQ@CM NPs. Adapted with permission from [119], Copyright© 2020 American Chemical Society.

Similarly, an investigation developed a macrophage cell membrane cloaked Cu₂−xSe-SC79 nano biomimetic (denoted as CSS@CM NPs) where the source of the macrophage cell membrane was RAW 264.7 macrophage cells, and SC79 is a specific unique agonist of Akt phosphorylation. SC79 was used to potentially target the neuronal damage by reducing neuron excitotoxicity. However, its precise brain delivery is a barrier to its therapeutic efficacy. Thus, Cu₂−xSe-SC79 was used as a carrier for SC79 due to its ultrasmall nature, and the release of copper ions can significantly facilitate neuronal proliferation and migration, which activates the Akt and ERK pathway. Therefore, CSS@CM NPs could significantly enhance neurite outgrowth and interactions of α4β1-VCAM-1, and reduce neuronal apoptosis and synaptic dysfunction in PD mice brain regions, such as substantia nigra and striatum [175].

Thus, there is a wide application of cell membrane-cloaked biomimetics for PD. This highlights a new era in targeted delivery systems. These biological deliveries are attractive candidates for precise delivery in the brain. The only drawback we are facing is related to the clinical translation of these bioscaffolds, which includes the scale-up processes, integrity and membrane coverage of NPs that needs to be addressed at its earliest.

4. Biomedical applications in neurodegenerative diseases

Biomimetic NPs represent a cutting-edge approach to biomedical applications, offering innovative solutions for various medical challenges. One particularly promising avenue is their application in the realm of neurodegenerative disorders, where the need for targeted and effective therapeutic interventions is paramount. Neurodegenerative diseases, such as AD and PD, pose significant challenges due to their complex etiology and the limited success of conventional treatments. Biomimetic NPs, designed to mimic biological entities' structural and functional aspects, present a revolutionary strategy to address these challenges [176]. These NPs, inspired by the intricate design of biological systems, exhibit unique properties that enhance their interaction with biological entities. In the context of neurodegenerative disorders, the biomimetic approach holds great promise for delivering therapeutic agents to specific regions of the brain with precision and efficacy. One noteworthy application involves using biomimetic NPs for drug delivery, enabling the targeted release of neuroprotective agents to combat the progression of neurodegeneration. Mimicking the natural mechanisms of cellular uptake, these NPs can traverse the BBB, ensuring that therapeutic payloads reach the affected areas of the brain [177].

Biomimetic NPs can be engineered to replicate the functions of endogenous cells, offering a multifaceted approach to neurodegenerative disorders. For instance, synthetic NPs can mimic the behavior of glial cells, playing a crucial role in maintaining neuronal health and function. By integrating features such as surface receptors and signaling molecules, these NPs can modulate the neuroinflammatory response, a key contributor to the progression of neurodegenerative diseases. This biomimetic modulation of the immune system can help attenuate the damaging effects of chronic inflammation on neural tissues. Various neurodegenerative disorders, including PD, AD, HD and multiple sclerosis (MS), will be elucidated. The exploration will delve into their distinct pathophysiological mechanisms and potential biomarkers, fostering a comprehensive understanding of each disease [141]. Not only in neurodegenerative disorders, but these biomimetics have also shown promise in other neurological disorders. For instance, recent research has focused on the role of targeting miRNAs in GBM. In this study, miRNA-nanosponge was developed and coated with BV2 cell membrane for crossing BBB which specifically targets miRNA-221, miRNA-215, miRNA-21 and miRNA-9 to modulate the immune system and inhibit GBM progression [178]. This highlights the importance of biomimetics in neurological disorders.

4.1. Parkinson's disease (PD)

PD is a progressive neurodegenerative disorder characterized by the selective loss of dopaminergic neurons in the substantia nigra pars compacta, a critical region of the brain responsible for motor control [179]. First described by James Parkinson in 1817, the pathophysiology involves the accumulation of misfolded alpha-synuclein protein in neuronal cytoplasmic inclusions, known as Lewy bodies. The resultant dopamine depletion in the basal ganglia leads to the cardinal motor symptoms of tremor, bradykinesia, rigidity and postural instability. While the etiology remains multifactorial, genetic predisposition and environmental factors are implicated in the complex interplay contributing to the disease's onset. Present therapeutic approaches focus on alleviating Parkinson’s symptoms through a combination of pharmacotherapy and surgical interventions. Furthermore, the existing treatment options continue to pose significant challenges, prompting the exploration of novel approaches for DDS development [180].

Investigation done by Liu, et al. on a novel therapeutic approach for PD was explored through the development of brain-targeted biomimetic nanodecoys, specifically RVG29-RBCm/Cur-NCs. The goal of this investigation was to increase the residence time in the systemic circulation, cross the BBB, and increase the anti-PD drug’s bioavailability. RVG29-RBCm/Cur-NCs were successfully introduced in the BBB, accumulated on neurons by binding to nAChR receptors, and demonstrated effective neuroprotective effects in a PD mouse model induced by MPTP/MPP⁺. These effects manifested as improvements in motor behavior, mitigation of the pathological decrease in TH⁺ neurons, inhibition of abnormal α-synuclein aggregation, elevation of dopamine levels, and reversal of mitochondrial dysfunction in the brain. Additionally, RVG29-RBCm/Cur-NCs exhibited commendable biocompatibility. The outcomes of this study highlight RVG29-RBCm/Cur-NCs as a promising strategy for targeted drug delivery to the brain in the context of PD and other neurodegenerative diseases [117].

In another study, the investigator engineered a therapeutic platform utilizing UCNP responsive to 980 nm NIR light, allowing controlled release of NO for the treatment of inflammatory PD. This system, enclosed within a cell membrane, demonstrated efficient penetration of the BBB and precise delivery to the inflamed region in the brain. Activation of UCNPs by NIR prompted the release of NO, swiftly diminishing proinflammatory factors in the affected area and achieving a therapeutic outcome [118].

Another study done by Liu et al. created biomimetic NPs based on ultrasmall Cu₂–xSe and demonstrated their efficacy in targeting and modulating microglia to enhance PD therapy. These Cu₂–xSe NPs, termed CSPQ, were endowed with exceptional multienzyme-like activities through modification with quercetin. Furthermore, they encapsulate them with the membrane of neuronal cells (MES23.5 cells), resulting in CSPQ-CM NPs with superior targeting capabilities and specificity. Studies reveal that the abundant expression of VCAM-1 on MES23.5 cell membranes facilitated the specific targeting of CSPQ-CM NPs to microglial α4β1 integrin through robust α4β1-VCAM-1 interactions. Additionally, these NPs induced microglial polarization toward the M2-like phenotype, exerting an anti-inflammatory effect by reducing neurotoxic IL-6 and increasing neuroprotective IL-10. Utilizing focused ultrasound assistance, CSPQ-CM NPs could efficiently and precisely reach the substantia nigra and striatum of the brain, effectively modulating microglia, and the brain environment by eliminating ROS. This led to increased dopamine levels in the cerebrospinal fluid, elevated expression of tyrosine hydroxylase, and reduced expression of IBA-1. The significant alleviation of PD symptoms, along with improvements in dyskinesia and cognition impairment in PD mice [119].

4.2. Alzheimer’s disease (AD)

AD is a progressive and incurable brain condition that gets worse over time. It causes problems with memory, thinking and behavior. People with AD may struggle with short-term memory, everyday activities, and how they see things and make decisions. There are also rare types of AD, like primary progressive aphasia, posterior cortical atrophy and the frontal variant of AD, which have different symptoms but still affect memory [181]. AD stands as the predominant form of dementia, constituting 60%–70% of all cases. Its incidence rises with age, affecting 10% of individuals aged 65 and older, and ∼50% of those aged 85 and above. Although less common, AD can also manifest in younger individuals, occasionally in their 20 s, often linked to genetic abnormalities. Projections estimate that the global prevalence of AD will reach 131 million by 2050, with the highest burden expected in middle-income and low-income countries. AD poses a significant societal and economic challenge, with annual healthcare costs rivalling those of cardiovascular disorders and exceeding those of cancer. It ranks as the 4th to 5th leading cause of death. To date, there is no curative intervention capable of slowing disease progression [182].

Some studies have shown the therapeutic use of biomimetic NPs. A study by Gao et al. focused on biomimetic engineered nanosystems in AD mice. They prepared biomimetic NPs (T807/TPP-RBC—NPs) that can deliver antioxidants into neuronal mitochondria. The biocompatibility and sustained circulation of T807/TPP-RBC—NPs result from the optimal physicochemical properties of the "inner core" in conjunction with the unique biological functions of the "outer shell." The incorporation of T807 and TPP on the "outer shell" facilitates the biomimetic-engineered nanosystem’s efficient traversal across the BBB and specific binding to neurons, ultimately reaching the mitochondria. While the current research data is preliminary, it collectively implies that T807/TPP-RBC—NPs hold promise as an intravenous delivery platform tailored for targeted mitochondrial delivery in AD treatment [183]. In another study, investigators engineered and synthesized CuxO-EM-K, a nanomaterial with active Aβ-targeting capabilities, aimed at enhancing protein adsorption resistance and minimizing immunogenicity. This design facilitates the selective and effective clearance of peripheral Aβ. The incorporation of RBC membrane coating, in conjunction with DSPE-PEG-K, serves as a nanocaptor for selectively capturing Aβ in the bloodstream. Simultaneously, the CuxO enzyme plays a crucial role in alleviating Aβ-induced membrane oxidative damage and stabilizing the RBC membrane, ensuring prolonged blood circulation essential for Aβ adsorption. Notably, the RBC membrane coating prevents the formation of protein coronas, preserving the Aβ-targeting capability of CuxO-EM-K in the bloodstream. Consequently, CuxO-EM-K effectively reduces Aβ burden in both the blood and the brain, ultimately reversing learning and memory impairments in 3xTg-AD mice. Importantly, blood biochemistry and histology analyses revealed no significant systemic toxicity associated with CuxO-EM-K. In summary, the biomimetic CuxO-EM-K presented in this study provides a safe and effective approach for the peripheral clearance of Aβ associated with AD [184]. Huo et al. developed a novel approach using exosomes loaded with silibinin (Exo-Slb) to simultaneously target Aβ aggregation and astrocyte activation, aiming to alleviate cognitive impairment in mice with AD. In this investigation, Exo-Slb exhibited high selectivity in binding to Aβ1–42, effectively inhibiting its aggregation. Moreover, Exo-Slb demonstrated the ability to suppress astrocyte activation, leading to a reduction in the secretion of inflammatory cytokines. Ultimately, Exo-Slb reversed neuronal damage and mitigated cognitive impairment in the AD mouse model induced by Aβ by modulating the NF-κB pathway. In summary, the design of silibinin-loaded exosomes derived from macrophages, serving as a multifunctional DDS, holds promise as a potentially beneficial treatment for AD in the future [185].

4.3. Multiple sclerosis (MS)

Inflammation subsequently causes demyelination and gliosis-associated neurodegeneration are the two key pathological characteristics of MS. However, the damage caused due to MS is limited to the CNS [186]. Several treatments are there like ocrelizumab, ofatumumab, natalizumab and mitoxantrone, but they have side effects including infusion-related reactions, upper respiratory tract infections, oral herpes infections, and urinary tract infections are frequently reported. FNb-1a (Rebif), IFNb-1a (Avonex) and PegIFNb-1a (Plegridy) are some more drugs that are used for MS but their mechanism of action has not been discovered yet, and here the biomimetic NPs play a crucial role in the treatment of MS [187].

MS is a serious condition affecting young individuals, and dimethyl fumarate (DMF) is among the oral treatment options for this disease. While DMF effectively reduces inflammatory conditions, its various side effects pose challenges to patient adherence and increase economic burdens. In this study, a novel approach was employed, using platelet membranes (PNs) and platelet membrane-coated nanoparticles (PCCN), to create a biomimetic DDS for managing MS. In vivo, results demonstrated that these prepared NPs significantly increased DMF concentrations in the brain compared to the free drug. Moreover, notable differences were observed between conventional nanoparticles (CN) and PCCN in brain concentration and AUC_0-t from the concentration-time curve. PNs exhibited a higher brain uptake clearance value than CN and PCCN, suggesting that PNs could enhance DMF penetration and accumulation in the brain. This study concludes that PNs present a promising cell-based biological approach for effectively delivering drugs used in the treatment of MS [188]. In another study, the interaction between myelin basic protein (MBP) and large unilamellar vesicles (LUV) with both native and experimental autoimmune encephalomyelitis (EAE) modified lipid compositions, investigating the subsequent liposomal assembly process. The affinity of LUV towards MBP with EAE-modified membranes was observed to be more vigorous than with innate membranes, leading to a heightened inclination for aggregation in EAE-mimicking LUV compared to native-like liposomes. While EAE-modified membranes remained stable in this aggregated state, native vesicles underwent a secondary reordering process over several days, resulting in LUV fusion and the formation of a significant amount of multilamellar structure. In investigations involving bending fluctuations of the LUV, liposomes with a native lipid composition exhibited higher bending rigidity than EAE-modified membranes. Additional experiments with oriented bilayers revealed the formation of a concentrated protein layer on top of the membranes for both lipid compositions. The surface excess of MBP on EAE lipid composition membranes exceeded that on native lipid composition membranes, indicating stronger MBP interactions with diseased membranes that caused structural perturbations. This manifested as membrane swelling and buffer insertion in the hydrocarbon section of the membrane [189]. The application and limitation of different cell membranes used to synthesize biomimetics is illustrated in Table 3.

Table 3.

Application and limitation of different cell membranes used to synthesize biomimetics.

Membrane type	Advantages	Disadvantages	Ref.
RBCs	Long circulation, simple approach for membrane functionalization.	Purification is time-consuming; there are proper protocols for preparation and purification	[190]
WBC	Regulation of inflammatory response, selectivity at specific disease site.	Complexity of WBC membrane	[191]
Platelets	Promising properties in treating hemorrhage and targeted delivery.	Complex purification routes	[190]
Exosomes	Favorable for long-distance cell-to-cell communications.	No proper standardized method to produce and isolate in the desired amount	[192]
Macrophage	Long circulation time, BBB penetrating ability, target specific.	Limited capacity for drug incorporation; Unfavorable or inappropriate drug release; Degradation of the drug within cells	[192]
T-cell	Long circulation time, targeting the specific site.	MHC restriction, targeted to limited site	[192]

5. Challenges and future perspectives

A key challenge in utilizing biomimetic nanomaterials stems from their inherent complexity. Understanding the significance of specific molecules in shaping nanomaterial behavior is challenging due to the multitude of molecules in the entire membrane, potentially numbering in the hundreds. Few studies have endeavored to block single membrane proteins using antibodies to observe changes in particle behavior without their functional contribution. However, redundancy among proteins with similar functions or unexpected contributions from other proteins may complicate the interpretation of such studies. Investigating this heightened complexity may necessitate future efforts involving innovative techniques for a more comprehensive understanding [53]. Efforts to expand knowledge and identify essential molecules crucial for desired particle functions could pave the way for producing nanomaterials that combine single proteins found in different cell types on a single platform. This approach would eliminate the need to include excessive unwanted cellular material, resulting in simpler formulations with more reproducible features and improved scalability. Despite the predominant focus on surface proteins in current research, it is crucial to recognize that the biological function of cell membranes is not solely attributed to surface proteins. The phospholipid bilayer and the complex sugars associated with proteins and lipids also play crucial roles. Surprisingly, there is limited research on these specific membrane components, and further exploration could unveil additional possibilities for innovative biomimetic nanosystems [53]. In a recent study by Liu et al. [37], an important issue was raised regarding the coating of NPs. The study demonstrated how processes such as extrusion, sonication, and the common ratios between particles and membranes used for coating can result in formulations with only a very small percentage of particles being completely coated. Interestingly, the authors found that cells internalize partially coated NPs using different mechanisms compared to fully coated ones. This discovery holds significance for improving current approaches to biomimetic membrane-coated nanovectors. Understanding this, researchers should aim to optimize NP coverage with cell membranes. This optimization could potentially lead to better stability, a prolonged presence of NPs in the body, and increased effectiveness due to the higher biological content carried by these NPs. Most studies so far have relied on qualitative techniques like TEM imaging, zeta potential assessment, or SDS-PAGE to confirm the presence of membrane proteins on NPs. Looking forward, researchers must incorporate quantitative methods in future formulations to accurately measure the extent of NP membrane coverage.

Furthermore, cell membrane-based biomimetics come with certain key concerns in terms of stability, storage, maintaining the functionality of embedded membrane proteins, potential issues with in vivo circulation and immune response, and ultimately, translating these systems to clinical applications, which can be significantly hindered by challenges related to production, scalability and consistent quality control [193]. Membrane proteins are extremely sensitive to environmental alterations and vulnerable to losing their native structure and function when extracted from the cell membrane, requiring optimization for stabilization during storage [194]. Lipid bilayer membranes are vulnerable to degradation over a long period, which also affects the integrity and stability of the biomimetic system. Cell membranes also tend to fuse or aggregate impacting the size distribution causing aggregation and disturbing their overall functionality [193]. Another important feature is maintaining the correct orientation and conformation of the membrane proteins within the biomimetic system, which is crucial for its overall function and can certainly challenge the fabrication process [193]. Another important issue is the ligand binding and signaling. Ensuring that receptors and proteins on the membrane retain the ability to interact with specific ligands and initiate downstream signaling pathways is crucial [195]. When the source of the cell membrane is different from the host source, immune responses play a major role. The foreign membrane can trigger immune responses leading to rapid clearance. Rapid clearance from the bloodstream may also occur by the RES due to their size and surface properties [196]. Many more modifications are required to address these critical challenges. Moreover, translating the pre-clinical evidence to the clinical level has various challenges including bio-compatibility issues, scalability, and regulatory hindrances [42]. On-going research is required to address the efficacy and safety profiles of these promising therapeutic tools.

Conflicts of interest

The authors declare that there are no conflicts of interest. The authors alone are responsible for the content and writing of this article.