Introduction

Nanotechnology is defined as a branch of science, engineering, and technology that involves molecules at the nanoscale (1–100 nm). To date, nanotechnology has contributed to several scientific fields, such as chemistry, physics, biology, and medicine. In particular, in biomedicine [1], many novel and promising nanoparticle (NP)-based drug delivery systems (DDSs) have been used for the safe and efficient transport of drugs or therapeutic genes in vivo [2]. Controlled distribution and drug release of NPs due to their nanoscale properties could improve bioavailability in vivo [2]. For example, because of their enhanced permeability and retention (EPR) effect [3,4,5,6], NPs can highly accumulate in tumor tissues. Even if NP delivery systems can achieve passive targeting, problems such as interaction with the reticuloendothelial system (RES), formation of a protein crown, accelerated blood clearance (ABC), and poor targeting ability toward specific cells remain unresolved [7,8,9,10,11]. Polyethylene glycol (PEG), designated as generally recognized as safe (GRAS) by the Food and Drug Administration (FDA), is widely used for the surface modification of NPs in order to extend their blood circulation time and enhance their targeting capabilities [12]. The PEG chains form a flexible polymer brush layer and create steric hindrance, which can cover up the NP surface charge [13, 14]. This significantly inhibits the adsorption of serum proteins, thereby reducing the recognition of macrophages and minimizing complement activation [15]. However, recent clinical research has indicated the existence of anti-PEG immunity, suggesting that PEGylation can also lead to the ABC phenomenon [16,17,49]. In industrial-scale production, homogenization combined with centrifugation is extensively used [37, 38, 49].

Principal types of NP templates

Different inner cores endow CMC-NPs with different properties. There are two main types of inner cores: organic and inorganic. Core selection according to the subsequent application is necessary.

Organic inner cores have better biocompatibility and biodegradability [50, 51]. The US FDA has approved the clinical application of gelatin, liposome, and poly(lactic-co-glycolic acid) (PLGA). Among all inner cores, PLGA is the most commonly used in the preparation of membrane biomimetic carriers and holds great promise for clinical applications [52]. Various membranes, including platelet membranes [21, 53], cancer cell membranes [22, 54], macrophage membranes [55], and stem cell membranes [56], can be modified on PLGA particles to prevent the formation of agglomerates on NPs and achieve better delivery efficiency. Another widely used inorganic inner core is a liposome, which resembles the cell membrane [57, 58]. Liposomes are biodegradable colloids capable of containing hydrophobic or hydrophilic pharmaceuticals [59, 60]. Moreover, they can penetrate in vivo barriers as they are flexible [60]. Cell membrane coating improves the stability of phospholipid membranes and achieves a longer circulation time without affecting the drug loading capacity [61, 62].

The stability of inorganic NPs and their resistance to enzymatic degradation are unmatched [63]. Moreover, by manipulating the form, size, composition, and surface qualities of inorganic NPs, their inherent electrical, optical, and magnetic capabilities can be enhanced to achieve full therapeutic potential [63]. For example, an innovative class of nanophotothermal transduction agents, Fe3O4 NPs, can be designed for use in photothermal therapy (PTT) [28]. Macrophage membrane-coated Fe3O4 NPs can specifically target cancer cells and selectively kill cells by increasing the ambient temperature when exposed to laser light [64]. Another example is the use of stem cell membrane-camouflaged superparamagnetic iron oxide (SPIO) NPs for thermomagnetic therapy. SPIO NPs can rapidly change their magnetic moments and thus generate heat under a high-frequency alternating magnetic field for hyperthermia therapy applications [65]. When using inorganic nanocarriers, toxicity and biodistribution continue to be key concerns. Changing the particle size is one solution [66]. For instance, micron-sized CuO could result in safe delivery; however, CuO NPs could cause DNA damage [66,67,68]. In the case of SiO2, an increase in particle size (from 30–40 to 100–150 nm) could significantly reduce cytotoxicity [8) [28].

Fig. 8
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Copyright 2018, Nano Letter

DiRL labeled liposomal nanoparticles (DiRL, LM-DiRL, TM-DiRL, and LTM-DiRL) (n = 4). a In vivo biodistribution of different groups after intravenous injection. b Quantitative analysis of fluorescence accumulation in the main organs. c Histogram of quantitative analysis of fluorescence accumulation in the main organs. Reproduced with permission [28]. DiRL, DiR-labeled liposomal nanoparticles; LM-DiRL, leukocyte membrane-coated DiRL; TM-DiRL, tumor cell membrane-coated DiRL; LTM-DiRL, leukocyte-tumor cell membrane-coated DiRL.

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LCMs can also be used for cancer detection, playing an important role in cancer monitoring and diagnosis. Some existing detection methods are not sensitive and accurate enough for capturing and detecting circulating tumor cells (CTCs). They fail to predict tumor metastasis in advance because of the low concentration of CTCs and interference from leukocytes [178]. However, LCMs can reduce interference from homologous leukocytes and have the ability of tumor region targeting, which can improve CTC isolation and detection. For instance, Ding et al. successfully built a nanoplatform with LCMs for highly efficient cancer detection [171]. The purity of captured CTCs in the LCM-coated NPs group was 96.96%, which was much higher than that in the bare NPs and monotypic cell membrane-coated NPs groups.

In conclusion, LCMs can be extensively used for disease treatment, particularly in cancer therapy. Leukocytes have also been confirmed to be a precursor of tumor metastasis in human bodies. Therefore, some studies have focused on the regulation of epigenetic expression of the parent cell by LCMs and expression of a specific antigen profile for performing immunotherapy in order to enable efficient removal of tumor cells and cancer treatment [89].

This section reviews the characteristics and advantages of various types of HCMNs. More applications and experiments of HCMNs are presented in Table 2 for better understanding. In summary, several reports have indicated that different cell membrane combinations play unique roles in the treatment of specific diseases. HCMNs can have multiple applications, use in liquid biopsy and cancer vaccines, targeting disease regions, use in combination with other treatments, and detoxification.

Prospects and challenges

Cell membrane coating utilizes natural components at the source to directly transfer natural properties displayed by source cells, thereby recreating complex biological functions and integrating functions that cannot be achieved through synthesis. In this review, the drug delivery capabilities of CMC-NPs are highlighted. Biologically derived raw materials offer a longer blood circulation time, better immune escape, and stronger targeting ability than bare NPs. Undeniably, CMC-NPs still have drawbacks and pose obstacles. Their prospects and challenges will be the main topics of this section.

Quality control

As CMC-NP is a novel drug delivery platform, its quality control needs to be further explored. By referring to the existing standards and quality control specifications for cellular medicines [179, 180], the quality control of CMC-NPs can be divided into three parts.

Cell collection and isolation process control

In the case of cellular raw materials used for preparing CMC-NPs, cell identification, survival and growth activity assessment, foreign pathogen detection, and basic cell characteristic assessment are necessary. Cell characteristics include specific populations of cell surface markers, expression products, and differentiation potentials.

In addition, standard operation and management procedures for the collection and separation of different cells should be formulated and strictly implemented based on GMP requirements. Moreover, each cell type requires standardized and well-established cell culture protocols so that its phenotype and purity can be maintained during passaging [181].

Manufacturing process and storage ability

More consideration needs to be given to the fusion process. Careful calculation and control of the membrane-to-NP ratio are essential to ensure complete coverage and reduce loss of cell membrane. Moreover, the preparation of HCMNs is complex (e.g., determination of the ratio of the two cell membranes and the membrane mixing type), making it difficult to determine an optimal HCMN preparation method suitable for a particular disease [25]. Furthermore, producers are required to use standard biotechnological production and purification techniques. The entire production process should not lead to further impurities other than those originating from the active substance.

Sterilization is another important part of manufacturing process control. The currently accepted sterility assurance level (SAL) is 10−6 [182]. Quality control systems need to guarantee that pyrogens, bacteria, virus endotoxins, or LPS do not contaminate CMC-NPs. Filter sterilization is a widely used technique for sterilizing nanoformulations [183, 184]. Specific standards for sterility and endotoxin testing can be formulated according to national quality control regulations.

During the storage process, biological sample storage is usually performed using the freeze–drying method [185]. The potential influence of the lyophilization process on finished product quality results in product-derived impurities, which need to be controlled using the established analytical methods. In addition, the purity and coverage of the preparation process can impact the storage stability of different cell membrane coating systems [24, 31]. Therefore, numerous pre-experiments on screening conditions in the early stage of mass production are required to improve the storage stability of certain CMC-NPs.

Product control, batch analysis, and product stability

For analyzing the active substance quality in CMC-NPs, therapeutic activity, encapsulation rate, and drug release rate are assessed. The precise ingredients in each CMC-NP primarily vary in two areas: safety and efficacy.

To ensure batch-to-batch repeatability during mass production, process parameters must be examined to determine the variables that could harm the product. Process variables include ambient conditions (temperature, pH, and pressure), formulation variables (cell types, component ratios, and solvents utilized), and formulation processes (time, speed, flow conditions, and power) [186]. Short-, medium-, and long-term stability must also be assessed.

Consideration for clinical applications

Although massive studies have resulted in different membrane-coated NP formulations, little research has progressed to clinical practice. This section focuses on the challenges in the clinical translation of CMC-NPs and tries to provide reliable solutions.

First, the in vivo mechanisms of both hybrid and monotypic CMC-NPs remain unknown. One of the main reasons why it is challenging to perform clinical trials for membrane biomimetic carriers is the intricacy and unpredictability of the intermediate process results in vivo. It is risky to assume that the CMC or HCMN would deliver drugs via the theoretical route after entering the human body. To apply membrane coatings beyond the current black box approach [8], researchers need to elucidate more physiological mechanisms, such as internalized mechanisms, intracellular release mechanisms, and subcellular-level actions. This requires a more fundamental understanding of cell biology, which is becoming more prevalent. Therefore, it is imperative to study the in vivo mechanism of membrane biomimetic carrier DDSs, their route of delivery, and their process as soon as possible.

Second, there are issues related to actual benefits. In vivo and in vitro experiments on various types of HCMNs have revealed that HCMNs can indeed exhibit the functional advantages of both types of CMC-NPs. Several experiments, however, have revealed that the mixed benefits of HCMNs are not as high as the unique benefits of monotypic cell membranes in terms of certain functions, such as targeting ability [86, 87] or prolonged blood circulation time [8, 27]. In other words, while the new HCMN DDS verifies and realizes the possibility of 1 + 1, this does not make it > 2.

Third, technical difficulties in acquiring source materials still exist. While cell membranes can be autologous, it may be more practical to obtain and store materials from types of matched donors [24]. However, heterologous cells may have toxicity, biological incompatibility, and immunogenicity. The optimization of protocols to remove unnecessary proteins and retain necessary ones remains to be explored. In addition, changes in membrane protein contents during storage remain another challenge [187, 188]. However, we believe that once a patient-specific cell membrane becomes available, precision medicine will dramatically advance. Addressing disease heterogeneity and establishing personalized therapeutics will then become an achievable goal.

Furthermore, cell membrane-coated platforms will encounter greater developmental opportunities through the integration of newer branches of science and biotechnology (e.g., synthetic biology and biomaterial science), leading to richer therapeutic possibilities. For instance, the use of CMC-NPs to develop vaccines is a novel method for the prevention and treatment of COVID-19, which has been continuously developed and transformed in recent years [189]. Moreover, a few studies have used the membrane from genetically engineered source cells. In these studies, the expression of specific surface markers has been induced or upregulated, optimizing the functionality for a given application [41, 190]. Although cell membranes are by far the main source of membrane coatings, more consideration could be given to other membrane sources, like organelle membranes [42].

Conclusion

Monotypic cell membrane coating or hybrid cell membrane coating confers unique biological properties to NPs, including immune escape, long circulation time, and targeted delivery, thereby enabling more efficient drug delivery. Consequently, cell membrane-coated DDSs have gradually become a novel research hotspot. However, more efforts are needed for the clinical transformation and application of CMC-NPs. Obstacles to the standard protocol, quality control, and large-scale production need to be overcome. Assessment of the mechanism and in vivo process will also guide further improvements in the design and preparation of biomimetic carriers.