A Closed Chondromimetic Environment within Magnetic-Responsive Liquified Capsules Encapsulating Stem Cells and Collagen II/TGF-β3 Microparticles

Stimuli-responsive microcarriers and their application in tissue repair: A review of magnetic and electroactive microcarrier

Microcarrier applications have made great advances in tissue engineering in recent years, which can load cells, drugs, and bioactive factors. These microcarriers can be minimally injected into the defect to help reconstruct a good microenvironment for tissue repair. In order to achieve more ideal performance and face more complex tissue damage, an increasing amount of effort has been focused on microcarriers that can actively respond to external stimuli. These microcarriers have the functions of directional movement, targeted enrichment, material release control, and providing signals conducive to tissue repair. Given the high controllability and designability of magnetic and electroactive microcarriers, the research progress of these microcarriers is highlighted in this review. Their structure, function and applications, potential tissue repair mechanisms, and challenges are discussed. In summary, through the design with clinical translation ability, meaningful and comprehensive experimental characterization, and in-depth study and application of tissue repair mechanisms, stimuli-responsive microcarriers have great potential in tissue repair.

In situ cell condensation-based cartilage tissue engineering via immediately implantable high-density stem cell core and rapidly degradable shell microgels

Formation of chondromimetic human mesenchymal stem cells (hMSCs) condensations typically required in vitro culture in defined environments. In addition, extended in vitro culture in differentiation media over several weeks is usually necessary prior to implantation, which is costly, time consuming and delays clinical treatment. Here, this study reports on immediately implantable core/shell microgels with a high-density hMSC-laden core and rapidly degradable hydrogel shell. The hMSCs in the core formed cell condensates within 12 hours and the oxidized and methacrylated alginate (OMA) hydrogel shells were completely degraded within 3 days, enabling spontaneous and precipitous fusion of adjacent condensed aggregates. By delivering transforming growth factor-β1 (TGF-β1) within the core, the fused condensates were chondrogenically differentiated and formed cartilage microtissues. Importantly, these hMSC-laden core/shell microgels, fabricated without any in vitro culture, were subcutaneously implanted into mice and shown to form cartilage tissue via cellular condensations in the core after 3 weeks. This innovative approach to form cell condensations in situ without in vitro culture that can fuse together with each other and with host tissue and be matured into new tissue with incorporated bioactive signals, allows for immediate implantation and may be a platform strategy for cartilage regeneration and other tissue engineering applications.

Viscous Microcapsules as Microbioreactors to Study Mesenchymal Stem/Stromal Cells Osteolineage Commitment

It is essential to design a multifunctional well-controlled platform to transfer mechanical cues to the cells in different magnitudes. This study introduces a platform, a miniaturized bioreactor, which enables to study the effect of shear stress in microsized compartmentalized structures. In this system, the well-established cell encapsulation system of liquefied capsules (LCs) is used as microbioreactors in which the encapsulated cells are exposed to variable core viscosities to experience different mechanical forces under a 3D dynamic culture. The LC technology is joined with electrospraying to produce such microbioreactors at high rates, thus allowing the application of microcapsules for high-throughput screening. Using this platform for osteogenic differentiation as an example, shows that microbioreactors with higher core viscosity which produce higher shear stress lead to significantly higher osteogenic characteristics. Moreover, in this system the forces experienced by cells in each LC are simulated by computational modeling. The maximum wall shear stress applied to the cells inside the bioreactor with low, and high core viscosity environment is estimated to be 297 and 1367 mPa, respectively, for the experimental setup employed. This work outlines the potential of LC microbioreactors as a reliable in vitro customizable platform with a wide range of applications.

Chitosan-Based Biomaterials: Insights into Chemistry, Properties, Devices, and Their Biomedical Applications

Chitosan is a marine-origin polysaccharide obtained from the deacetylation of chitin, the main component of crustaceans’ exoskeleton, and the second most abundant in nature. Although this biopolymer has received limited attention for several decades right after its discovery, since the new millennium chitosan has emerged owing to its physicochemical, structural and biological properties, multifunctionalities and applications in several sectors. This review aims at providing an overview of chitosan properties, chemical functionalization, and the innovative biomaterials obtained thereof. Firstly, the chemical functionalization of chitosan backbone in the amino and hydroxyl groups will be addressed. Then, the review will focus on the bottom-up strategies to process a wide array of chitosan-based biomaterials. In particular, the preparation of chitosan-based hydrogels, organic–inorganic hybrids, layer-by-layer assemblies, (bio)inks and their use in the biomedical field will be covered aiming to elucidate and inspire the community to keep on exploring the unique features and properties imparted by chitosan to develop advanced biomedical devices. Given the wide body of literature that has appeared in past years, this review is far from being exhaustive. Selected works in the last 10 years will be considered.

Liquefied Microcapsules Compartmentalizing Macrophages and Umbilical Cord-Derived Cells for Bone Tissue Engineering

Extraordinary capabilities underlie the potential use of immune cells, particularly macrophages, in bone tissue engineering. Indeed, the depletion of macrophages during bone repair often culminates in disease scenarios. Inspired by the native dynamics between immune and skeletal systems, this work proposes a straightforward in vitro method to bioengineer biomimetic bone niches using biological waste. For that, liquefied and semipermeable reservoirs generated by electrohydrodynamic atomization and layer-by-layer techniques are developed to coculture umbilical cord-derived human cells, namely monocyte-derived macrophages, mesenchymal-derived stromal cells (MSCs), and human umbilical vein endothelial cells (HUVECs). Poly(ε-caprolactone) microparticles are also added to the liquefied core to act as cell carriers. The fabricated microcapsules grant the successful development of viable microtissues, ensuring the high diffusion of bioactive factors. Interestingly, macrophages within the bioengineered microcapsules increase the release of osteocalcin, osteoprotegerin, and vascular endothelial growth factor. The cytokines profile variation indicates macrophages' polarization into a prohealing phenotype. Altogether, the incorporation of macrophages within the fabricated microcapsules allows to recreate an appropriate bone microenvironment for developing new bone mineralized microtissues. The proposed bioencapsulation protocol is a powerful self-regulated system, which might find great applicability in bone tissue engineering based on bottom-up approaches or disease modeling.

Progress of 3D Printing Techniques for Nasal Cartilage Regeneration

Once cartilage is damaged, its self-repair capacity is very limited. The strategy of tissue engineering has brought a new idea for repairing cartilage defect and cartilage regeneration. In particular, nasal cartilage regeneration is a challenge because of the steady increase in nasal reconstruction after oncologic resection, trauma, or rhinoplasty. From this perspective, three-dimensional (3D) printing has emerged as a promising technology to address the complexity of nasal cartilage regeneration, using patient's image data and computer-aided deposition of cells and biomaterials to precisely fabricate complex, personalized tissue-engineered constructs. In this review, we summarized the major progress of three prevalent 3D printing approaches, including inkjet-based printing, extrusion-based printing and laser-assisted printing. Examples are highlighted to illustrate 3D printing for nasal cartilage regeneration, with special focus on the selection of seeded cell, scaffolds and growth factors. The purpose of this paper is to systematically review recent research about the challenges and progress and look forward to the future of 3D printing techniques for nasal cartilage regeneration.Level of Evidence III This journal requires that authors assign a level of evidence to each submission to which Evidence-Based Medicine rankings are applicable. This excludes Review Articles, Book Reviews, and manuscripts that concern Basic Science, Animal Studies, Cadaver Studies, and Experimental Studies. For a full description of these Evidence-Based Medicine ratings, please refer to the Table of Contents or the online Instructions to Authors https://www.springer.com/00266 .

Hipster microcarriers: exploring geometrical and topographical cues of non-spherical microcarriers in biomedical applications

Structure and organisation are key aspects of the native tissue environment, which ultimately condition cell fate via a myriad of processes, including the activation of mechanotransduction pathways. By modulating the formation of integrin-mediated adhesions and consequently impacting cell contractility, engineered geometrical and topographical cues may be introduced to activate downstream signalling and ultimately control cell morphology, proliferation, and differentiation. Microcarriers appear as attractive vehicles for cell-based tissue engineering strategies aiming to modulate this 3D environment, but also as vehicles for cell-free applications, given the ease in tuning their chemical and physical properties. In this review, geometry and topography are highlighted as two preponderant features in actively regulating interactions between cells and the extracellular matrix. While most studies focus on the 2D environment, we focus on how the incorporation of these strategies in 3D systems could be beneficial. The techniques applied to design 3D microcarriers with unique geometries and surface topographical cues are covered, as well as specific tissue engineering approaches employing these microcarriers. In fact, successfully achieving a functional histoarchitecture may depend on a combination of fine-tuned geometrically shaped microcarriers presenting intricately tailored topographical cues. Lastly, we pinpoint microcarrier geometry as a key player in cell-free biomaterial-based strategies, and its impact on drug release kinetics, the production of steerable microcarriers to target tumour cells, and as protein or antibody biosensors.

Core-Shell Microcapsules: Biofabrication and Potential Applications in Tissue Engineering and Regenerative Medicine

The construction of biomaterial scaffolds that accurately recreate the architecture of living tissues in vitro is a major challenge in the field of tissue engineering and regenerative medicine. Core-shell microcapsules...

An Immunomodulatory Miniaturized 3D Screening Platform Using Liquefied Capsules

A critical determinant of successful clinical outcomes is the host's response to the biomaterial. Therefore, the prediction of the immunomodulatory bioperformance of biomedical devices following implantation is of utmost importance. Herein, liquefied capsules are proposed as immunomodulatory miniaturized 3D platforms for the high-content combinatorial screening of different polymers that could be used generically in scaffolds. Additionally, the confined and liquefied core of capsules affords a cell-mediated 3D assembly with bioinstructive microplatforms, allowing to study the potential synergistic effect that cells in tissue engineering therapies have on the immunological environment before implantation. As a proof-of-concept, three different polyelectrolytes, ranging in charge density and source, are used. Poly(L-lysine)-, alginate-, and chitosan-ending capsules with or without encapsulated mesenchymal stem/stromal cells (MSCs) are placed on top of a 2D culture of macrophages. Results show that chitosan-ending capsules, as well as the presence of MSCs, favor the balance of macrophage polarization toward a more regenerative profile, through the up-regulation of anti-inflammatory markers, and the release of pro-regenerative cytokines. Overall, the developed system enables the study of the immunomodulatory bioperformance of several polymers in a cost-effective and scalable fashion, while the paracrine signaling between encapsulated cells and the immunological environment can be simultaneously evaluated.

Minimalist Tissue Engineering Approaches Using Low Material-Based Bioengineered Systems

From an "over-engineering" era in which biomaterials played a central role, now it is observed to the emergence of "developmental" tissue engineering (TE) strategies which rely on an integrative cell-material perspective that paves the way for cell self-organization. The current challenge is to engineer the microenvironment without hampering the spontaneous collective arrangement ability of cells, while simultaneously providing biochemical, geometrical, and biophysical cues that positively influence tissue healing. These efforts have resulted in the development of low-material based TE strategies focused on minimizing the amount of biomaterial provided to the living key players of the regenerative process. Through a "minimalist-engineering" approach, the main idea is to fine-tune the spatial balance occupied by the inanimate region of the regenerative niche toward maximum actuation of the key living components during the healing process.

Potential and recent advances of microcarriers in repairing cartilage defects

Articular cartilage regeneration is one of the challenges faced by orthopedic surgeons. Microcarrier applications have made great advances in cartilage tissue engineering in recent years and enable cost-effective cell expansion, thus providing permissive microenvironments for cells. In addition, microcarriers can be loaded with proteins, factors, and drugs for cartilage regeneration. Some microcarriers also have the advantages of injectability and targeted delivery. The application of microcarriers with these characteristics can overcome the limitations of traditional methods and provide additional advantages. In terms of the transformation potential, microcarriers have not only many advantages, such as providing sufficient and beneficial cells, factors, drugs, and microenvironments for cartilage regeneration, but also many application characteristics; for example, they can be injected to reduce invasiveness, transplanted after microtissue formation to increase efficiency, or combined with other stents to improve mechanical properties. Therefore, this technology has enormous potential for clinical transformation. In this review, we focus on recent advances in microcarriers for cartilage regeneration. We compare the characteristics of microcarriers with other methods for repairing cartilage defects, provide an overview of the advantages of microcarriers, discuss the potential of microcarrier systems, and present an outlook for future development.

Translational potential of this article

We reviewed the advantages and recent advances of microcarriers for cartilage regeneration. This review could give many scholars a better understanding of microcarriers, which can provide doctors with potential methods for treating patients with cartilage injure.

In Situ Cross-Linking of Artificial Basement Membranes in 3D Tissues and Their Size-Dependent Molecular Permeability

In the human body, highly organized tissues rely on the compartmentalization effect of basement membranes (BMs) that separate different types of cells. We recently reported an artificial basement membrane (A-BM) composed of type-IV collagen and laminin (Col-IV/LM), which are the main components of natural BMs, for cell compartmentalization in three-dimensional (3D) tissues. However, such compartmentalized structures can be maintained only for 3 days, probably due to the degradation issues. In this study, a robust A-BM was fabricated by in situ cross-linking the Col-IV/LM layer-by-layer (LbL) nanofilms in 3D tissues by transglutaminase. The effects of molecular size and configuration on the permeability of obtained A-BMs were comprehensively studied using polystyrene nanoparticles (PS NPs) and dextran with various hydrodynamic diameters, as well as albumin. The findings agreed well with the known size-selective behavior of the glomerular basement membrane. Cross-linked Col-IV/LM nanofilms demonstrate improved stability and a more powerful barrier effect to maintain cell compartmentalization for organized 3D tissues. This in vitro A-BM exhibit great potentials for the design of more complex compartmentalized 3D tissues, for understanding the unique cell-cell cross talk through BMs, and for providing a more reliable 3D tissue model for new drug screening and other in vitro physiological studies.

Polyelectrolyte Multilayer Capsule (PEMC)-Based Scaffolds for Tissue Engineering

Tissue engineering (TE) is a highly multidisciplinary field that focuses on novel regenerative treatments and seeks to tackle problems relating to tissue growth both in vitro and in vivo. These issues currently involve the replacement and regeneration of defective tissues, as well as drug testing and other related bioapplications. The key approach in TE is to employ artificial structures (scaffolds) to support tissue development; these constructs should be capable of hosting, protecting and releasing bioactives that guide cellular behaviour. A straightforward approach to integrating bioactives into the scaffolds is discussed utilising polyelectrolyte multilayer capsules (PEMCs). Herein, this review illustrates the recent progress in the use of CaCO3 vaterite-templated PEMCs for the fabrication of functional scaffolds for TE applications, including bone TE as one of the main targets of PEMCs. Approaches for PEMC integration into scaffolds is addressed, taking into account the formulation, advantages, and disadvantages of such PEMCs, together with future perspectives of such architectures.

Stimuli-Responsive Delivery of Growth Factors for Tissue Engineering

Growth factors (GFs) play a crucial role in directing stem cell behavior and transmitting information between different cell populations for tissue regeneration. However, their utility as therapeutics is limited by their short half-life within the physiological microenvironment and significant side effects caused by off-target effects or improper dosage. "Smart" materials that can not only sustain therapeutic delivery over a treatment period but also facilitate on-demand release upon activation are attracting significant interest in the field of GF delivery for tissue engineering. Three properties are essential in engineering these "smart" materials: 1) the cargo vehicle protects the encapsulated therapeutic; 2) release is targeted to the site of injury; 3) cargo release can be modulated by disease-specific stimuli. The aim of this review is to summarize the current research on stimuli-responsive materials as intelligent vehicles for controlled GF delivery; Five main subfields of tissue engineering are discussed: skin, bone and cartilage, muscle, blood vessel, and nerve. Challenges in achieving such "smart" materials and perspectives on future applications of stimuli-responsive GF delivery for tissue regeneration are also discussed.

Advanced Bottom-Up Engineering of Living Architectures

Bottom-up tissue engineering is a promising approach for designing modular biomimetic structures that aim to recapitulate the intricate hierarchy and biofunctionality of native human tissues. In recent years, this field has seen exciting progress driven by an increasing knowledge of biological systems and their rational deconstruction into key core components. Relevant advances in the bottom-up assembly of unitary living blocks toward the creation of higher order bioarchitectures based on multicellular-rich structures or multicomponent cell-biomaterial synergies are described. An up-to-date critical overview of long-term existing and rapidly emerging technologies for integrative bottom-up tissue engineering is provided, including discussion of their practical challenges and required advances. It is envisioned that a combination of cell-biomaterial constructs with bioadaptable features and biospecific 3D designs will contribute to the development of more robust and functional humanized tissues for therapies and disease models, as well as tools for fundamental biological studies.

Biphasic Hydrogels Integrating Mineralized and Anisotropic Features for Interfacial Tissue Engineering

The innate graded structural and compositional profile of musculoskeletal tissue interfaces is disrupted and replaced by fibrotic tissue in the context of disease and degeneration. Tissue engineering strategies focused on the restoration of the transitional complexity found in those junctions present special relevance for regenerative medicine. Herein, we developed a gelatin-based multiphasic hydrogel system, where sections with distinct composition and microstructure were integrated in a single unit. In each phase, hydroxyapatite particles or cellulose nanocrystals (CNC) were incorporated into an enzymatically cross-linked gelatin network to mimic bone or tendon tissue, respectively. Stiffer hydrogels were produced with the incorporation of mineralized particles, and magnetic alignment of CNC resulted in anisotropic structure formation. The evaluation of the biological commitment with human adipose-derived stem cells toward the tendon-to-bone interface revealed an aligned cell growth and higher synthesis and deposition of tenascin in the anisotropic phase, while the activity of the secreted alkaline phosphatase and the expression of osteopontin were induced in the mineralized phase. These results highlight the potential versatility offered by gelatin-transglutaminase enzyme tandem for the development of strategies that mimic the graded, composite, and complex intersections of the connective tissues.

Liquefied Microcapsules as Dual-Microcarriers for 3D+3D Bottom-Up Tissue Engineering

Cell encapsulation systems must ensure the diffusion of molecules to avoid the formation of necrotic cores. The architectural design of hydrogels, the gold standard tissue engineering strategy, is thus limited to a microsize range. To overcome such a limitation, liquefied microcapsules encapsulating cells and microparticles are proposed. Microcapsules with controlled sizes with average diameters of 608.5 ± 122.3 µm are produced at high rates by electrohydrodynamic atomization, and arginyl-glycyl-aspartic acid (RGD) domains are introduced in the multilayered membrane. While cells and microparticles interact toward the production of confined microaggregates, on the outside cell-mediated macroaggregates are formed due to the aggregation of microcapsules. The concept of simultaneous aggregation is herein termed as 3D+3D bottom-up tissue engineering. Microcapsules are cultured alone (microcapsule¹ ) or on top of 2D cell beds composed of human umbilical vein endothelial cells (HUVECs) alone (microcapsule² ) or cocultured with fibroblasts (microcapsule³ ). Microcapsules are able to support cell encapsulation shown by LiveDead, 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulphofenyl)-2H-tetrazolium (MTS), and dsDNA assays. Only microcapsule³ are able to form macroaggregates, as shown by F-actin immunofluorescence. The bioactive 3D system also presented alkaline phosphatase activity, thus allowing osteogenic differentiation. Upon implantation using the chick chorioallontoic membrane (CAM) model, microcapsules recruit a similar number of vessels with alike geometric parameters in comparison with CAMs supplemented with basic fibroblast growth factor (bFGF).

Cell encapsulation in liquified compartments: Protocol optimization and challenges

Cell encapsulation is a widely used technique in the field of Tissue Engineering and Regenerative Medicine (TERM). However, for the particular case of liquefied compartmentalised systems, only a limited number of studies have been reported in the literature. We have been exploring a unique cell encapsulation system composed by liquefied and multilayered capsules. This system transfigured the concept of 3D scaffolds for TERM, and was already successfully applied for bone and cartilage regeneration. Due to a number of appealing features, we envisage that it can be applied in many other fields, including in advanced therapies or as disease models for drug discovery. In this review, we intend to highlight the advantages of this new system, while discussing the methodology, and sharing the protocol optimization and results. The different liquefied systems for cell encapsulation reported in the literature will be also discussed, considering the different encapsulation matrixes as core templates, the types of membranes, and the core liquefaction treatments.

Microparticles in Contact with Cells: From Carriers to Multifunctional Tissue Modulators

For several decades microparticles have been exclusively and extensively explored as spherical drug delivery vehicles and large-scale cell expansion carriers. More recently, microparticulate structures gained interest in broader bioengineering fields, integrating myriad strategies that include bottom-up tissue engineering, 3D bioprinting, and the development of tissue/disease models. The concept of bulk spherical micrometric particles as adequate supports for cell cultivation has been challenged, and systems with finely tuned geometric designs and (bio)chemical/physical features are current key players in impacting technologies. Herein, we critically review the state of the art and future trends of biomaterial microparticles in contact with cells and tissues, excluding internalization studies, and with emphasis on innovative particle design and applications.

Biomedical Applications of Layer-by-Layer Self-Assembly for Cell Encapsulation: Current Status and Future Perspectives

Encapsulating living cells within multilayer functional shells is a crucial extension of cellular functions and a further development of cell surface engineering. In the last decade, cell encapsulation has been widely utilized in many cutting-edge biomedical fields. Compared with other techniques for cell encapsulation, layer-by-layer (LbL) self-assembly technology, due to the versatility and tunability to fabricate diverse multilayer shells with controllable compositions and structures, is considered as a promising approach for cell encapsulation. This review summarizes the state-of-the-art and potential future biomedical applications of LbL cell encapsulation. First of all, a brief introduction to the LbL self-assembly technique, including assembly mechanisms and technologies, is made. Next, different cell encapsulation strategies by LbL self-assembly techniques are explained. Then, the biomedical applications of LbL cell encapsulation in cell-based biosensors, cell transplantation, cell/molecule delivery, and tissue engineering, are highlighted. Finally, discussions on the current limitations and future perspectives of LbL cell encapsulation are also provided.

Design Principles and Multifunctionality in Cell Encapsulation Systems for Tissue Regeneration

Cell encapsulation systems are being increasingly applied as multifunctional strategies to regenerate tissues. Lessons afforded with encapsulation systems aiming to treat endocrine diseases seem to be highly valuable for the tissue engineering and regenerative medicine (TERM) systems of today, in which tissue regeneration and biomaterial integration are key components. Innumerous multifunctional systems for cell compartmentalization are being proposed to meet the specific needs required in the TERM field. Herein is reviewed the variable geometries proposed to produce cell encapsulation strategies toward tissue regeneration, including spherical and fiber-shaped systems, and other complex shapes and arrangements that better mimic the highly hierarchical organization of native tissues. The application of such principles in the TERM field brings new possibilities for the development of highly complex systems, which holds tremendous promise for tissue regeneration. The complex systems aim to recreate adequate environmental signals found in native tissue (in particular during the regenerative process) to control the cellular outcome, and conferring multifunctional properties, namely the incorporation of bioactive molecules and the ability to create smart and adaptative systems in response to different stimuli. The new multifunctional properties of such systems that are being employed to fulfill the requirements of the TERM field are also discussed.

The Potential of Liquid Marbles for Biomedical Applications: A Critical Review

Liquid marbles (LM) are freestanding droplets covered by micro/nanoparticles with hydrophobic/hydrophilic properties, which can be manipulated as a soft solid. The phenomenon that generates these soft structures is regarded as a different method to generate a superhydrophobic behavior in the liquid/solid interface without modifying the surface. Several applications for the LM have been reported in very different fields, however the developments for biomedical applications are very recent. At first, the LM properties are reviewed, namely shell structure, LM shape, evaporation, floatability and robustness. The different strategies for LM manipulation are also described, which make use of magnetic, electrostatic and gravitational forces, ultraviolet and infrared radiation, and approaches that induce LM self-propulsion. Then, very distinctive applications for LM in the biomedical field are presented, namely for diagnostic assays, cell culture, drug screening and cryopreservation of mammalian cells. Finally, a critical outlook about the unexplored potential of LM for biomedical applications is presented, suggesting possible advances on this emergent scientific area.

In vivo osteogenic differentiation of stem cells inside compartmentalized capsules loaded with co-cultured endothelial cells

Capsules coated with polyelectrolytes and co-encapsulating adipose stem (ASCs) and endothelial (ECs) cells with surface modified microparticles are developed. Microparticles and cells are freely dispersed in a liquified core, responsible to maximize the diffusion of essential molecules and allowing the geometrical freedom for the autonomous three-dimensional (3D) organization of cells. While the membrane wraps all the instructive cargo elements within a single structure, the microparticles provide a solid 3D substrate for the encapsulated cells. Our hypothesis is that inside this isolated biomimetic 3D environment, ECs would lead ASCs to differentiate into the osteogenic lineage to ultimately generate a mineralized tissue in vivo. For that, capsules encapsulating only ASCs (MONO capsules) or co-cultured with ECs (CO capsules) are subcutaneously implanted in nude mice up to 6weeks. Capsules implanted immediately after production or after 21days of in vitro osteogenic stimulation are tested. The most valuable outcome of the present study is the mineralized tissue in CO capsules without in vitro pre-differentiation, with similar levels compared to the pre-stimulated capsules in vitro. We believe that the proposed bioencapsulation strategy is a potent self-regulated system, which might find great applicability in bone tissue engineering.

STATEMENT OF SIGNIFICANCE

The diffusion efficiency of essential molecules for cell survival is a main issue in cell encapsulation. Former studies reported the superior biological outcome of encapsulated cells within liquified systems. However, most cells used in TE are anchorage-dependent, requiring a solid substrate to perform main cellular processes. We hypothesized that liquified capsules encapsulating microparticles are a promising attempt. Inspired by the multiphenotypic cellular environment of bone, we combine the concept of liquified capsules with co-cultures of stem and endothelial cells. After implantation, results show that co-cultured capsules without in vitro stimulation were able to form a mineralized tissue in vivo. We believe that the present ready-to-use TE strategy requiring minimum in vitro manipulation will find great applicability in bone tissue engineering.

Engineering Cell Surfaces with Polyelectrolyte Materials for Translational Applications

Engineering cell surfaces with natural or synthetic materials is a unique and powerful strategy for biomedical applications. Cells exhibit more sophisticated migration, control, and functional capabilities compared to nanoparticles, scaffolds, viruses, and other engineered materials or agents commonly used in the biomedical field. Over the past decade, modification of cell surfaces with natural or synthetic materials has been studied to exploit this complexity for both fundamental and translational goals. In this review we present the existing biomedical technologies for engineering cell surfaces with one important class of materials, polyelectrolytes. We begin by introducing the challenges facing the cell surface engineering field. We then discuss the features of polyelectrolytes and how these properties can be harnessed to solve challenges in cell therapy, tissue engineering, cell-based drug delivery, sensing and tracking, and immune modulation. Throughout the review, we highlight opportunities to drive the field forward by bridging new knowledge of polyelectrolytes with existing translational challenges.