Bioactive Microsphere-Based Scaffolds Containing Decellularized Cartilage

Development of bioactive solid-foam scaffolds from decellularized cartilage with chondrogenic and osteogenic properties

Full osteochondral regeneration remains a major clinical challenge. Among other experimental cartilage regenerative approaches, decellularized cartilage (DCC) is considered a promising material for generating potentially implantable scaffolds useful as cartilage repair strategy. In this work, we focus on screening and comparing different decellularization methods, aiming to generate DCC potentially useful in biomedical context, and therefore, with biological activity and functional properties in terms of induction of differentiation and regeneration. Data indicates that enzymatic and detergents-based decellularization methods differentially affect ECM components, and that it has consequences in further biological behavior. SDS-treated DCC powder is not useful to be further processed in 2D or 3D structures, because these structures tend to rapidly solubilize, or disaggregate, in physiologic media conditions. Conversely, Trypsin-treated DCC powders can be processed to mechanically stable 2D films and 3D solid-foam scaffolds, presumably due to partial digestion of collagens during decellularization, which would ease crosslinking at DCC during solubilization and processing. In vitro cell culture studies indicate that these structures are biocompatible and induce and potentiate chondrogenic differentiation. In vivo implantation of DCC derived 3D porous scaffolds in rabbit osteochondral defects induce subchondral bone regeneration and fibrocartilage tissue formation after implantation. Therefore, this work defines an optimal cartilage tissue decellularization protocol able to generate DCC powders processable to biocompatible and bioactive 2D and 3D structures. These structures are useful for in vitro cartilage research and in vivo subchondral bone regeneration, while hyaline cartilage regeneration with DCC alone as implantable material remains elusive.

Chondrogenesis of mesenchymal stromal cells on the 3D printed polycaprolactone/fibrin/decellular cartilage matrix hybrid scaffolds in the presence of piascledine

Nowadays, cartilage tissue engineering (CTE) is considered important due to lack of repair of cartilaginous lesions and the absence of appropriate methods for treatment. In this study, polycaprolactone (PCL) scaffolds were fabricated by three-dimensional (3D) printing and were then coated with fibrin (F) and acellular solubilized extracellular matrix (ECM). After extracting adipose-derived stem cells (ADSCs), 3D-printed scaffolds were characterized and compared to hydrogel groups. After inducing the chondrogenic differentiation in the presence of Piascledine and comparing it with TGF-β3 for 28 days, the expression of genes involved in chondrogenesis (AGG, COLII) and the expression of the hypertrophic gene (COLX) were examined by real-time PCR. The expression of proteins COLII and COLX was also determined by immunohistochemistry. Glycosaminoglycan was measured by toluidine blue staining. 3D-printed scaffolds clearly improved cell proliferation, viability, water absorption and compressive strength compared to the hydrogel groups. Moreover, the use of compounds such as ECM and Piascledine in the process of ADSCs chondrogenesis induction increased cartilage-specific markers and decreased the hypertrophic marker compared to TGF-β3. In Piascledine groups, the expression of COLL II protein, COLL II and Aggrecan genes, and the amount of glycosaminoglycan showed a significant increase in the PCL/F/ECM compared to the PCL and PCL/F groups.

Decellularization of cartilage microparticles: Effects of temperature, supercritical carbon dioxide and ultrasound on biochemical, mechanical, and biological properties

One of the approaches to restoring the structure of damaged cartilage tissue is an intra-articular injection of tissue-engineered medical products (TEMPs) consisting of biocompatible matrices loaded with cells. The most interesting are the absorbable matrices from decellularized tissues, provided that the cellular material is completely removed from them with the maximum possible preservation of the structure and composition of the natural extracellular matrix. The present study investigated the mechanical, biochemical, and biological properties of decellularized porcine cartilage microparticles (DCMps) obtained by techniques, differing only in physical treatments, such as freeze-thaw cycling (Protocol 1), supercritical carbon dioxide fluid (Protocol 2) and ultrasound (Protocol 3). Full tissue decellularization was achieved, as confirmed by the histological analysis and DNA quantification, though all the resultant DCMps had reduced glycosaminoglycans (GAGs) and collagen. The elastic modulus of all DCMp samples was also significantly reduced. Most notably, DCMps prepared with Protocol 3 significantly outperformed other samples in viability and the chondroinduction of the human adipose-derived stem cells (hADSCs), with a higher GAG production per DNA content. A positive ECM staining for type II collagen was also detected only in cartilage-like structures based on ultrasound-treated DCMps. The biocompatibility of a xenogenic DCMps obtained with Protocol 3 has been confirmed for a 6-month implantation in the thigh muscle tissue of mature rats (n = 18). Overall, the results showed that the porcine cartilage microparticles decellularized by a combination of detergents, ultrasound and DNase could be a promising source of scaffolds for TEMPs for cartilage reconstruction.

Calcium Phosphate Silicate Microspheres with Soybean Lecithin as a Sustained-Release Bone Morphogenetic Protein-Delivery System for Bone Tissue Regeneration

Bone morphogenetic protein (BMP) is a growth factor that effectively promotes osteogenesis. Microsphere-based drug-delivery systems can facilitate an increase in the local concentration of BMP, thus promoting bone formation. In this study, calcium phosphate silicate (CPS) microspheres were used as drug-loading systems for BMP. Three groups─CPS, CPS + BMP, and CPS + BMP + soy lecithin (SL)─were set up, where SL was used to prolong the osteogenic effect of the microsphere system. Bone marrow mesenchymal stem cells and femoral defects in rats were used to compare the osteogenic ability of the three groups. The results indicated that CPS microspheres were good carriers of BMP, facilitating a smoother release into the cells and tissues. SL loading improved the loading rate of BMP, which promoted the osteogenic effect of the microspheres with BMP. We propose CPS microspheres as potential drug-delivery systems that can be effectively used in the treatment of bone defects.

Chondrocytes In Vitro Systems Allowing Study of OA

Osteoarthritis (OA) is an extremely complex disease, as it combines both biological-chemical and mechanical aspects, and it also involves the entire joint consisting of various types of tissues, including cartilage and bone. This paper describes the methods of conducting cell cultures aimed at searching for the mechanical causes of OA development, therapeutic solutions, and methods of preventing the disease. It presents the systems for the cultivation of cartilage cells depending on the level of their structural complexity, and taking into account the most common solutions aimed at recreating the most important factors contributing to the development of OA, that is mechanical loads. In-vitro systems used in tissue engineering to investigate the phenomena associated with OA were specified depending on the complexity and purposefulness of conducting cell cultures.

The Role of Growth Factors in Bioactive Coatings

With increasing obesity and an ageing population, health complications are also on the rise, such as the need to replace a joint with an artificial one. In both humans and animals, the integration of the implant is crucial, and bioactive coatings play an important role in bone tissue engineering. Since bone tissue engineering is about designing an implant that maximally mimics natural bone and is accepted by the tissue, the search for optimal materials and therapeutic agents and their concentrations is increasing. The incorporation of growth factors (GFs) in a bioactive coating represents a novel approach in bone tissue engineering, in which osteoinduction is enhanced in order to create the optimal conditions for the bone healing process, which crucially affects implant fixation. For the application of GFs in coatings and their implementation in clinical practice, factors such as the choice of one or more GFs, their concentration, the coating material, the method of incorporation, and the implant material must be considered to achieve the desired controlled release. Therefore, the avoidance of revision surgery also depends on the success of the design of the most appropriate bioactive coating. This overview considers the integration of the most common GFs that have been investigated in in vitro and in vivo studies, as well as in human clinical trials, with the aim of applying them in bioactive coatings. An overview of the main therapeutic agents that can stimulate cells to express the GFs necessary for bone tissue development is also provided. The main objective is to present the advantages and disadvantages of the GFs that have shown promise for inclusion in bioactive coatings according to the results of numerous studies.

Application of bone and cartilage extracellular matrices in articular cartilage regeneration

Articular cartilage has an avascular structure with a poor ability for self-repair; therefore, many challenges arise in cases of trauma or disease. It is of utmost importance to identify the proper biomaterial for tissue repair that has the capability to direct cell recruitment, proliferation, differentiation, and tissue integration by imitating the natural microenvironment of cells and transmitting an orchestra of intracellular signals. Cartilage extracellular matrix (cECM) is a complex nanostructure composed of divergent proteins and glycosaminoglycans (GAGs), which regulate many functions of resident cells. Numerous studies have shown the remarkable capacity of ECM-derived biomaterials for tissue repair and regeneration. Moreover, given the importance of biodegradability, biocompatibility, 3D structure, porosity, and mechanical stability in the design of suitable scaffolds for cartilage tissue engineering, demineralized bone matrix (DBM) appears to be a promising biomaterial for this purpose, as it possesses the aforementioned characteristics inherently. To the best of the authors' knowledge, no comprehensive review study on the use of DBM in cartilage tissue engineering has previously been published. Since so much work is needed to address DBM limitations such as pore size, cell retention, and so on, we decided to draw the attention of researchers in this field by compiling a list of recent publications. This review discusses the implementation of composite scaffolds of natural or synthetic origin functionalized with cECM or DBM in cartilage tissue engineering. Cutting-edge advances and limitations are also discussed in an attempt to provide guidance to researchers and clinicians.

Development of novel apoptosis-assisted lung tissue decellularization methods

Decellularized tissues hold great potential for both regenerative medicine and disease modeling applications. The acellular extracellular matrix (ECM)-enriched scaffolds can be recellularized with patient-derived cells prior to transplantation, or digested to create thermally-gelling ECM hydrogels for 3D cell culture. Current methods of decellularization clear cellular components using detergents, which can result in loss of ECM proteins and tissue architectural integrity. Recently, an alternative approach utilizing apoptosis to decellularize excised murine sciatic nerves resulted in superior ECM preservation, cell removal, and immune tolerance in vivo. However, this apoptosis-assisted decellularization approach has not been optimized for other tissues with a more complex geometry, such as lungs. To this end, we developed an apoptosis-assisted lung tissue decellularization method using a combination of camptothecin and sulfobetaine-10 (SB-10) to induce apoptosis and facilitate gentle and effective removal of cell debris, respectively. Importantly, combination of the two agents resulted in superior cell removal and ECM preservation compared to either of the treatments alone, presumably because of pulmonary surfactants. In addition, our method was superior in cell removal compared to a previously established detergent-based decellularization protocol. Furthermore, thermally-gelling lung ECM hydrogels supported high viability of rat lung epithelial cells for up to 2 weeks in culture. This work demonstrates that apoptosis-based lung tissue decellularization is a superior technique that warrants further utilization for both regenerative medicine and disease modeling purposes.

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.

Development and Biocompatibility Analysis of NBD Peptide Sustained- Release Microsphere Scaffold Nanoparticle SP-Sr-CaS/NBD

BACKGROUND

In clinical treatment, it is difficult to carry out effective bone tissue transplantation and anti-inflammatory treatment at the same time due to bone defects and osteomyelitis where the tissue is contaminated or infected. As a downstream target of TNF-α, NF-κB has an inhibition effect on the proliferation and differentiation of cells surrounding the lesion. As a negative effect, it leads to a reduction in bone growth and development.

METHODS

In this study, the small molecule NBD polypeptide and bone conduction matrix Sr-CaS are microspheres, formed to prepare Sr-CaS, NBD drug-loaded sustained-release microspheres in order to achieve a Sr-CaS/NBD peptide drug-loaded sustained release microsphere scaffold material (SP-Sr-CaS/NBD). We prepared the microspheres and optimized the production process to obtain particles with stable morphological properties and sustained release properties.

RESULT

In vitro experiments demonstrated that SP-Sr-CaS/NBD could reduce TNF-α-induced cell growth inhibition, caspase-3 activity and NF-κB transcriptional activation as the function of continuous NBD peptide dosing regimen.

CONCLUSION

Also, the introduction of the Sr-CaS matrix potentiates microspheres to promote cell proliferation and provides a basis to become a promising 3D bone scaffold material in the future.

Tissue-Specific Decellularization Methods: Rationale and Strategies to Achieve Regenerative Compounds

The extracellular matrix (ECM) is a complex network with multiple functions, including specific functions during tissue regeneration. Precisely, the properties of the ECM have been thoroughly used in tissue engineering and regenerative medicine research, aiming to restore the function of damaged or dysfunctional tissues. Tissue decellularization is gaining momentum as a technique to obtain potentially implantable decellularized extracellular matrix (dECM) with well-preserved key components. Interestingly, the tissue-specific dECM is becoming a feasible option to carry out regenerative medicine research, with multiple advantages compared to other approaches. This review provides an overview of the most common methods used to obtain the dECM and summarizes the strategies adopted to decellularize specific tissues, aiming to provide a helpful guide for future research development.

Decellularized tissues as platforms for in vitro modeling of healthy and diseased tissues

Biomedical engineers are at the forefront of developing novel treatments to improve human health, however, many products fail to translate to clinical implementation. In vivo pre-clinical animal models, although the current best approximation of complex disease conditions, are limited by reproducibility, ethical concerns, and poor accurate prediction of human response. Hence, there is a need to develop physiologically relevant, low cost, scalable, and reproducible in vitro platforms to provide reliable means for testing drugs, biomaterials, and tissue engineered products for successful clinical translation. One emerging approach of developing physiologically relevant in vitro models utilizes decellularized tissues/organs as biomaterial platforms for 2D and 3D models of healthy and diseased tissue. Decellularization is a process that removes cellular content and produces tissue-specific extracellular matrix scaffolds that can more accurately recapitulate an organ/tissue's native microenvironment compared to other natural or synthetic materials. Decellularized tissues hold enormous potential for in vitro modeling of various disease phenotypes and tissue responses to drugs or external conditions such as aging, toxin exposure, or even implantation. In this review, we highlight the need for in vitro models, the advantages and limitations of implementing decellularized tissues, and considerations of the decellularization process. We discuss current research efforts towards applying decellularized tissues as platforms to generate in vitro models of healthy and diseased tissues, and where we foresee the field progressing. A variety of organs/tissues are discussed, including brain, heart, kidney, large intestine, liver, lung, skeletal muscle, skin, and tongue. STATEMENT OF SIGNIFICANCE: Many biomedical products fail to reach clinical translation due to animal model limitations. Development of physiologically relevant in vitro models can provide a more economic, scalable, and reproducible means of testing drugs/therapeutics for successful clinical translation. The use of decellularized tissues as platforms for in vitro models holds promise, as these scaffolds can effectively replicate native tissue complexity, but is not widely explored. This review discusses the need for in vitro models, the promise of decellularized tissues as biomaterial substrates, and the current research applying decellularized tissues towards the creation of in vitro models. Further, this review provides insights into the current limitations and future of such in vitro models.

Intra-articular injection of decellularized extracellular matrices in the treatment of osteoarthritis in rabbits

Background

We investigated the role of decellularized cartilage matrix in osteoarthritis to seek a new treatment for this disease.

Methods

Knee cartilage from rabbits was decellularized and the degree of decellularization was assessed. A grinder was used to turn acellular cartilage into particles, which were then used in a suspension. Thirty New Zealand white rabbits were subjected to an operation on their anterior cruciate ligament for the osteoarthritis model. The success of the animal model of osteoarthritis was evaluated using results from six rabbits. The remaining 24 rabbits were randomly divided into four groups (groups A, B, C, and D). Rabbits in groups A, B, C, and D were injected with 200 µl of normal saline, 200 µl of 10% (w/v) cartilage decellularized suspension, 200 µl of 20% (w/v) cartilage decellularized suspension, and 200 µl of 40% (w/v) cartilage decellularized suspension into the knee joints, respectively. Macroscopic and microscopic assessments were performed three months after surgery to assess the degree of osteoarthritic changes.

Results

Histological and biochemical analysis revealed that the cartilage decellularized matrix removed cells after decellularization but retained components of collagen and glycosaminoglycan. Group A exhibited the most significant changes from osteophyte and cartilage erosion, which was macroscopically observable on the surface of the femoral cartilage. HE staining in group A revealed damage to the cartilage surface, disorganized chondrocytes, and spontaneous fibrocartilage formation. Safranin O-fast green staining revealed a cavity formed at the osteochondral junction in group A that did not appear in other groups.

Conclusion

Our study shows that decellularized cartilage matrix has a certain therapeutic effect on osteoarthritis and provides new insights in the treatment of osteoarthritis.

Hybrid and Composite Scaffolds Based on Extracellular Matrices for Cartilage Tissue Engineering

IMPACT STATEMENT

Scaffolds fabricated from extracellular matrix (ECM) derivatives are composed of conducive structures for cell attachment, proliferation, and differentiation, but generally do not have proper mechanical properties and load-bearing capacity. In contrast, scaffolds based on synthetic biomaterials demonstrate appropriate mechanical strength, but the absence of desirable biological properties is one of their main disadvantages. To integrate mechanical strength and biological cues, these ECM derivatives can be conjugated with synthetic biomaterials. Hence, hybrid scaffolds comprising both advantages of synthetic polymers and ECM derivatives can be considered a robust vehicle for tissue engineering applications.

Decellularized cartilage as a prospective scaffold for cartilage repair

Articular cartilage lacks self-healing capacity, and there is no effective therapy facilitating cartilage repair. Osteoarthritis (OA) due to cartilage defects represents large and increasing healthcare burdens worldwide. Nowadays, the generation of scaffolds to preserve bioactive factors and the biophysical environment has received increasing attention. Furthermore, improved decellularization technology has provided novel insights into OA treatment. This review provides a comparative account of different cartilage defect therapies. Furthermore, some recent effective decellularization protocols have been discussed. In particular, this review focuses on the decellularization ratio of each protocol. Moreover, these protocols were compared particularly on the basis of immunogenicity and mechanical functionality. Further, various recellularization methods have been enlisted and the reparative capacity of decellularized cartilage scaffolds is evaluated herein. The advantages and limitations of different recellularization processes have been described herein. This provides a basis for the generation of decellularized cartilage scaffolds, thereby potentially promoting the possibility of decellularization as a clinical therapeutic target.

Autologous liquid platelet rich fibrin: A novel drug delivery system

There is currently widespread interest within the biomaterial field to locally deliver biomolecules for bone and cartilage regeneration. Substantial work to date has focused on the potential role of these biomolecules during the healing process, and the carrier system utilized is a key factor in their effectiveness. Platelet rich fibrin (PRF) is a naturally derived fibrin scaffold that is easily obtained from peripheral blood following centrifugation. Slower centrifugation speeds have led to the commercialization of a liquid formulation (liquid-PRF) resulting in an upper plasma layer composed of liquid fibrinogen/thrombin prior to clot formation that remains in its liquid phase for approximately 15 min until injected into bodily tissues. Herein, we introduce the use of liquid PRF as an advanced local delivery system for small and large biomolecules. Potential target molecules including large (growth factors/cytokines and morphogenetic/angiogenic factors), as well as small (antibiotics, peptides, gene therapy and anti-osteoporotic) molecules are considered potential candidates for enhanced bone/cartilage tissue regeneration. Furthermore, liquid-PRF is introduced as a potential carrier system for various cell types and nano-sized particles that are capable of limiting/by-passing the immune system and minimizing potential foreign body reactions within host tissues following injection.

STATEMENT OF SIGNIFICANCE

There is currently widespread interest within the biomaterial field to locally deliver biomolecules for bone and cartilage regeneration. This review article focuses on the use of a liquid version of platelet rich fibrin (PRF) composed of liquid fibrinogen/thrombin as a drug delivery system. Herein, we introduce the use of liquid PRF as an advanced local delivery system for small and large biomolecules including growth factors, cytokines and morphogenetic/angiogenic factors, as well as antibiotics, peptides, gene therapy and anti-osteoporotic molecules as potential candidates for enhanced bone/cartilage tissue regeneration.

Functionality of decellularized matrix in cartilage regeneration: A comparison of tissue versus cell sources

Increasing evidence indicates that decellularized extracellular matrices (dECMs) derived from cartilage tissues (T-dECMs) or chondrocytes/stem cells (C-dECMs) can support proliferation and chondrogenic differentiation of cartilage-forming cells. However, few review papers compare the differences between these dECMs when they serve as substrates for cartilage regeneration. In this review, after an introduction of cartilage immunogenicity and decellularization methods to prepare T-dECMs and C-dECMs, a comprehensive comparison focuses on the effects of T-dECMs and C-dECMs on proliferation and chondrogenic differentiation of chondrocytes/stem cells in vitro and in vivo. Key factors within dECMs, consisting of microarchitecture characteristics and micromechanical properties as well as retained insoluble and soluble matrix components, are discussed in-depth for potential mechanisms underlying the functionality of these dECMs in regulating chondrogenesis. With this information, we hope to benefit dECM based cartilage engineering and tissue regeneration for future clinical application.

STATEMENT OF SIGNIFICANCE

The use of decellularized extracellular matrix (dECM) is becoming a promising approach for tissue engineering and regeneration. Compared to dECM derived from cartilage tissue, recently reported dECM from cell sources exhibits a distinct role in cell based cartilage regeneration. In this review paper, for the first time, tissue and cell based dECMs are comprehensively compared for their functionality in cartilage regeneration. This information is expected to provide an update for dECM based cartilage regeneration.

Effects of tissue processing on bioactivity of cartilage matrix-based hydrogels encapsulating osteoconductive particles

In the treatment of severe traumatic brain injury (TBI), decompressive craniectomy is commonly used to remove a large portion of calvarial bone to allow unimpeded brain swelling. Hydrogels have the potential to revolutionize TBI treatment by permitting a single-surgical intervention, remaining pliable during brain swelling, and tuned to regenerate bone after swelling has subsided. With this motivation, our goal is to present a pliable material capable of regenerating calvarial bone across a critical size defect. We therefore proposed the use of a methacrylated solubilized decellularized cartilage (MeSDCC) hydrogel encapsulating synthetic osteogenic particles of hydroxyapatite nanofibers, bioglass microparticles, or added rat bone marrow-derived mesenchymal stem cells (rMSCs) for bone regeneration in critical-size rat calvarial defects. Fibrin hydrogels were employed as a control material for the study. MeSDCC hydrogels exhibited sufficient rheological performance for material placement before crosslinking ([Formula: see text] > 500 Pa), and sufficient compressive moduli post-crosslinking (E > 150 kPa). In vitro experiments suggested increased calcium deposition for cells seeded on the MeSDCC material; however, in vivo bone regeneration was minimal in both MeSDCC and fibrin groups, even with colloidal materials or added rMSCs. Minimal bone regeneration in the MeSDCC test groups may potentially be attributed to cartilage solubilization after decellularization, in which material signals may have degraded from enzymatic treatment. Looking to the future, an improvement in the bioactivity of the material will be crucial to the success of bone regeneration strategies for TBI treatment.

Microspheres containing decellularized cartilage induce chondrogenesis in vitro and remain functional after incorporation within a poly(caprolactone) filament useful for fabricating a 3D scaffold

In this study, articular cartilage was decellularized preserving a majority of the inherent proteins, cytokines, growth factors and sGAGs. The decellularized cartilage matrix (dCM) was then encapsulated in poly(lactic acid) microspheres (MS + dCM) via double emulsion. Blank microspheres without dCM, MS(-), were also produced. The microspheres were spherical in shape and protein encapsulation efficiency within MS + dCM was 63.4%. The sustained release of proteins from MS + dCM was observed over 4 weeks in vitro. Both MS + dCM and MS(-) were cytocompatible. The sustained delivery of retained growth factors and cytokines from MS + dCM promoted cell migration in contrast to MS(-). Subsequently, chondrogenesis of human mesenchymal stem cells was upregulated in presence of MS + dCM as evidenced from immunohistochemistry, biochemical quantification and qPCR studies. Specifically, collagen II, aggrecan and SOX 9 gene expression were increased in the presence of MS + dCM by an order or more in magnitude compared to MS(-) with concomitant downregulation of hypertrophic genes (COL X) despite being cultured in the absence of chondrogenic media, (p < 0.05). Lastly, microspheres containing alkaline phosphatase (MS + ALP), a surrogate to assess the thermal stability of dCM proteins, incorporated within poly(caprolactone) filaments showed that the enzyme remained functional after filament production by melt extrusion. The establishment of a novel, thermally stable process for producing filaments containing chondroinductive microspheres provides evidence supporting subsequent development of a clinically-relevant, 3D scaffold fabricated from them for osteochondral regeneration and repair.

Microsphere-Based Scaffolds in Regenerative Engineering

Microspheres have long been used in drug delivery applications because of their controlled release capabilities. They have increasingly served as the fundamental building block for fabricating scaffolds for regenerative engineering because of their ability to provide a porous network, offer high-resolution control over spatial organization, and deliver growth factors/drugs and/or nanophase materials. Because they provide physicochemical gradients via spatiotemporal release of bioactive factors and nanophase ceramics, microspheres are a desirable tool for engineering complex tissues and biological interfaces. In this review we describe various methods for microsphere fabrication and sintering, and elucidate how these methods influence both micro- and macroscopic scaffold properties, with a special focus on the nature of sintering. Furthermore, we review key applications of microsphere-based scaffolds in regenerating various tissues. We hope to inspire researchers to join a growing community of investigators using microspheres as tissue engineering scaffolds so that their full potential in regenerative engineering may be realized.

Colloidal Gels with Extracellular Matrix Particles and Growth Factors for Bone Regeneration in Critical Size Rat Calvarial Defects

Colloidal gels encapsulating natural materials and exhibiting paste-like properties for placement are promising for filling complex geometries in craniofacial bone regeneration applications. Colloidal materials have demonstrated modest clinical outcomes as bone substitutes in orthopedic applications, but limited success in craniofacial applications. As such, development of a novel colloidal gel will fill a void in commercially available products for use in craniofacial reconstruction. One likely application for this technology is cranial reconstruction. Currently, traumatic brain injury (TBI) is often treated with a hemi-craniectomy, a procedure in which half the cranium is removed to allow the injured brain to swell and herniate beyond the enclosed cranial vault. The use of colloidal gels would allow for the design of a pliable material capable of expansion during brain swelling and facilitate cranial bone regeneration alleviating the need for a second surgery to replace the previously removed hemi-cranium. In the current study, colloidal nanoparticles of hydroxyapatite (HAp), demineralized bone matrix (DBM), and decellularized cartilage (DCC) were combined with hyaluronic acid (HA) to form colloidal gels with desirable rheological properties ([Formula: see text] ≥ 100 Pa). BMP-2 and VEGF growth factors were included to assess extracellular matrix (ECM) contribution of DBM and DCC. The HA-HAp (BMP-2) and HA-HAp-DCC group had 89 and 82% higher bone regeneration compared to the sham group, respectively (p < 0.01). Material retention issues observed may be alleviated by implementing chemical crosslinking. Overall, DCC may be a promising material for bone regeneration in general, and colloidal gels may hold significant potential in craniofacial applications.

Hyaluronic-Acid-Hydroxyapatite Colloidal Gels Combined with Micronized Native ECM as Potential Bone Defect Fillers

One of the grand challenges in translational regenerative medicine is the surgical placement of biomaterials. For bone regeneration in particular, malleable and injectable colloidal gelsare frequently designed to exhibit self-assembling and shear-response behavior which facilitates biomaterial placement in tissue defects. The current study demonstrated that by combining native extracellular matrix (ECM) microparticles, i.e., demineralized bone matrix (DBM) and decellularized cartilage (DCC), with hyaluronic acid (HA) and hydroxyapatite (HAP) nanoparticles, a viscoelastic colloidal gel consisting exclusively of natural materials was achieved. Rheological testing of HA-ECM suspensions and HA-HAP-ECM colloidal gels concluded either equivalent or substantially higher storage moduli (G' ≈ 100-10 000 Pa), yield stresses (τ_y ≈ 100-1000 Pa), and viscoelastic recoveries (G'_recovery ≥ 87%) in comparison with controls formulated without ECM, which indicated a previously unexplored synergy in fluid properties between ECM microparticles and HA-HAP colloidal networks. Notable rheological differences were observed between respective DBM and DCC formulations, specifically in HA-HAP-DBM mixtures, which displayed a mean 3-fold increase in G' and a mean 4-fold increase in τ_y from corresponding DCC mixtures. An initial in vitro assessment of these potential tissue fillers as substrates for cell growth revealed that all formulations of HA-ECM and HA-HAP-ECM showed no signs of cytotoxicity and appeared to promote cell viability. Both DBM and DCC colloidal gels represent promising platforms for future studies in bone and cartilage tissue engineering. Overall, the current study identified colloidal gels constructed exclusively of natural materials, with viscoelastic properties that may facilitate surgical placement for a wide variety of therapeutic applications.

Microsphere-based scaffolds encapsulating chondroitin sulfate or decellularized cartilage

Extracellular matrix materials such as decellularized cartilage (DCC) and chondroitin sulfate (CS) may be attractive chondrogenic materials for cartilage regeneration. The goal of the current study was to investigate the effects of encapsulation of DCC and CS in homogeneous microsphere-based scaffolds, and to test the hypothesis that encapsulation of these extracellular matrix materials would induce chondrogenesis of rat bone marrow stromal cells. Four different types of homogeneous scaffolds were fabricated from microspheres of poly(D,L-lactic-co-glycolic acid): Blank (poly(D,L-lactic-co-glycolic acid) only; negative control), transforming growth factor-β3 encapsulated (positive control), DCC encapsulated, and CS encapsulated. These scaffolds were then seeded with rat bone marrow stromal cells and cultured for 6 weeks. The DCC and CS encapsulation altered the morphological features of the microspheres, resulting in higher porosities in these groups. Moreover, the mechanical properties of the scaffolds were impacted due to differences in the degree of sintering, with the CS group exhibiting the highest compressive modulus. Biochemical evidence suggested a mitogenic effect of DCC and CS encapsulation on rat bone marrow stromal cells with the matrix synthesis boosted primarily by the inherently present extracellular matrix components. An important finding was that the cell seeded CS and DCC groups at week 6 had up to an order of magnitude higher glycosaminoglycan contents than their acellular counterparts. Gene expression results indicated a suppressive effect of DCC and CS encapsulation on rat bone marrow stromal cell chondrogenesis with differences in gene expression patterns existing between the DCC and CS groups. Overall, DCC and CS were easily included in microsphere-based scaffolds; however, there is a requirement to further refine their concentrations to achieve the differentiation profiles we seek in vitro.

Microsphere-based scaffolds encapsulating tricalcium phosphate and hydroxyapatite for bone regeneration

Bioceramic mixtures of tricalcium phosphate (TCP) and hydroxyapatite (HAp) are widely used for bone regeneration because of their excellent cytocompatibility, osteoconduction, and osteoinduction. Therefore, we hypothesized that incorporation of a mixture of TCP and HAp in microsphere-based scaffolds would enhance osteogenesis of rat bone marrow stromal cells (rBMSCs) compared to a positive control of scaffolds with encapsulated bone-morphogenic protein-2 (BMP-2). Poly(D,L-lactic-co-glycolic acid) (PLGA) microsphere-based scaffolds encapsulating TCP and HAp mixtures in two different ratios (7:3 and 1:1) were fabricated with the same net ceramic content (30 wt%) to evaluate how incorporation of these ceramic mixtures would affect the osteogenesis in rBMSCs. Encapsulation of TCP/HAp mixtures impacted microsphere morphologies and the compressive moduli of the scaffolds. Additionally, TCP/HAp mixtures enhanced the end-point secretion of extracellular matrix components relevant to bone tissue compared to the "blank" (PLGA-only) microsphere-based scaffolds as evidenced by the biochemical, gene expression, histology, and immunohistochemical characterization. Moreover, the TCP/HAp mixture groups even surpassed the BMP-2 positive control group in some instances in terms of matrix synthesis and gene expression. Lastly, gene expression data suggested that the rBMSCs responded differently to different TCP/HAp ratios presented to them. Altogether, it can be concluded that TCP/HAp mixtures stimulated the differentiation of rBMSCs toward an osteoblastic phenotype, and therefore may be beneficial in gradient microsphere-based scaffolds for osteochondral regeneration.

Microsphere-Based Osteochondral Scaffolds Carrying Opposing Gradients Of Decellularized Cartilage And Demineralized Bone Matrix

Extracellular matrix (ECM) "raw materials" such as demineralized bone matrix (DBM) and cartilage matrix have emerged as leading scaffolding materials for osteochondral regeneration owing to their capacity to facilitate progenitor/resident cell recruitment, infiltration, and differentiation without adding growth factors. Scaffolds comprising synthetic polymers are sturdy yet generally lack cues for guiding cell differentiation. We hypothesized that opposing gradients of decellularized cartilage (DCC) and DBM in polymeric microsphere-based scaffolds would provide superior regeneration compared to polymer-only scaffolds in vivo. Poly(D,L-lactic-co-glycolic acid) (PLGA) microsphere-based scaffolds were fabricated, either with opposing gradients of DCC and DBM encapsulated (GRADIENT) or without DCC and DBM (BLANK control), and implanted into rabbit osteochondral defects in medial femoral condyles. After 12 weeks, gross morphological evaluation showed that the repair tissue in about 30% of the implants was either slightly or significantly depressed, hinting toward rapid polymer degradation in scaffolds from both of the groups. Additionally, no differences were observed in gross morphology of the repair tissue between the BLANK and GRADIENT groups. Mechanical testing revealed no significant differences in model parameter values between the two groups. Histological observations demonstrated that the repair tissue in both of the groups was fibrous in nature with the cells demonstrating notable proliferation and matrix deposition activity. No adverse inflammatory response was observed in any of the implants from the two groups. Overall, the results emphasize the need to improve the technology in terms of altering the DBM and DCC concentrations, and tailoring the polymer degradation to these concentrations.

Fibrin hydrogels functionalized with cartilage extracellular matrix and incorporating freshly isolated stromal cells as an injectable for cartilage regeneration

Freshly isolated stromal cells can potentially be used as an alternative to in vitro expanded cells in regenerative medicine. Their use requires the development of bioactive hydrogels or scaffolds which provide an environment to enhance their proliferation and tissue-specific differentiation in vivo. The goal of the current study was to develop an injectable fibrin hydrogel functionalized with cartilage ECM microparticles and transforming growth factor (TGF)-β3 as a putative therapeutic for articular cartilage regeneration. ECM microparticles were produced by cryomilling and freeze-drying porcine articular cartilage. Up to 2% (w/v) ECM could be incorporated into fibrin without detrimentally affecting its capacity to form stable hydrogels. To access the chondroinductivity of cartilage ECM, we compared chondrogenesis of infrapatellar fat pad-derived stem cells in fibrin hydrogels functionalized with either particulated ECM or control gelatin microspheres. Cartilage ECM particles could be used to control the delivery of TGF-β3 to IFP-derived stem cells within fibrin hydrogels in vitro, and furthermore, led to higher levels of sulphated glycosaminoglycan (sGAG) and collagen accumulation compared to control constructs loaded with gelatin microspheres. In vivo, freshly isolated stromal cells generated a more cartilage-like tissue within fibrin hydrogels functionalized with cartilage ECM particles compared to the control gelatin loaded constructs. These tissues stained strongly for type II collagen and contained higher levels of sGAGs. These results support the use of fibrin hydrogels functionalized with cartilage ECM components in single-stage, cell-based therapies for joint regeneration.

STATEMENT OF SIGNIFICANCE

An alternative to the use of in vitro expanded cells in regenerative medicine is the use of freshly isolated stromal cells, where a bioactive scaffold or hydrogel is used to provide an environment that enhances their proliferation and tissue-specific differentiation in vivo. The objective of this study was to develop an injectable fibrin hydrogel functionalized with cartilage ECM micro-particles and the growth factor TGF-β3 as a therapeutic for articular cartilage regeneration. This study demonstrates that freshly isolated stromal cells generate cartilage tissue in vivo when incorporated into such a fibrin hydrogels functionalized with cartilage ECM particles. These findings open up new possibilities for in-theatre, single-stage, cell-based therapies for joint regeneration.

Engineering Tissues That Mimic the Zonal Nature of Articular Cartilage Using Decellularized Cartilage Explants Seeded with Adult Stem Cells

Articular cartilage (AC) possesses uniquely complex mechanical properties; for example its stiffness increases with depth through the tissue and it softens when compressed. These properties are integral to the function of AC and can be attributed to the tissue's collagen network and how it interacts with negatively charged proteoglycans. In this study, scaffolds containing arrays of channels were produced from decellularized AC explants derived from skeletally immature and mature pigs. These scaffolds were then repopulated with human infrapatellar fat pad derived stem cells (FPSCs). After 4 weeks in culture, FPSCs filled channels within the decellularized explants with a matrix rich in proteoglycans and collagen. Cellular and neo-matrix alignment within these scaffolds appeared to be influenced by the underlying collagen architecture of the decellularized cartilage. Repopulating scaffolds derived from decellularized skeletally mature cartilage with FPSCs led to the development of engineered cartilage with depth-dependent mechanical properties mimicking aspects of native tissue. Furthermore, these constructs displayed the characteristic strain softening behavior of AC. These findings highlight the importance of the collagen network to engineering mechanically functional cartilage grafts.

Induction of mesenchymal stem cell chondrogenic differentiation and functional cartilage microtissue formation for in vivo cartilage regeneration by cartilage extracellular matrix-derived particles

We propose a method of preparing a novel cell carrier derived from natural cartilage extracellular matrix (ECM), designated cartilage ECM-derived particles (CEDPs). Through a series of processes involving pulverization, sieving, and decellularization, fresh cartilage was made into CEDPs with a median diameter of 263 ± 48 μm. Under microgravity culture conditions in a rotary cell culture system (RCCS), bone marrow stromal cells (BMSCs) can proliferate rapidly on the surface of CEDPs with high viability. Histological evaluation and gene expression analysis indicated that BMSCs were differentiated into mature chondrocytes after 21 days of culture without the use of exogenous growth factors. Functional cartilage microtissue aggregates of BMSC-laden CEDPs formed as time in culture increased. Further, the microtissue aggregates were directly implanted into trochlear cartilage defects in a rat model (CEDP+MSC group). Gait analysis and histological results indicated that the CEDP+MSC group obtained better and more rapid joint function recovery and superior cartilage repair compared to the control groups, in which defects were treated with CEDPs alone or only fibrin glue, at both 6 and 12 weeks after surgery. In conclusion, the innovative cell carrier derived from cartilage ECM could promote chondrogenic differentiation of BMSCs, and the direct use of functional cartilage microtissue facilitated cartilage regeneration. This strategy for cell culture, stem cell differentiation and one-step surgery using cartilage microtissue for cartilage repair provides novel prospects for cartilage tissue engineering and may have further broad clinical applications.

STATEMENT OF SIGNIFICANCE

We proposed a method to prepare a novel cell carrier derived from natural cartilage ECM, termed cartilage ECM-derived particles (CEDPs), which can support proliferation of MSCs and facilitate their chondrogenic differentiation. Further, the direct use of functional cartilage microtissue of MSC-laden CEDP aggregates for cartilage repair in vivo induced hyaline-like articular cartilage repair. This strategy for cell culture, stem cell differentiation and the one-step surgery for cartilage repair provide novel prospects for cartilage tissue engineering and may have further broad clinical applications.

A Road Map to Commercialization of Cartilage Therapy in the United States of America

Despite numerous efforts in cartilage regeneration, few products see the light of clinical translation as the commercialization process is opaque, financially demanding, and requires collaboration with people of varied skill sets. The aim of this review is to introduce, to an academic audience, the different paradigms involved in the commercialization of cartilage regeneration technology, elucidate the different hurdles associated with the use of cells and materials in developing new technologies, discuss potential commercialization strategies, and inform the reader about the current trends observed in both the clinical and laboratory setting for establishing clinical trials. Although there are review articles on articular cartilage tissue engineering, independent reports provided by the Food and Drug Administration, and separate review articles on animal models, this is the first review that encompasses all of these facets and is presented in a format favorable to the academic investigator interested in clinical translation from bench to bedside.