Transforming growth factor-β1 release from oligo(poly(ethylene glycol) fumarate) hydrogels in conditions that model the cartilage wound healing environment

ZIF-8-modified hydrogel sequentially delivers angiogenic and osteogenic growth factors to accelerate vascularized bone regeneration

To realize high-quality vascularized bone regeneration, we developed a multifunctional hydrogel (SHPP-ZB) by incorporating BMP-2@ZIF-8/PEG-NH2 nanoparticles (NPs) into a sodium alginate/hydroxyapatite/polyvinyl alcohol hydrogel loaded with PDGF-BB, allowing for the sequential release of angiogenic and osteogenic growth factors (GFs) during bone repair. ZIF-8 served as a protective host for BMP-2 from degradation, ensuring high encapsulation efficiency and long-term bioactivity. The SHPP-ZB hydrogel exhibited enhanced mechanical strength and injectability, making it suitable for complex bone defects. It provided a swelling interface for tissue interlocking and the early release of Zn2+ and tannin acid (TA) to exert antioxidant and antibacterial effects, followed by the sequential release of angiogenic and osteogenic GFs to promote high-quality vascularized bone regeneration. In vitro experiments demonstrated the superior angiogenic and osteogenic properties of SHPP-ZB compared to other groups. In vivo experiments indicated that the sequential delivery of GFs via SHPP-ZB hydrogel could improve vascularized bone regeneration. Further, RNA sequencing analysis of regenerative bone tissue revealed that SHPP-ZB hydrogel promoted vascularized bone regeneration by regulating JUN, MAPK, Wnt, and calcium signaling pathways in vivo. This study presented a promising approach for efficient vascularized bone regeneration in large-scale bone defects.

O-alg-THAM/gel hydrogels functionalized with engineered microspheres based on mesenchymal stem cell secretion recruit endogenous stem cells for cartilage repair

Lacking self-repair abilities, injuries to articular cartilage can lead to cartilage degeneration and ultimately result in osteoarthritis. Tissue engineering based on functional bioactive scaffolds are emerging as promising approaches for articular cartilage regeneration and repair. Although the use of cell-laden scaffolds prior to implantation can regenerate and repair cartilage lesions to some extent, these approaches are still restricted by limited cell sources, excessive costs, risks of disease transmission and complex manufacturing practices. Acellular approaches through the recruitment of endogenous cells offer great promise for in situ articular cartilage regeneration. In this study, we propose an endogenous stem cell recruitment strategy for cartilage repair. Based on an injectable, adhesive and self-healable o-alg-THAM/gel hydrogel system as scaffolds and a biophysio-enhanced bioactive microspheres engineered based on hBMSCs secretion during chondrogenic differentiation as bioactive supplement, the as proposed functional material effectively and specifically recruit endogenous stem cells for cartilage repair, providing new insights into in situ articular cartilage regeneration.

Enhancing cartilage repair with optimized supramolecular hydrogel-based scaffold and pulsed electromagnetic field

Functional tissue engineering strategies provide innovative approach for the repair and regeneration of damaged cartilage. Hydrogel is widely used because it could provide rapid defect filling and proper structure support, and is biocompatible for cell aggregation and matrix deposition. Efforts have been made to seek suitable scaffolds for cartilage tissue engineering. Here Alg-DA/Ac-β-CD/gelatin hydrogel was designed with the features of physical and chemical multiple crosslinking and self-healing properties. Gelation time, swelling ratio, biodegradability and biocompatibility of the hydrogels were systematically characterized, and the injectable self-healing adhesive hydrogel were demonstrated to exhibit ideal properties for cartilage repair. Furthermore, the new hydrogel design introduces a pre-gel state before photo-crosslinking, where increased viscosity and decreased fluidity allow the gel to remain in a semi-solid condition. This granted multiple administration routes to the hydrogels, which brings hydrogels the ability to adapt to complex clinical situations. Pulsed electromagnetic fields (PEMF) have been recognized as a promising solution to various health problems owing to their noninvasive properties and therapeutic potentials. PEMF treatment offers a better clinical outcome with fewer, if any, side effects, and wildly used in musculoskeletal tissue repair. Thereby we propose PEMF as an effective biophysical stimulation to be 4th key element in cartilage tissue engineering. In this study, the as-prepared Alg-DA/Ac-β-CD/gelatin hydrogels were utilized in the rat osteochondral defect model, and the potential application of PEMF in cartilage tissue engineering were investigated. PEMF treatment were proven to enhance the quality of engineered chondrogenic constructs in vitro, and facilitate chondrogenesis and cartilage repair in vivo. All of the results suggested that with the injectable self-healing adhesive hydrogel and PEMF treatment, this newly proposed tissue engineering strategy revealed superior clinical potential for cartilage defect treatment.

Gelatin as It Is: History and Modernity

The data concerning the synthesis and physicochemical characteristics of one of the practically important proteins-gelatin, as well as the possibilities of its practical application, are systematized and discussed. When considering the latter, emphasis is placed on the use of gelatin in those areas of science and technology that are associated with the specifics of the spatial/molecular structure of this high-molecular compound, namely, as a binder for the silver halide photographic process, immobilized matrix systems with a nano-level organization of an immobilized substance, matrices for creating pharmaceutical/dosage forms and protein-based nanosystems. It was concluded that the use of this protein is promising in the future.

Recombinant protein drugs-based intra articular drug delivery systems for osteoarthritis therapy

Osteoarthritis (OA) is the most prevalent chronic degenerative joint disease. It weakens the motor function of patients and imposes a significant economic burden on society. The current medications commonly used in clinical practice do not meet the need for the treatment of OA. Recombinant protein drugs (RPDs) can treat OA by inhibiting inflammatory pathways, regulating catabolism/anabolism, and promoting cartilage repair, thereby showing promise as disease-modifying OA drugs (DMOADs). However, the rapid clearance and short half-life of them in the articular cavity limit their clinical translation. Therefore, the reliable drug delivery systems for extending drug treatment are necessary for the further development. This review introduces RPDs with therapeutic potential for OA, and summarizes their research progress on related drug delivery systems, and make proper discussion on the certain keys for optimal development of this area.

Poly(vinyl alcohol)/Premna Oblongifolia Merr. Extract/Glutaraldehyde/Carbon Nanotube (VOGC)-Based Composite Hydrogel: A Potential Candidate for Controlled-Release Materials

A hydrogel based on poly(vinyl alcohol) (V), Premna Oblongifolia Merr. extract (O), glutaraldehyde (G), and carbon nanotubes (C) has been synthesized in search of candidates to develop controlled-release fertilizers (CRF). Referring to previous studies, O and C can be considered as two materials that have potential as modifiers in synthesizing CRF. This work is comprised of hydrogel synthesis, their characterisation, including measuring swelling ratio (SR) and water retention (WR) of VOGm , VOGe , VOGm C3 , VOGm C5 , VOGm C7 , VOGm C7 -KCl, and release behaviour of KCl from VOGm C7 -KCl. We found that C interacts physically with VOG, increased the surface roughness of VOGm , and reduced the VOGm crystallite size. The addition of KCl into VOGm C7 reduced the pore size and increased the structural density of VOGm C7 . The thickness and the C content of VOG affected its SR and WR. The addition of KCl into VOGm C7 reduced its SR, but did not significantly affect its WR.

Evaluating the physicochemical effects of conjugating peptides into thermogelling hydrogels for regenerative biomaterials applications

Thermogelling hydrogels, such as poly(N-isopropylacrylamide) [P(NiPAAm)], provide tunable constructs leveraged in many regenerative biomaterial applications. Recently, our lab developed the crosslinker poly(glycolic acid)-poly(ethylene glycol)-poly(glycolic acid)-di(but-2-yne-1,4-dithiol), which crosslinks P(NiPAAm-co-glycidyl methacrylate) via thiol-epoxy reaction and can be functionalized with azide-terminated peptides via alkyne-azide click chemistry. This study's aim was to evaluate the impact of peptides on the physicochemical properties of the hydrogels. The physicochemical properties of the hydrogels including the lower critical solution temperature, crosslinking times, swelling, degradation, peptide release and cytocompatibility were evaluated. The gels bearing peptides increased equilibrium swelling indicating hydrophilicity of the hydrogel components. Comparable sol fractions were found for all groups, indicating that inclusion of peptides does not impact crosslinking. Moreover, the inclusion of a matrix metalloproteinase-sensitive peptide allowed elucidation of whether release of peptides from the network was driven by hydrolysis or enzymatic cleavage. The hydrophilicity of the network determined by the swelling behavior was demonstrated to be the most important factor in dictating hydrogel behavior over time. This study demonstrates the importance of characterizing the impact of additives on the physicochemical properties of hydrogels. These characteristics are key in determining design considerations for future in vitro and in vivo studies for tissue regeneration.

Carboxylated-xyloglucan and peptide amphiphile co-assembly in wound healing

Hydrogel wound dressings can play critical roles in wound healing protecting the wound from trauma or contamination and providing an ideal environment to support the growth of endogenous cells and promote wound closure. This work presents a self-assembling hydrogel dressing that can assist the wound repair process mimicking the hierarchical structure of skin extracellular matrix. To this aim, the co-assembly behaviour of a carboxylated variant of xyloglucan (CXG) with a peptide amphiphile (PA-H3) has been investigated to generate hierarchical constructs with tuneable molecular composition, structure, and properties. Transmission electron microscopy and circular dichroism at a low concentration shows that CXG and PA-H3 co-assemble into nanofibres by hydrophobic and electrostatic interactions and further aggregate into nanofibre bundles and networks. At a higher concentration, CXG and PA-H3 yield hydrogels that have been characterized for their morphology by scanning electron microscopy and for the mechanical properties by small-amplitude oscillatory shear rheological measurements and compression tests at different CXG/PA-H3 ratios. A preliminary biological evaluation has been carried out both in vitro with HaCat cells and in vivo in a mouse model.

Development of a modular, biocompatible thiolated gelatin microparticle platform for drug delivery and tissue engineering applications

The field of biomaterials has advanced significantly in the past decade. With the growing need for high-throughput manufacturing and screening, the need for modular materials that enable streamlined fabrication and analysis of tissue engineering and drug delivery schema has emerged. Microparticles are a powerful platform that have demonstrated promise in enabling these technologies without the need to modify a bulk scaffold. This building block paradigm of using microparticles within larger scaffolds to control cell ratios, growth factors and drug release holds promise. Gelatin microparticles (GMPs) are a well-established platform for cell, drug and growth factor delivery. One of the challenges in using GMPs though is the limited ability to modify the gelatin post-fabrication. In the present work, we hypothesized that by thiolating gelatin before microparticle formation, a versatile platform would be created that preserves the cytocompatibility of gelatin, while enabling post-fabrication modification. The thiols were not found to significantly impact the physicochemical properties of the microparticles. Moreover, the thiolated GMPs were demonstrated to be a biocompatible and robust platform for mesenchymal stem cell attachment. Additionally, the thiolated particles were able to be covalently modified with a maleimide-bearing fluorescent dye and a peptide, demonstrating their promise as a modular platform for tissue engineering and drug delivery applications.

Swelling Behaviors of 3D Printed Hydrogel and Hydrogel-Microcarrier Composite Scaffolds

The present study sought to demonstrate the swelling behavior of hydrogel-microcarrier composite constructs to inform their use in controlled release and tissue engineering applications. In this study, gelatin methacrylate (GelMA) and GelMA-gelatin microparticle (GMP) composite constructs were three-dimensionally printed, and their swelling and degradation behavior was evaluated over time and as a function of the degree of crosslinking of included GMPs. GelMA-only constructs and composite constructs loaded with GMPs crosslinked with 10 mM (GMP-10) or 40 mM (GMP-40) glutaraldehyde were swollen in phosphate-buffered saline for up to 28 days to evaluate changes in swelling and polymer loss. In addition, scaffold reswelling capacity was evaluated under five successive drying-rehydration cycles. All printed materials demonstrated shear thinning behavior, with microparticle additives significantly increasing viscosity relative to the GelMA-only solution. Swelling results demonstrated that for GelMA/GMP-10 and GelMA/GMP-40 scaffolds, fold and volumetric swelling were statistically higher and lower, respectively, than for GelMA-only scaffolds after 28 days, and the volumetric swelling of GelMA and GelMA/GMP-40 scaffolds decreased over time. After 5 drying-rehydration cycles, GelMA scaffolds demonstrated higher fold swelling than both GMP groups while also showing lower volumetric swelling than GMP groups. Although statistical differences were not observed in the swelling of GMP-10 and GMP-40 particles alone, the interaction of GelMA/GMP demonstrated a significant effect on the swelling behaviors of composite scaffolds. These results demonstrate an example hydrogel-microcarrier composite system's swelling behavior and can inform the future use of such a composite system for controlled delivery of bioactive molecules in vitro and in vivo in tissue engineering applications. Impact statement In this study, porous three-dimensional printed (3DP) hydrogel constructs with and without natural polymer microcarriers were fabricated to observe swelling and degradation behavior under continuous swelling and drying-rehydration cycle conditions. Inclusion of microcarriers with different crosslinking densities led to distinct swelling behaviors for each biomaterial ink tested. 3DP hydrogel and hydrogel-microcarrier composite scaffolds have been commonly used in tissue engineering for the delivery of biomolecules. This study demonstrates the swelling behavior of porous hydrogel and hydrogel-microcarrier scaffolds that may inform later use of such materials for controlled release applications in a variety of fields including materials development and tissue regeneration.

Exquisite design of injectable Hydrogels in Cartilage Repair

Cartilage damage is still a threat to human beings, yet there is currently no treatment available to fully restore the function of cartilage. Recently, due to their unique structures and properties, injectable hydrogels have been widely studied and have exhibited high potential for applications in therapeutic areas, especially in cartilage repair. In this review, we briefly introduce the properties of cartilage, some articular cartilage injuries, and now available treatment strategies. Afterwards, we propose the functional and fundamental requirements of injectable hydrogels in cartilage tissue engineering, as well as the main advantages of injectable hydrogels as a therapy for cartilage damage, including strong plasticity and excellent biocompatibility. Moreover, we comprehensively summarize the polymers, cells, and bioactive molecules regularly used in the fabrication of injectable hydrogels, with two kinds of gelation, i.e., physical and chemical crosslinking, which ensure the excellent design of injectable hydrogels for cartilage repair. We also include novel hybrid injectable hydrogels combined with nanoparticles. Finally, we conclude with the advances of this clinical application and the challenges of injectable hydrogels used in cartilage repair.

Bioactive Factors-imprinted Scaffold Vehicles for Promoting Bone Healing: The Potential Strategies and the Confronted Challenges for Clinical Production

Abstract Wound repair of bone is a complicated multistep process orchestrated by inflammation, angiogenesis, callus formation, and bone remodeling. Many bioactive factors (BFs) including cytokine and growth factors (GFs) have previously been reported to be involved in regulating wound healing of bone and some exogenous BFs such as bone morphogenetic proteins (BMPs) were proven to be helpful for improving bone healing. In this regard, the BFs reported for boosting bone repair were initially categorized according to their regulatory mechanisms. Thereafter, the challenges including short half-life, poor stability, and rapid enzyme degradation and deactivation for these exogenous BFs in bone healing are carefully outlined in this review. For these issues, BFs-imprinted scaffold vehicles have recently been reported to promote the stability of BFs and enhance their half-life in vivo. This review is focused on the incorporation of BFs into the modulated biomaterials with various forms of bone tissue engineering applications: firstly, rigid bone graft substitutes (BGSs) were used to imprint BFs for large scale bone defect repair; secondly, the soft sponge-like scaffold carrying BFs is discussed as filling materials for the cavity of bone defects; thirdly, various injectable vehicles including hydrogel, nanoparticles, and microspheres for the delivery of BFs were also introduced for irregular bone fracture repair. Meanwhile, the challenges for BFs-imprinted scaffold vehicles are also analyzed in this review.

Improving the in-vivo biological activity of fingolimod loaded PHBV nanoparticles by using hydrophobically modified alginate

Uncontrolled distribution of nanoparticles (NPs) within the body can significantly decrease the efficiency of drug therapy and is considered among the main restrictions of NPs application. The aim of this study was to develop a depot combination delivery system (CDS) containing fingolimod loaded poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) NPs dispersed into a matrix of oleic acid-grafted-aminated alginate (OA-g-AAlg) to minimize the nonspecific biodistribution (BD) of PHBV NPs. OA-g-AAlg was synthesized in two step; First, Alg was aminated by using adipic dihydrazide (ADH). The degree of hyrazide group substitution of Alg was determined by trinitro-benzene-sulfonic acid (TNBS) assay. Second, OA was attached to AAlg through formation of an amide bond. Chemical structure of OA-g-AAlg was confirmed with FTIR and HNMR spectroscopy. Furthermore, rheological properties of OA-g-AAlg with different grafting ratios were evaluated. In-vitro release studies indicated that 47% of fingolimod was released from the CDS within 28 days. Blood and tissue samples were analyzed using liquid chromatography/tandem mass spectrometry following subcutaneous (SC) injection of fingolimod-CDS into Wistar rats. The elimination phase half-life of CDS-fingolimod was significantly higher than that of fingolimod (∼32 d vs. ∼20 h). To investigate the therapeutic efficacy, lymphocyte count was assessed over a 40 day period in Wistar rats. Peripheral blood lymphocyte count decreased from baseline by 27 ± 8% in 2 days after injection. Overall, the designed CDS represented promising results in improving the pharmacokinetic properties of fingolimod. Therefore, we believe that this sustained release formulation has a great potential to be applied to delivery of various therapeutics.

Preparation and drug delivery of dextran-drug complex

Dextran as a drug carrier for inhibiting cancer cells effectively reduces the toxic and side effects of the drug in the biological body. Targeting improves the concentration of active substance around the target tissue, which reduces damage to other heavy organs and other normal tissues. Dextran will be a potential carrier for the delivery of antitumor drugs in the future, which provides the possibility of slow-release chemotherapy and targeted drug delivery. Herein, the preparation and drug delivery of dextran-drug complex were summarized and discussed in detail.

Aspects of Nanomaterials in Wound Healing

Wound infections impose a remarkable clinical challenge that has a considerable influence on morbidity and mortality of patients, influencing the cost of treatment. The unprecedented advancements in molecular biology have come up with new molecular and cellular targets that can be successfully applied to develop smarter therapeutics against diversified categories of wounds such as acute and chronic wounds. However, nanotechnology-based diagnostics and treatments have achieved a new horizon in the arena of wound care due to its ability to deliver a plethora of therapeutics into the target site, and to target the complexity of the normal wound-healing process, cell type specificity, and plethora of regulating molecules as well as pathophysiology of chronic wounds. The emerging concepts of nanobiomaterials such as nanoparticles, nanoemulsion, nanofibrous scaffolds, graphene-based nanocomposites, etc., and nano-sized biomaterials like peptides/proteins, DNA/RNA, oligosaccharides have a vast application in the arena of wound care. Multi-functional, unique nano-wound care formulations have acquired major attention by facilitating the wound healing process. In this review, emphasis has been given to different types of nanomaterials used in external wound healing (chronic cutaneous wound healing); the concepts of basic mechanisms of wound healing process and the promising strategies that can help in the field of wound management.

Photo-curable poly-(ethylene glycol)-fumarate elastomers with controlled structural composition and their evaluation as eluting systems

Elastomeric poly-ester materials have extraordinary potential for soft tissue engineering applications. In connection, in the last 10 years, cross-linkable oligo-(polyethylene glycol fumarate)s emerged as promising materials for obtaining hydrogels for bone tissue engineering applications. In this work we prepared a new family of photo-curable poly-(ethylene glycol)–fumarate elastomers with controlled structural composition. These novel elastomers were obtained by photo-curing of fumarate pre-polymers based on diethylene glycol and oligo-ethylene glycols (PEGs 200 and 400), under extremely mild experimental conditions using a low power UV source. The synthesis of fumarate pre-polymers, which were obtained by thermal poly-condensation, and the photo-curing process, were both here discussed on the basis of their structural differences and proposed operating mechanisms. Finally, the photo-radical cross-linking reactions were performed in the presence of anti-cancer drugs (doxorubicin and paclitaxel), in order to evaluate the potential application of the elastomers as new eluting systems. Thus, different release profiles were obtained for hydrophilic (doxorubicin) and hydrophobic (paclitaxel) anticancer drugs, and these differences are discussed on the basis of the structure of the elastomers.

Elastomeric poly-ester materials have extraordinary potential for soft tissue engineering applications.

Sulfated zwitterionic poly(sulfobetaine methacrylate) hydrogels promote complete skin regeneration

Skin wound healing is a still long-history challenging problem and impeded by the foreign-body reaction including severe inflammation response, poor neovascularization, incomplete re-epithelialization and defective ECM remodeling. Development of biocompatible polymers, in combination with specific drugs or growth factors, has been considered as a promising strategy to treat skin wounds. Significant research efforts have been made to develop poly(ethylene glycol) PEG-based polymers for wound healing, however less efforts has been paid to zwitterionic materials, some of which have demonstrated their super low-fouling property in vitro and anti-inflammatory property in vivo. Here, we synthesized ultra-low-fouling zwitterionic sulfated poly(sulfobetaine methacrylate) (polySBMA) hydrogels and applied them to full-thickness cutaneous wounds in mice. The healing effects of SBMA hydrogels on the wound closure, re-epithelialization ratio, ECM remodeling, angiogenesis, and macrophage responses during wound healing processes were histologically evaluated by in vivo experiments. Collective results indicate that SBMA hydrogels promote full-thickness excisional acute wound regeneration in mice by enhancing angiogenesis, decreasing inflammation response, and modulating macrophage polarization. Consistently, the incorporation of SBMA into PEG hydrogels also improved the overall wound healing efficiency as compared to pure PEG hydrogels. This work demonstrates zwitterionic SBMA hydrogels as promising wound dressings for treating full-thickness excisional skin wounds.

STATEMENT OF SIGNIFICANCE

Development of highly effective wound regeneration system is practically important for biomedical applications. Here, we synthesized ultra-low-fouling zwitterionic sulfated poly(sulfobetaine methacrylate) (polySBMA) hydrogels and applied it to full-thickness cutaneous wounds in mice, in comparison with PEG hydrogels as a control. We are the first to examine and reveal the difference between zwitterionic SBMA hydrogels and PEG hydrogels using a full-thickness excisional mice model. Overall, a series of in vivo systematic tests demonstrated that zwitterionic SBMA hydrogels exhibited superior wound healing property in almost all aspects as compared to PEG hydrogels.

Structured Macroporous Hydrogels: Progress, Challenges, and Opportunities

Structured macroporous hydrogels that have controllable porosities on both the nanoscale and the microscale offer both the swelling and interfacial properties of bulk hydrogels as well as the transport properties of "hard" macroporous materials. While a variety of techniques such as solvent casting, freeze drying, gas foaming, and phase separation have been developed to fabricate structured macroporous hydrogels, the typically weak mechanics and isotropic pore structures achieved as well as the required use of solvent/additives in the preparation process all limit the potential applications of these materials, particularly in biomedical contexts. This review highlights recent developments in the field of structured macroporous hydrogels aiming to increase network strength, create anisotropy and directionality within the networks, and utilize solvent-free or additive-free fabrication methods. Such functional materials are well suited for not only biomedical applications like tissue engineering and drug delivery but also selective filtration, environmental sorption, and the physical templating of secondary networks.

Pullulan microbeads/Si-HPMC hydrogel injectable system for the sustained delivery of GDF-5 and TGF-β1: new insight into intervertebral disc regenerative medicine

Discogenic low back pain is considered a major health concern and no etiological treatments are today available to tackle this disease. To clinically address this issue at early stages, there is a rising interest in the stimulation of local cells by in situ injection of growth factors targeting intervertebral disc (IVD) degenerative process. Despite encouraging safety and tolerability results in clinic, growth factors efficacy may be further improved. To this end, the use of a delivery system allowing a sustained release, while protecting growth factors from degradation appears of particular interest. We propose herein the design of a new injectable biphasic system, based on the association of pullulan microbeads (PMBs) into a cellulose-based hydrogel (Si-HPMC), for the TGF-β1 and GDF-5 growth factors sustained delivery. We present for the first time the design and mechanical characterization of both the PMBs and the called biphasic system (PMBs/Si-HPMC). Their loading and release capacities were also studied and we were able to demonstrate a sustained release of both growth factors, for up to 28 days. Noteworthy, the growth factors biological activity on human cells was maintained. Altogether, these data suggest that this PMBs/Si-HPMC biphasic system may be a promising candidate for the development of an innovative bioactive delivery system for IVD regenerative medicine.

Engineering cell aggregates through incorporated polymeric microparticles

Ex vivo cell aggregates must overcome significant limitations in the transport of nutrients, drugs, and signaling proteins compared to vascularized native tissue. Further, engineered extracellular environments often fail to sufficiently replicate tethered signaling cues and the complex architecture of native tissue. Co-cultures of cells with microparticles (MPs) is a growing field directed towards overcoming many of these challenges by providing local and controlled presentation of both soluble and tethered proteins and small molecules. Further, co-cultured MPs offer a mechanism to better control aggregate architecture and even to report key characteristics of the local microenvironment such as pH or oxygen levels. Herein, we provide a brief introduction to established and developing strategies for MP production including the choice of MP materials, fabrication techniques, and techniques for incorporating additional functionality. In all cases, we emphasize the specific utility of each approach to form MPs useful for applications in cell aggregate co-culture. We review established techniques to integrate cells and MPs. We highlight those strategies that promote targeted heterogeneity or homogeneity, and we describe approaches to engineer cell-particle and particle-particle interactions that enhance aggregate stability and biological response. Finally, we review advances in key application areas of MP aggregates and future areas of development.

STATEMENT OF SIGNIFICANT

Cell-scaled polymer microparticles (MPs) integrated into cellular aggregates have been shown to be a powerful tool to direct cell response. MPs have supported the development of healthy cartilage, islets, nerves, and vasculature by the maintenance of soluble gradients as well as by the local presentation of tethered cues and diffusing proteins and small molecules. MPs integrated with pluripotent stem cells have directed in vivo expansion and differentiation. Looking forward, MPs are expected to support both the characterization and development of in vitro tissue systems for applications such as drug testing platforms. However, useful co-cultures must be designed keeping in mind the limitations and attributes of each material strategy within the context of the overall tissue biology. The present review integrates prospectives from materials development, drug delivery, and tissue engineering to provide a toolbox for the development and application of MPs useful for long-term co-culture within cell aggregates.

Extracellular Matrix (ECM) Multilayer Membrane as a Sustained Releasing Growth Factor Delivery System for rhTGF-β3 in Articular Cartilage Repair

Recombinant human transforming growth factor beta-3 (rhTGF-β₃) is a key regulator of chondrogenesis in stem cells and cartilage formation. We have developed a novel drug delivery system that continuously releases rhTGF-β₃ using a multilayered extracellular matrix (ECM) membrane. We hypothesize that the sustained release of rhTGF-β₃ could activate stem cells and result in enhanced repair of cartilage defects. The properties and efficacy of the ECM multilayer-based delivery system (EMLDS) are investigated using rhTGF-β₃ as a candidate drug. The bioactivity of the released rhTGF-ß3 was evaluated through chondrogenic differentiation of mesenchymal stem cells (MSCs) using western blot and circular dichroism (CD) analyses in vitro. The cartilage reparability was evaluated through implanting EMLDS with endogenous and exogenous MSC in both in vivo and ex vivo models, respectively. In the results, the sustained release of rhTGF-ß3 was clearly observed over a prolonged period of time in vitro and the released rhTGF-β₃ maintained its structural stability and biological activity. Successful cartilage repair was also demonstrated when rabbit MSCs were treated with rhTGF-β3-loaded EMLDS ((+) rhTGF-β3 EMLDS) in an in vivo model and when rabbit chondrocytes and MSCs were treated in ex vivo models. Therefore, the multilayer ECM membrane could be a useful drug delivery system for cartilage repair.

A comparative study of the mechanical properties of hybrid double-network hydrogels in swollen and as-prepared states

Significant efforts have been made to develop very tough hydrogels at both swelling and as-prepared states towards many scientific and industrial applications.

In vitro culture of hFOB1.19 osteoblast cells on TGF-β1-SF-CS three-dimensional scaffolds

The aim of the present study was to examine the biocompatibility of transforming growth factor-β1-silk fibroin-chitosan (TGF-β1-SF-CS) scaffolds. In order to provide an ideal scaffold for use in bone tissue engineering, TGF-β1 was introduced into the SF-CS scaffold in order to reconstruct a three dimensional scaffold, following which hFOB1.19 osteoblast cells were seeded onto TGF-β1-SF-CS and SF-CS scaffolds. On the TGF-β1-SF-CS and SF-CS scaffolds, the cell adhesion rate increased in a time-dependent manner. Scanning electron microscopy revealed that the cells grew actively and exhibited normal morphological features with multiple fissions, and granular and filamentous substrates were observed surrounding the cells. In addition, the cell microfilaments were closely connected with the scaffolds. The cells exhibited attached growth on the surfaces of the scaffolds, however, the growth also extended into the scaffolds. Cell Counting Kit-8 and ALP analyses revealed that TGF-β1 significantly promoted the growth and proliferation of the hFOB1.19 osteoblast cells in the SF-CS scaffolds, and the enhancement of osteoblast cell proliferation and activity by TGF-β1 occurred in a time-dependent manner. The TGF-β1-SF-CS composite material may offer potential as an ideal scaffold material for bone tissue engineering.

Cytocompatible in situ forming chitosan/hyaluronan hydrogels via a metal-free click chemistry for soft tissue engineering

Injectable hydrogels are important cell scaffolding materials for tissue engineering and regenerative medicine. Here, we report a new class of biocompatible and biodegradable polysaccharide hydrogels derived from chitosan and hyaluronan via a metal-free click chemistry, without the addition of copper catalyst. For the metal-free click reaction, chitosan and hyaluronan were modified with oxanorbornadiene (OB) and 11-azido-3,6,9-trioxaundecan-1-amine (AA), respectively. The gelation is attributed to the triazole ring formation between OB and azido groups of polysaccharide derivatives. The molecular structures were verified by FT-IR spectroscopy and elemental analysis, giving substitution degrees of 58% and 47% for chitosan-OB and hyaluronan-AA, respectively. The in vitro gelation, morphologies, equilibrium swelling, compressive modulus and degradation of the composite hydrogels were examined. The potential of the metal-free hydrogel as a cell scaffold was demonstrated by encapsulation of human adipose-derived stem cells (ASCs) within the gel matrix in vitro. Cell culture showed that this metal-free hydrogel could support survival and proliferation of ASCs. A preliminary in vivo study demonstrated the usefulness of the hydrogel as an injectable scaffold for adipose tissue engineering. These characteristics provide a potential opportunity to use the metal-free click chemistry in preparation of biocompatible hydrogels for soft tissue engineering applications.

Strategies for controlled delivery of biologics for cartilage repair

The delivery of biologics is an important component in the treatment of osteoarthritis and the functional restoration of articular cartilage. Numerous factors have been implicated in the cartilage repair process, but the uncontrolled delivery of these factors may not only reduce their full reparative potential but can also cause unwanted morphological effects. It is therefore imperative to consider the type of biologic to be delivered, the method of delivery, and the temporal as well as spatial presentation of the biologic to achieve the desired effect in cartilage repair. Additionally, the delivery of a single factor may not be sufficient in guiding neo-tissue formation, motivating recent research toward the delivery of multiple factors. This review will discuss the roles of various biologics involved in cartilage repair and the different methods of delivery for appropriate healing responses. A number of spatiotemporal strategies will then be emphasized for the controlled delivery of single and multiple bioactive factors in both in vitro and in vivo cartilage tissue engineering applications.

Gelatin methacrylate microspheres for controlled growth factor release

Gelatin has been commonly used as a delivery vehicle for various biomolecules for tissue engineering and regenerative medicine applications due to its simple fabrication methods, inherent electrostatic binding properties, and proteolytic degradability. Compared to traditional chemical cross-linking methods, such as the use of glutaraldehyde (GA), methacrylate modification of gelatin offers an alternative method to better control the extent of hydrogel cross-linking. Here we examined the physical properties and growth factor delivery of gelatin methacrylate (GMA) microparticles (MPs) formulated with a wide range of different cross-linking densities (15-90%). Less methacrylated MPs had decreased elastic moduli and larger mesh sizes compared to GA MPs, with increasing methacrylation correlating to greater moduli and smaller mesh sizes. As expected, an inverse correlation between microparticle cross-linking density and degradation was observed, with the lowest cross-linked GMA MPs degrading at the fastest rate, comparable to GA MPs. Interestingly, GMA MPs at lower cross-linking densities could be loaded with up to a 10-fold higher relative amount of growth factor than conventional GA cross-linked MPs, despite the GA MPs having an order of magnitude greater gelatin content. Moreover, a reduced GMA cross-linking density resulted in more complete release of bone morphogenic protein 4 and basic fibroblast growth factor and accelerated release rate with collagenase treatment. These studies demonstrate that GMA MPs provide a more flexible platform for growth factor delivery by enhancing the relative binding capacity and permitting proteolytic degradation tunability, thereby offering a more potent controlled release system for growth factor delivery.

Mechanically strong hybrid double network hydrogels with antifouling properties

The development of mechanically tough and biocompatible polymer hydrogels has great potential and promise for many applications.

Synthetic biodegradable hydrogel delivery of demineralized bone matrix for bone augmentation in a rat model

There exists a strong clinical need for a more capable and robust method to achieve bone augmentation, and a system with fine-tuned delivery of demineralized bone matrix (DBM) has the potential to meet that need. As such, the objective of the present study was to investigate a synthetic biodegradable hydrogel for the delivery of DBM for bone augmentation in a rat model. Oligo(poly(ethylene glycol) fumarate) (OPF) constructs were designed and fabricated by varying the content of rat-derived DBM particles (either 1:3, 1:1 or 3:1 DBM:OPF weight ratio on a dry basis) and using two DBM particle size ranges (50-150 or 150-250 μm). The physical properties of the constructs and the bioactivity of the DBM were evaluated. Selected formulations (1:1 and 3:1 with 50-150 μm DBM) were evaluated in vivo compared to an empty control to investigate the effect of DBM dose and construct properties on bone augmentation. Overall, 3:1 constructs with higher DBM content achieved the greatest volume of bone augmentation, exceeding 1:1 constructs and empty implants by 3- and 5-fold, respectively. As such, we have established that a synthetic, biodegradable hydrogel can function as a carrier for DBM, and that the volume of bone augmentation achieved by the constructs correlates directly to the DBM dose.

Controlled release of transforming growth factor-β3 from cartilage-extra-cellular-matrix-derived scaffolds to promote chondrogenesis of human-joint-tissue-derived stem cells

The objective of this study was to develop a scaffold derived from cartilaginous extracellular matrix (ECM) that could be used as a growth factor delivery system to promote chondrogenesis of stem cells. Dehydrothermal crosslinked scaffolds were fabricated using a slurry of homogenized porcine articular cartilage, which was then seeded with human infrapatellar-fat-pad-derived stem cells (FPSCs). It was found that these ECM-derived scaffolds promoted superior chondrogenesis of FPSCs when the constructs were additionally stimulated with transforming growth factor (TGF)-β3. Cell-mediated contraction of the scaffold was observed, which could be limited by the additional use of 1-ethyl-3-3dimethyl aminopropyl carbodiimide (EDAC) crosslinking without suppressing cartilage-specific matrix accumulation within the construct. To further validate the utility of the ECM-derived scaffold, we next compared its chondro-permissive properties to a biomimetic collagen-hyaluronic acid (HA) scaffold optimized for cartilage tissue engineering (TE) applications. The cartilage-ECM-derived scaffold supported at least comparable chondrogenesis to the collagen-HA scaffold, underwent less contraction and retained a greater proportion of synthesized sulfated glycosaminoglycans. Having developed a promising scaffold for TE, with superior chondrogenesis observed in the presence of exogenously supplied TGF-β3, the final phase of the study explored whether this scaffold could be used as a TGF-β3 delivery system to promote chondrogenesis of FPSCs. It was found that the majority of TGF-β3 that was loaded onto the scaffold was released in a controlled manner over the first 10days of culture, with comparable long-term chondrogenesis observed in these TGF-β3-loaded constructs compared to scaffolds where the TGF-β3 was continuously added to the media. The results of this study support the use of cartilage-ECM-derived scaffolds as a growth factor delivery system for use in articular cartilage regeneration.

Strategies to balance covalent and non-covalent biomolecule attachment within collagen-GAG biomaterials

Strategies to integrate instructive biomolecular signals into a biomaterial are becoming increasingly complex and bioinspired. While a large majority of reports still use repeated treatments with soluble factors, this approach can be prohibitively costly and difficult to translate in vivo for applications where spatial control over signal presentation is necessary. Recent efforts have explored the use of covalent immobilization of biomolecules to the biomaterial, via both bulk (ubiquitous) as well as spatially-selective light-based crosslinking, as a means to both enhance stability and bioactivity. However, little is known about how processing conditions during immobilization impact the degree of unintended non-covalent interactions, or fouling, that takes place between the biomaterial and the biomolecule of interest. Here we demonstrate the impact of processing conditions for bulk carbodiimide (EDC) and photolithography-based benzophenone (BP) crosslinking on specific attachment vs. fouling of a model protein (Concanavalin A, ConA) within collagen-glycosaminoglycan (CG) scaffolds. Collagen source significantly impacts the selectivity of biomolecule immobilization. EDC crosslinking intensity and ligand concentration significantly impacted selective immobilization. For benzophenone photoimmobilization we observed that increased UV exposure time leads to increased ConA immobilization. Immobilization efficiency for both EDC and BP strategies was maximal at physiological pH. Increasing ligand concentration during immobilization process led to enhanced immobilization for EDC chemistry, no impact on BP immobilization, but significant increases in non-specific fouling. Given recent efforts to covalently immobilize biomolecules to a biomaterial surface to enhance bioactivity, improved understanding of the impact of crosslinking conditions on selective attachment versus non-specific fouling will inform the design of instructive biomaterials for applications across tissue engineering.

Osteochondral defect repair using bilayered hydrogels encapsulating both chondrogenically and osteogenically pre-differentiated mesenchymal stem cells in a rabbit model

OBJECTIVE

To investigate the ability of cell-laden bilayered hydrogels encapsulating chondrogenically and osteogenically (OS) pre-differentiated mesenchymal stem cells (MSCs) to effect osteochondral defect repair in a rabbit model. By varying the period of chondrogenic pre-differentiation from 7 (CG7) to 14 days (CG14), the effect of chondrogenic differentiation stage on osteochondral tissue repair was also investigated.

METHODS

Rabbit MSCs were subjected to either chondrogenic or osteogenic pre-differentiation, encapsulated within respective chondral/subchondral layers of a bilayered hydrogel construct, and then implanted into femoral condyle osteochondral defects. Rabbits were randomized into one of four groups (MSC/MSC, MSC/OS, CG7/OS, and CG14/OS; chondral/subchondral) and received two similar constructs bilaterally. Defects were evaluated after 12 weeks.

RESULTS

All groups exhibited similar overall neo-tissue filling. The delivery of OS cells when compared to undifferentiated MSCs in the subchondral construct layer resulted in improvements in neo-cartilage thickness and regularity. However, the addition of CG cells in the chondral layer, with OS cells in the subchondral layer, did not augment tissue repair as influenced by the latter when compared to the control. Instead, CG7/OS implants resulted in more irregular neo-tissue surfaces when compared to MSC/OS implants. Notably, the delivery of CG7 cells, when compared to CG14 cells, with OS cells stimulated morphologically superior cartilage repair. However, neither osteogenic nor chondrogenic pre-differentiation affected detectable changes in subchondral tissue repair.

CONCLUSIONS

Cartilage regeneration in osteochondral defects can be enhanced by MSCs that are chondrogenically and osteogenically pre-differentiated prior to implantation. Longer chondrogenic pre-differentiation periods, however, lead to diminished cartilage repair.

Dual growth factor delivery from bilayered, biodegradable hydrogel composites for spatially-guided osteochondral tissue repair

The present work investigated the use of biodegradable hydrogel composite scaffolds, based on the macromer oligo(poly(ethylene glycol) fumarate) (OPF), to deliver growth factors for the repair of osteochondral tissue in a rabbit model. In particular, bilayered OPF composites were used to mimic the structural layers of the osteochondral unit, and insulin-like growth factor-1 (IGF-1) and bone morphogenetic protein-2 (BMP-2) were loaded into gelatin microparticles and embedded within the OPF hydrogel matrix in a spatially controlled manner. Three different scaffold formulations were implanted in a medial femoral condyle osteochondral defect: 1) IGF-1 in the chondral layer, 2) BMP-2 in the subchondral layer, and 3) IGF-1 and BMP-2 in their respective separate layers. The quantity and quality of osteochondral repair was evaluated at 6 and 12 weeks with histological scoring and micro-computed tomography (micro-CT). While histological scoring results at 6 weeks showed no differences between experimental groups, micro-CT analysis revealed that the delivery of BMP-2 alone increased the number of bony trabecular islets formed, an indication of early bone formation, over that of IGF-1 delivery alone. At 12 weeks post-implantation, minimal differences were detected between the three groups for cartilage repair. However, the dual delivery of IGF-1 and BMP-2 had a higher proportion of subchondral bone repair, greater bone growth at the defect margins, and lower bone specific surface than the single delivery of IGF-1. These results suggest that the delivery of BMP-2 enhances subchondral bone formation and that, while the dual delivery of IGF-1 and BMP-2 in separate layers does not improve cartilage repair under the conditions studied, they may synergistically enhance the degree of subchondral bone formation. Overall, bilayered OPF hydrogel composites demonstrate potential as spatially-guided, multiple growth factor release vehicles for osteochondral tissue repair.

Gelatin carriers for drug and cell delivery in tissue engineering

The ability of gelatin to form complexes with different drugs has been investigated for controlled release applications. Gelatin parameters, such as crosslinking density and isoelectric point, have been tuned in order to optimize gelatin degradation and drug delivery kinetics. In recent years, focus has shifted away from the use of gelatin in isolation toward the modification of gelatin with functional groups and the fabrication of material composites with embedded gelatin carriers. In this review, we highlight some of the latest work being performed in these areas and comment on trends in the field. Specifically, we discuss gelatin modifications for immune system evasion, drug stabilization, and targeted delivery, as well as gelatin composite systems based on ceramics, naturally-occurring polymers, and synthetic polymers.

Generation of osteochondral tissue constructs with chondrogenically and osteogenically predifferentiated mesenchymal stem cells encapsulated in bilayered hydrogels

This study investigated the ability of chondrogenic and osteogenic predifferentiation of mesenchymal stem cells (MSCs) to play a role in the development of osteochondral tissue constructs using injectable bilayered oligo(poly(ethylene glycol) fumarate) (OPF) hydrogel composites. We hypothesized that the combinatorial approach of encapsulating cell populations of both chondrogenic and osteogenic lineages in a spatially controlled manner within bilayered constructs would enable these cells to maintain their respective phenotypes via the exchange of biochemical factors even without the influence of external growth factors. During monolayer expansion prior to hydrogel encapsulation, it was found that 7 (CG7) and 14 (CG14) days of MSC exposure to TGF-β3 allowed for the generation of distinct cell populations with corresponding chondrogenic maturities as indicated by increasing aggrecan and type II collagen/type I collagen expression. Chondrogenic and osteogenic cells were then encapsulated within their respective (chondral/subchondral) layers in bilayered hydrogel composites to include four experimental groups. Encapsulated CG7 cells within the chondral layer exhibited enhanced chondrogenic phenotype when compared to other cell populations based on stronger type II collagen and aggrecan gene expression and higher glycosaminoglycan-to-hydroxyproline ratios. Osteogenic cells that were co-cultured with chondrogenic cells (in the chondral layer) showed higher cellularity over time, suggesting that chondrogenic cells stimulated the proliferation of osteogenic cells. Groups with osteogenic cells displayed mineralization in the subchondral layer, confirming the effect of osteogenic predifferentiation. In summary, it was found that MSCs that underwent 7 days, but not 14 days, of chondrogenic predifferentiation most closely resembled the phenotype of native hyaline cartilage when combined with osteogenic cells in a bilayered OPF hydrogel composite, indicating that the duration of chondrogenic preconditioning is an important factor to control. Furthermore, the respective chondrogenic and osteogenic phenotypes were maintained for 28 days in vitro without the need for external growth factors, demonstrating the exciting potential of this novel strategy for the generation of osteochondral tissue constructs for cartilage engineering applications.

A factorial analysis of the combined effects of hydrogel fabrication parameters on the in vitro swelling and degradation of oligo(poly(ethylene glycol) fumarate) hydrogels

In this study, a full factorial approach was used to investigate the effects of poly(ethylene glycol) (PEG) molecular weight (MW; 10,000 vs. 35,000 nominal MW), crosslinker-to-macromer carbon-carbon double bond ratio (DBR; 40 vs. 60), crosslinker type (PEG-diacrylate (PEGDA) vs. N,N'-methylene bisacrylamide (MB)), crosslinking extent of incorporated gelatin microparticles (low vs. high), and incubation medium composition (with or without collagenase) on the swelling and degradation characteristics of oligo[(poly(ethylene glycol) fumarate)] (OPF) hydrogel composites as indicated by the swelling ratio and the percentage of mass remaining, respectively. Each factor consisted of two levels, which were selected based on previous in vitro and in vivo studies utilizing these hydrogels for various tissue engineering applications. Fractional factorial analyses of the main effects indicated that the mean swelling ratio and the mean percentage of mass remaining of OPF composite hydrogels were significantly affected by every factor. In particular, increasing the PEG chain MW of OPF macromers significantly increased the mean swelling ratio and decreased the mean percentage of mass remaining by 5.7 ± 0.3 and 17.2 ± 0.6%, respectively. However, changing the crosslinker from MB to PEGDA reduced the mean swelling ratio and increased the mean percentage of mass remaining of OPF composite hydrogels by 4.9 ± 0.2 and 9.4 ± 0.9%, respectively. Additionally, it was found that the swelling characteristics of hydrogels fabricated with higher PEG chain MW or with MB were more sensitive to increases in DBR. Collectively, the main and cross effects observed between factors enables informed tuning of the swelling and degradation properties of OPF-based hydrogels for various tissue engineering applications. © 2013 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 102A: 3477-3487, 2014.