Characterization of photo-cross-linked oligo[poly(ethylene glycol) fumarate] hydrogels for cartilage tissue engineering

Open-Spaced Ridged Hydrogel Scaffolds Containing TiO2-Self-Assembled Monolayer of Phosphonates Promote Regeneration and Recovery Following Spinal Cord Injury

The spinal cord has a poor ability to regenerate after an injury, which may be due to cell loss, cyst formation, inflammation, and scarring. A promising approach to treating a spinal cord injury (SCI) is the use of biomaterials. We have developed a novel hydrogel scaffold fabricated from oligo(poly(ethylene glycol) fumarate) (OPF) as a 0.08 mm thick sheet containing polymer ridges and a cell-attractive surface on the other side. When the cells are cultured on OPF via chemical patterning, the cells attach, align, and deposit ECM along the direction of the pattern. Animals implanted with the rolled scaffold sheets had greater hindlimb recovery compared to that of the multichannel scaffold control, which is likely due to the greater number of axons growing across it. The immune cell number (microglia or hemopoietic cells: 50-120 cells/mm² in all conditions), scarring (5-10% in all conditions), and ECM deposits (Laminin or Fibronectin: approximately 10-20% in all conditions) were equal in all conditions. Overall, the results suggest that the scaffold sheets promote axon outgrowth that can be guided across the scaffold, thereby promoting hindlimb recovery. This study provides a hydrogel scaffold construct that can be used in vitro for cell characterization or in vivo for future neuroprosthetics, devices, or cell and ECM delivery.

Acoustic Force Elastography Microscopy

OBJECTIVE

Hydrogel scaffolds have attracted attention to develop cellular therapy and tissue engineering platforms for regenerative medicine applications. Among factors, local mechanical properties of scaffolds drive the functionalities of cell niche. Dynamic mechanical analysis (DMA), the standard method to characterize mechanical properties of hydrogels, restricts development in tissue engineering because the measurement provides a single elasticity value for the sample, requires direct contact, and represents a destructive evaluation preventing longitudinal studies on the same sample. We propose a novel technique, acoustic force elastography microscopy (AFEM), to evaluate elastic properties of tissue engineering scaffolds.

RESULTS

AFEM can resolve localized and two-dimensional (2D) elastic properties of both transparent and opaque materials with advantages of being non-contact and non-destructive. Gelatin hydrogels, neat synthetic oligo[poly(ethylene glycol)fumarate] (OPF) scaffolds, OPF hydroxyapatite nanocomposite scaffolds and ex vivo biological tissue were examined with AFEM to evaluate the elastic modulus. These measurements of Young's modulus range from approximately 2 kPa to over 100 kPa were evaluated and are in good agreement with finite element simulations, surface wave measurements, and DMA tests.

CONCLUSION

The AFEM can resolve localized and 2D elastic properties of hydrogels, scaffolds and thin biological tissues. These materials can either be transparent or non-transparent and their evaluation can be done in a non-contact and non-destructive manner, thereby facilitating longitudinal evaluation.

SIGNIFICANCE

AFEM is a promising technique to quantify elastic properties of scaffolds for tissue engineering and will be applied to provide new insights for exploring elastic changes of cell-laden scaffolds for tissue engineering and material science.

Current Understanding of the Applications of Photocrosslinked Hydrogels in Biomedical Engineering

Hydrogel materials have great application value in biomedical engineering. Among them, photocrosslinked hydrogels have attracted much attention due to their variety and simple convenient preparation methods. Here, we provide a systematic review of the biomedical-engineering applications of photocrosslinked hydrogels. First, we introduce the types of photocrosslinked hydrogel monomers, and the methods for preparation of photocrosslinked hydrogels with different morphologies are summarized. Subsequently, various biomedical applications of photocrosslinked hydrogels are reviewed. Finally, some shortcomings and development directions for photocrosslinked hydrogels are considered and proposed. This paper is designed to give researchers in related fields a systematic understanding of photocrosslinked hydrogels and provide inspiration to seek new development directions for studies of photocrosslinked hydrogels or related materials.

Size-dependent osteogenesis of black phosphorus in nanocomposite hydrogel scaffolds

A promising new strategy emerged in bone tissue engineering is to incorporate black phosphorus (BP) into polymer scaffolds, fabricating nanocomposite hydrogel platforms with biocompatibility, degradation controllability, and osteogenic capacity. BP quantum dot is a new concept and stands out recently among the BP family due to its tiny structure and a series of excellent characteristics. In this study, BP was processed into nanosheets of three different sizes via different exfoliation strategies and then incorporated into cross-linkable oligo[poly(ethylene glycol) fumarate] (OPF) to produce nanocomposite hydrogels for bone regeneration. The three different BP nanosheets were designated as BP-L, BP-M, and BP-S, with a corresponding diameter of 242.3 ± 90.0, 107.1 ± 47.9, and 18.8 ± 4.6 nm. The degradation kinetics and osteogenic capacity of MC3T3 pre-osteoblasts in vitro were both dependent on the BP size. BP exhibited a controllable degradation rate, which increased with the decrease of the size of the nanosheets, coupled with the release of phosphate in vitro. The osteogenic capacity of the hydrogels was promoted with the addition of all BP nanosheets, compared with OPF hydrogel alone. The smallest BP quantum dots was shown to be optimal in enhancing MC3T3 cell behaviors, including spreading, distribution, proliferation, and differentiation on the OPF hydrogels. These results reinforced that the supplementation of BP quantum dots into OPF nanocomposite hydrogel scaffolds could potentially find application in the restoration of bone defects.

Polymeric Hydrogels for Controlled Drug Delivery to Treat Arthritis

Rheumatoid arthritis (RA) and osteoarthritis (OA) are disabling musculoskeletal disorders that affect joints and cartilage and may lead to bone degeneration. Conventional delivery of anti-arthritic agents is limited due to short intra-articular half-life and toxicities. Innovations in polymer chemistry have led to advancements in hydrogel technology, offering a versatile drug delivery platform exhibiting tissue-like properties with tunable drug loading and high residence time properties This review discusses the advantages and drawbacks of polymeric materials along with their modifications as well as their applications for fabricating hydrogels loaded with therapeutic agents (small molecule drugs, immunotherapeutic agents, and cells). Emphasis is given to the biological potentialities of hydrogel hybrid systems/micro-and nanotechnology-integrated hydrogels as promising tools. Applications for facile tuning of therapeutic drug loading, maintaining long-term release, and consequently improving therapeutic outcome and patient compliance in arthritis are detailed. This review also suggests the advantages, challenges, and future perspectives of hydrogels loaded with anti-arthritic agents with high therapeutic potential that may alter the landscape of currently available arthritis treatment modalities.

Injectable pH-responsive adhesive hydrogels for bone tissue engineering inspired by the underwater attachment strategy of marine mussels

A major challenge in tissue engineering is the development of alternatives to traditional bone autografts and allografts that can regenerate critical-sized bone defects. Here we present the design of injectable pH-responsive double-crosslinked adhesive hydrogels inspired by the molecular mechanism and environmental post-processing of marine mussel adhesive. Nine adhesive hydrogel formulations were developed through the conjugation of crosslinkable catechol functional groups (DOPA) and the synthetic oligomer oligo[poly(ethylene glycol) fumarate] (OPF), varying the DOPA content (w/w%) and molecular weight (MW) of the OPF backbone to produce formulations with a range of swelling ratios, porosities, and crosslink densities. DOPA incorporation altered the surface chemistry, mechanical properties, and surface topography of hydrogels, resulting in an increase in material stiffness, slower degradation, and enhanced pre-osteoblast cell attachment and proliferation. When injected within simulated bone defects, DOPA-mediated interfacial adhesive interactions also prevented the displacement of scaffolds, an effect that was maintained even after swelling within physiological conditions. Taken together, OPF-DOPA hydrogels represent a promising new material to enhanced tissue integration and the prevention of the post-implantation migration of scaffolds that can occur due to biomechanical loading in vivo.

Black phosphorus incorporation modulates nanocomposite hydrogel properties and subsequent MC3T3 cell attachment, proliferation, and differentiation

A promising strategy that emerged in tissue engineering is to incorporate two-dimensional (2D) materials into polymer scaffolds, producing materials with desirable mechanical properties and surface chemistries, which also display broad biocompatibility. Black phosphorus (BP) is a 2D material that has sparked recent scientific interest due to its unique structure and electrochemical characteristics. In this study, BP nanosheets (BPNSs) were incorporated into a cross-linkable oligo[poly(ethylene glycol) fumarate] (OPF) hydrogel to produce a new nanocomposite for bone regeneration. BPNSs exhibited a controllable degradation rate coupled with the release of phosphate in vitro. MTS assay results together with live/dead images confirmed that the introduction of BPNSs into OPF hydrogels enhanced MC3T3-E1 cell proliferation. Moreover, the morphology parameters indicated better attachments of cells in the BPNSs containing group. Immunofluorescence images as well as intercellular ALP and OCN activities showed that adding a certain amount of BPNSs to OPF hydrogel could greatly improve differentiation of pre-osteoblasts on the hydrogel. Additionally, embedding black phosphorous into a neutral polymer network helped to control its cytotoxicity, with optimal cell growth observed at BP concentrations as high as 500 ppm. These results reinforced that the supplementation of OPF with BPNSs can increase the osteogenic capacity of polymer scaffolds for use in bone tissue engineering.

SDF-1α/OPF/BP Composites Enhance the Migrating and Osteogenic Abilities of Mesenchymal Stem Cells

In situ cell recruitment is a promising regenerative medicine strategy with the purpose of tissue regeneration without stem cell transplantation. This chemotaxis-based strategy is aimed at ensuring a restorative environment through the release of chemokines that promote site-specific migration of healing cell populations. Stromal cell-derived factor-1α (SDF-1α) is a critical chemokine that can regulate the migration of mesenchymal stem cells (MSCs). Accordingly, here, SDF-1α-loaded microporous oligo[poly(ethylene glycol) fumarate]/bis[2-(methacryloyloxy)ethyl] phosphate composites (SDF-1α/OPF/BP) were engineered and probed. SDF-1α/OPF/BP composites were loaded with escalating SDF-1α concentrations, namely, 0 ng/ml, 50 ng/ml, 100 ng/ml, and 200 ng/ml, and were cocultured with MSC. Scratching assay, Transwell assay, and three-dimensional migration model were utilized to assess the migration response of MSCs. Immunofluorescence staining of Runx2 and osteopontin (OPN), ELISA assay of osteocalcin (OCN) and alkaline phosphatase (ALP), and Alizarin Red S staining were conducted to assess the osteogenesis of MSCs. All SDF-1α/OPF/BP composites engendered a release of SDF-1α (>80%) during the first four days. SDF-1α released from the composites significantly promoted migration and osteogenic differentiation of MSCs documented by upregulated expression of osteogenic-related proteins, ALP, Runx2, OCN, and OPN. SDF-1α at 100 ng/ml was optimal for enhanced migration and osteogenic proficiency. Thus, designed SDF-1α/OPF/BP composites were competent in promoting the homing and osteogenesis of MSCs and thus offer a promising bioactive scaffold candidate for on-demand bone tissue regeneration.

Acid Scavenger Free Synthesis of Oligo(Poly(Ethylene Glycol) Fumarate) Utilizing Inert Gas Sparging

The macromolecule oligo(poly(ethylene glycol) fumarate) (OPF) exhibits promising attributes for creating suitable three-dimensional hydrogel environments to study cell behavior, deliver therapeutics, and serve as a degradable, nonfouling material. However, traditional synthesis techniques are time consuming, contain salt contaminants, and generate significant waste. These issues have been overcome with an alternative, one-pot approach that utilizes inert gas sparging. Departing from previous synthetic schemes that require acid scavengers, inert gas sparging removes byproducts in situ, eliminating significant filtration and postprocessing steps, while allowing a more uniform product. Characterized by nuclear magnetic resonance, gel permeation chromatography, and differential scanning calorimetry, nitrogen sparge synthesis yields an OPF product with greater polymer length than traditional acid scavenger synthesis methods. Furthermore, nitrogen-sparged OPF readily crosslinks using either ultraviolet or thermal initiator methods with or without the addition of short-chain diacrylate units, allowing for greater tunability in hydrogel properties with little to no cytotoxicity. Overall, inert gas sparging provides a longer chain and cleaner polymer product for hydrogel material studies while maintaining degradable characteristics. Impact statement Using nitrogen sparging, we have demonstrated that oligo(poly(ethylene glycol) fumarate) (OPF) can be produced with decreased postprocessing, increased product purity, greater oligomerization, and cell viability. These properties lead to greater tunability in mechanical properties and a more versatile hydrogel for biomedical applications. The simplification of synthesis and elimination of impurities will expand the utility of OPF as a degradable hydrogel for cell culture, tissue engineering, regenerative medicine, and therapeutic delivery, among other applications.

Promoting Neuronal Outgrowth Using Ridged Scaffolds Coated with Extracellular Matrix Proteins

Spinal cord injury (SCI) results in cell death, demyelination, and axonal loss. The spinal cord has a limited ability to regenerate, and current clinical therapies for SCI are not effective in helping promote neurologic recovery. We have developed a novel scaffold biomaterial that is fabricated from the biodegradable hydrogel oligo(poly(ethylene glycol)fumarate) (OPF). We have previously shown that positively charged OPF scaffolds (OPF+) in an open spaced, multichannel design can be loaded with Schwann cells to support axonal generation and functional recovery following SCI. We have now developed a hybrid OPF+ biomaterial that increases the surface area available for cell attachment and that contains an aligned microarchitecture and extracellular matrix (ECM) proteins to better support axonal regeneration. OPF+ was fabricated as 0.08 mm thick sheets containing 100 μm high polymer ridges that self-assemble into a spiral shape when hydrated. Laminin, fibronectin, or collagen I coating promoted neuron attachment and axonal outgrowth on the scaffold surface. In addition, the ridges aligned axons in a longitudinal bipolar orientation. Decreasing the space between the ridges increased the number of cells and neurites aligned in the direction of the ridge. Schwann cells seeded on laminin coated OPF+ sheets aligned along the ridges over a 6-day period and could myelinate dorsal root ganglion neurons over 4 weeks. This novel scaffold design, with closer spaced ridges and Schwann cells, is a novel biomaterial construct to promote regeneration after SCI.

Polyglycerol-based hydrogels and nanogels: from synthesis to applications

Hydrogels and nanogels have emerged as promising materials for biomedical applications owing to their large surface area and tunable mechanical and chemical properties. Their large surface area is well suited for bioconjugation, whilst the interior porous network can be utilized for the transport of valuable biomolecules. The use of biocompatible hydrophilic building blocks/linkers for the preparation of hydrogels and nanogels not only avoids undesired side effects within the biological system, but also retains high water content, thereby creating an environment which is very similar to extracellular matrix. Their tunable multivalency and hydrophilicity and excellent biocompatibility, together with ease of functionalization, makes polyglycerol macromonomers well suited for synthesizing cross-linked networks that can be used as extracellular matrix mimics. Here we provide an overview of the synthesis of polyglycerol-based hydrogels and nanogels for various biomedical applications.

Amelioration of imiquimod-induced psoriasis-like dermatitis in mice by DSW therapy inspired hydrogel

Psoriasis is a long-lasting and recurrent autoimmune disease which is incurable so far. Dead Sea water (DSW) therapy is an effective approach to help control the symptoms of psoriasis due to the abundant mineral ions in DSW, which inspired the material formulation in this study. Rubidium-Sodium alginate/Polyacrylamide hydrogels (Rb-SA/PAAm gels) composed of sodium alginate and polyacrylamide interpenetrating network structure with different concentrations of rubidium and certain magnesium and zinc content were prepared for the treatment of psoriasis. The obtained results suggest the good mechanical properties of the Rb-SA/PAAm gels including toughness and swelling performance. In terms of in vitro tests, the Rb-SA/PAAm gels not only show nontoxicity to human keratinocyte cell line (Hacats) but also inhibits the activity against inflammatory NF-κβ signaling pathway. Meanwhile, they can release Rb+ which enable the Rb-SA/PAAm gels have better antibacterial ability to Streptococcus and Escherichia coli. The results obtained from in vivo tests indicate that these hydrogels could alleviate the symptoms of psoriasis caused by Imiquimod (IMQ) in mice by reducing the inflammatory factor in STAT3 pathway and therefore reduce the immune stimulation of the spleen. In conclusion, the 100Rb-SA/PAAm gel has demonstrated great potential to be a topical wettable dressing for psoriasis treatment.

Mesenchymal stem cell spheroids incorporated with collagen and black phosphorus promote osteogenesis of biodegradable hydrogels

Mesenchymal stem cell (MSC)-spheroids have sparked significant interest in bone tissue engineering due to their resemblance to natural bone tissue, especially in terms of cell-cell and cell-extracellular matrix interactions. Many biomaterials or biomolecules have been incorporated into MSC-spheroids to enhance their osteogenic abilities. In this respect, we assessed the osteogenic responses of MSC spheroids leveraged through the unique combination of collagen and black phosphorus (BP). The MSC spheroids were successfully constructed with 6 μg/mL collagen and/or a concentration gradient (0 μg/mL, 4 μg/mL, 8 μg/mL, and 16 μg/mL) of BP and were evaluated for MSC viability and their osteogenic differentiation over a time period of 14 days. Improved MSC viability and osteogenic ability were observed for the spheroids with collagen and BP at the concentration of 4 μg/mL and 8 μg/mL. Next, blank spheroids (Control) or the optimized MSC spheroids with 6 μg/mL collagen and 4 μg/mL BP (Col+BP4) were further encapsulated into two types of hydrogel scaffolds: porous oligo[poly(ethylene glycol) fumarate] (OPF) hydrogel and hydroxyapatite-collagen I scaffold (HE-COL). The osteogenic abilities of these four groups were evaluated after 14 and 21 days of osteogenic induction. The MSC spheroids incorporated with collagen and BP implanted into OPF porous hydrogel (Col+BP/OPF) elicited a higher expression of Runx2, osteopontin, and alkaline phosphatase than blank spheroids implanted into OPF porous hydrogel (Control/OPF). Enhanced osteogenesis was also observed in the Col+BP/HE-COL group as compared to Control/HE-COL. Taken together, the results from this study showed the perspectives of collagen and BP incorporated MSC spheroids for the development of injectable cellular therapies for bone regeneration.

Injectable Electrical Conductive and Phosphate Releasing Gel with Two-Dimensional Black Phosphorus and Carbon Nanotubes for Bone Tissue Engineering

Injectable hydrogels have unique advantages for the repair of irregular tissue defects. In this study, we report a novel injectable carbon nanotube (CNT) and black phosphorus (BP) gel with enhanced mechanical strength, electrical conductivity, and continuous phosphate ion release for tissue engineering. The gel utilized biodegradable oligo(poly(ethylene glycol) fumarate) (OPF) polymer as the cross-linking matrix, with the addition of cross-linkable CNT-poly(ethylene glycol)-acrylate (CNTpega) to grant mechanical support and electric conductivity. Two-dimensional (2D) black phosphorus nanosheets were also infused to aid in tissue regeneration through the steady release of phosphate that results from environmental oxidation of phosphorus in situ. This newly developed BP-CNTpega-gel was found to enhance the adhesion, proliferation, and osteogenic differentiation of MC3T3 preosteoblast cells. With electric stimulation, the osteogenesis of preosteoblast cells was further enhanced with elevated expression of several key osteogenic pathway genes. As monitored with X-ray imaging, the BP-CNTpega-gel demonstrated excellent in situ gelation and cross-linking to fill femur defects, vertebral body cavities, and posterolateral spinal fusion sites in the rabbit. Together, these results indicate that this newly developed injectable BP-CNTpega-gel owns promising potential for future bone and broad types of tissue engineering applications.

Hydrosoluble collagen based biodegradable hybrid hydrogel for biomedical scaffold

Hydrogel scaffolds are explored as efficient methods to repair damaged organs or tissues. In this study, we developed a hybrid hydrogel system based on collagen (Col) and PEG-derived polymer (PEGF) for biomedical scaffold. The Col-PEGF hybrid hydrogel, in which different materials were combined and sequential interpenetrating networks were built, achieved significantly enhanced mechanical strength and viscoelasticity compared with the corresponding Col hydrogel or PEGF hydrogel. Degradation test indicated that Col enabled the hybrid hydrogel to be broken down via enzymatic degradation while PEGF contributed to the anti-degradation of the hydrogel. This balanced biodegradability of Col-PEGF hydrogel would be advantageous to the application for tissue engineering and regenerative medicine. Moreover, the Col-PEGF hybrid hydrogel with micron-sized pores and variable moisture performed good biocompatibility to NIH-3T3 cells, and supplied a favorable environment for cell growth and proliferation. Therefore, the Col-PEGF hydrogel will provide a promising biomedical scaffold for the therapy of tissue defects.

Phosphate functionalization and enzymatic calcium mineralization synergistically enhance oligo[poly(ethylene glycol) fumarate] hydrogel osteoconductivity for bone tissue engineering

A current approach in bone tissue engineering is the implantation of polymeric scaffolds that promote osteoblast attachment and growth as well as biomineralization. One promising polymer is oligo[poly(ethylene glycol) fumarate] (OPF), a polyethylene glycol-based material that is biocompatible, injectable, and biodegradable, but in its native form does not support robust bone cell attachment or growth. To address this issue, this study evaluated the osteoconductivity of bis[02-(methacryloyloxy)ethyl] phosphate (BP) functionalized OPF hydrogels (OPF-BP) using MC3T3-E1 pre-osteoblast cells, both before and after enzymatic mineralization with a calcium solution. The inclusion of negatively charged functional groups allowed for the tailored uptake and release of calcium, while also altering the mechanical properties and surface topography of the hydrogel surface. In cell culture, OPF-BP hydrogels with 20 and 30% (w/w) BP optimized osteoblast attachment, proliferation, and differentiation after a 21-day in vitro period. In addition, the OPF-BP30 treatment, when mineralized with calcium, exhibited a 128% increase in osteocalcin expression when compared with the non-mineralized treatment. These findings suggest that phosphate functionalization and enzymatic calcium mineralization can act synergistically to enhance the osteoconductivity of OPF hydrogels, making this processed material an attractive candidate for bone tissue engineering applications.

Hydrogel scaffolds for tissue engineering: the importance of polymer choice

We explore the design and synthesis of hydrogel scaffolds for tissue engineering from the perspective of the underlying polymer chemistry. The key polymers, properties and architectures used, and their effect on tissue growth are discussed.

Effect of Biomaterial Electrical Charge on Bone Morphogenetic Protein-2-Induced In Vivo Bone Formation

IMPACT STATEMENT

Biomaterials can play a dual role in bone regeneration: they enable local sustained delivery of growth factors, such as bone morphogenetic protein-2 (BMP-2), while they provide structural support as scaffold. By better imitating the properties of native bone tissue, scaffolds may be both osteoconductive and osteoinductive. The latter can be achieved by modifying the electrical charge of the surface. The present work uses tunable oligo[(polyethylene glycol) fumarate] hydrogel and demonstrates that negative charge enhances BMP-2-induced bone formation compared with neutral or positive charge. Altogether, this indicates that tissue-specific surface charge modifications of biomaterials hold great promise in the field of tissue regeneration.

In Vitro and In Vivo Correlation of Bone Morphogenetic Protein-2 Release Profiles from Complex Delivery Vehicles

Local sustained delivery of bioactive molecules from biomaterials is a promising strategy to enhance bone regeneration. To optimize delivery vehicles for bone formation, the design characteristics are tailored with consequential effect on bone morphogenetic protein-2 (BMP-2) release and bone regeneration. Complying with the 3R principles (Replacement, Reduction, and Refinement), the growth factor release is often investigated in vitro using several buffers to mimic the in vivo physiological environment. However, this remains an unmet need. Therefore, this study investigates the in vitro-in vivo correlation (IVIVC) of BMP-2 release from complex delivery vehicles in several commonly used in vitro buffers: cell culture model, phosphate buffered saline, and a strong desorption buffer. The results from this study showed that the release environment affected the BMP-2 release profiles, creating distinct relationships between release versus time and differences in extent of release. According to the guidance set by the U.S. Food and Drug Administration (FDA), IVIVC resulted in level A internal predictability for individual composites. Since the IVIVC was influenced by the BMP-2 loading method and composite surface chemistry, the external predictive value of the IVIVCs was limited. These results show that the IVIVCs can be used for predicting the release of an individual composite. However, the models cannot be used for predicting in vivo release for different composite formulations since they lack external predictability. Potential confounding effects of drug type, delivery vehicle formulations, and application site should be added to the equation to develop one single IVIVC applicable for complex delivery vehicles. Altogether, these results imply that more sophisticated in vitro systems should be used in bone regeneration to accurately discriminate and predict in vivo BMP-2 release from different complex delivery vehicles.

Biocompatibility of synthetic ultraviolet radiation cross-linked polymers - Subcutaneous implantation study

Photo-cross-linked polymers have attracted a lot of attention in the biomedical field. The main benefits of these materials are related to the fact that they are most of the time viscous liquids or pastes that adapt a custom and fixed shape on demand of the user. Present study deals specifically with the biological response upon subcutaneous implantation of four different materials in rabbits. In the study 20 rabbits were divided into four groups (each five rabbits): Groups 1-3 were implanted with tested new obtained by us macromonomers (P1838-DMA; P1838-UR; PDEGA-UR - respectively), while group 4 (control) was implanted with the mesh (PLA) routinely used for surgical treatment of a hernia. The new compounds were polarized earlier using ultraviolet radiation to obtain cross-linked networks. The polymers in the form of discs were then implanted subcutaneously in dorsal region of rabbits. After 28 days polymers were explanted and examined. Microscopic observation evaluated: thickness of the connective tissue capsule around the discs, cells of inflammatory response, disc surface erosion, spectroscopic analysis. The examined materials cause no chronic inflammation, abscesses or tissue necrosis, and the biological response is similar to observed in control group. Therefore, new synthetic materials could be considered as biocompatible and safe. Materials undergo slow degradation of ester bonds and surface erosion and degradation products could be eliminated probably by phagocytosis. On the basis on the afore mentioned knowledge, we formulated hypothesis, that the new polymers are well tolerated by the adjacent tissues. The aim of the following study was to examine reaction of the tissue on new types of prepolymerized material implanted subcutaneously. The obtained results suggest, that the new UV cross-linked polymers do not affect negatively on the connective tissue that is in the contact with the implants. Furthermore, the used materials are in the liquid form, thus they could be easily performed in in minimally invasive laparoscopic treatment of abdominal hernias. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 1889-1897, 2019.

Cell-Surface Engineering for Advanced Cell Therapy

Stem cells opened great opportunity to overcome diseases that conventional therapy had only limited success. Use of scaffolds made from biomaterials not only helps handling of stem cells for delivery or transplantation but also supports enhanced cell survival. Likewise, cell encapsulation can provide stability for living animal cells even in a state of separateness. Although various chemical reactions were tried to encapsulate stolid microbial cells such as yeasts, a culture environment for the growth of animal cells allows only highly biocompatible reactions. Therefore, the animal cells were mostly encapsulated in hydrogels, which resulted in enhanced cell survival. Interestingly, major findings of chemistry on biological interfaces demonstrate that cell encapsulation in hydrogels have a further a competence for modulating cell characteristics that can go beyond just enhancing the cell survival. In this review, we present a comprehensive overview on the chemical reactions applied to hydrogel-based cell encapsulation and their effects on the characteristics and behavior of living animal cells.

Current advanced therapy cell-based medicinal products for type-1-diabetes treatment

In the XXI century diabetes mellitus has become one of the main threats to human health with higher incidence in regions such as Europe and North America. Type 1 diabetes mellitus (T1DM) occurs as a consequence of the immune-mediated destruction of insulin producing β-cells located in the endocrine part of the pancreas, the islets of Langerhans. The administration of exogenous insulin through daily injections is the most prominent treatment for T1DM but its administration is frequently associated to failure in glucose metabolism control, finally leading to hyperglycemia episodes. Other approaches have been developed in the past decades, such as whole pancreas and islet allotransplantation, but they are restricted to patients who exhibit frequent episodes of hypoglycemia or renal failure because the lack of donors and islet survival. Moreover, patients transplanted with either whole pancreas or islets require of immune suppression to avoid the rejection of the transplant. Currently, advanced therapy medicinal products (ATMP), such as implantable devices, have been developed in order to reduce immune rejection response while increasing cell survival. To overcome these issues, ATMPs must promote vascularization, guaranteeing the nutritional contribution, while providing O₂ until vasculature can surround the device. Moreover, it should help in the immune-protection to avoid acute and chronic rejection. The transplanted cells or islets should be embedded within biomaterials with tunable properties like injectability, stiffness and porosity mimicking natural ECM structural characteristics. And finally, an infinitive cell source that solves the donor scarcity should be found such as insulin producing cells derived from mesenchymal stem cells (MSCs), embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs). Several companies have registered their ATMPs and future studies envision new prototypes. In this review, we will discuss the mechanisms and etiology of diabetes, comparing the clinical trials in the last decades in order to define the main characteristics for future ATMPs.

Cross-linkable graphene oxide embedded nanocomposite hydrogel with enhanced mechanics and cytocompatibility for tissue engineering

Graphene oxide (GO) is an attractive material that can be utilized to enhance the modulus and conductivities of substrates and hydrogels. To covalently cross-link graphene oxide sheets into hydrogels, abundant cross-linkable double bonds were introduced to synthesize the graphene-oxide-tris-acrylate sheet (GO-TrisA). Polyacrylamide (PAM) nanocomposite hydrogels were then fabricated with inherent covalently and permanently cross-linked GO-TrisA sheets. Results showed that the covalently cross-linked GO-TrisA/PAM nanocomposite hydrogel had enhanced mechanical strength, thermo stability compared with GO/PAM hydrogel maintained mainly by hydrogen bonding between PAM chains and GO sheets. In vitro cell study showed that the covalently cross-linked rGO-TrisA/PAM nanocomposite hydrogel had excellent cytocompatibility after in situ reduction. These results suggest that rGO-TrisA/PAM nanocomposite hydrogel holds great potential for tissue engineering applications. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 1247-1257, 2018.

Current and novel injectable hydrogels to treat focal chondral lesions: Properties and applicability

Focal chondral lesions and early osteoarthritis (OA) are responsible for progressive joint pain and disability in millions of people worldwide, yet there is currently no surgical joint preservation treatment available to fully restore the long term functionality of cartilage. Limitations of current treatments for cartilage defects have prompted the field of cartilage tissue engineering, which seeks to integrate engineering and biological principles to promote the growth of new cartilage to replace damaged tissue. Toward improving cartilage repair, hydrogel design has advanced in recent years to improve their utility. Injectable hydrogels have emerged as a promising scaffold due to their wide range of properties, the ability to encapsulate cells within the material, and their ability to provide cues for cell differentiation. Some of these advances include the development of improved control over in situ gelation (e.g., light), new techniques to process hydrogels (e.g., multi-layers), and better incorporation of biological signals (e.g., immobilization, controlled release, and tethering). This review summarises the innovative approaches to engineer injectable hydrogels toward cartilage repair. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:64-75, 2018.

Electrically conductive nanocomposite hydrogels embedded with functionalized carbon nanotubes for spinal cord injury

Electrically conductive hydrogels incorporated with CNTs support PC12 cell growth and differentiation and hold promise for nerve regeneration.

Local Heterogeneities Improve Matrix Connectivity in Degradable and Photoclickable Poly(ethylene glycol) Hydrogels for Applications in Tissue Engineering

Hydrolytically degradable poly(ethylene glycol) (PEG) hydrogels are promising platforms for cell encapsulation and tissue engineering. However, hydrolysis leads to bulk degradation and a decrease in hydrogel mechanical integrity. Despite these challenges, hydrolytically degradable hydrogels have supported macroscopic neotissue growth. The goal of this study was to combine experimental methods with a multiscale mathematical model to analyze hydrogel degradation concomitant with neocartilage growth in PEG hydrogels. Primary bovine chondrocytes were encapsulated at increasing densities (50, 100, and 150 million cells/mL of precursor solution) in a radical-mediated photoclickable hydrogel formed from 8-arm PEG-co-caprolactone end-capped with norbornene and cross-linked with PEG dithiol. Two observations were made in the experimental system: (1) the cell distribution was not uniform and cell clustering was evident, which increased with increasing cell density and (2) a significant decrease in the initial hydrogel compressive modulus was observed with increasing cell concentration. By introducing heterogeneities in the form of cell clusters and spatial variations in the network structure around cells, the mathematical model explained the drop in initial modulus and captured the experimentally observed spatial evolution of ECM and the construct modulus as a function of cell density and culture time. Overall, increasing cell density led to improved ECM formation, ECM connectivity, and overall modulus. This study strongly points to the importance of heterogeneities within a cell-laden hydrogel in retaining mechanical integrity as the construct transitions from hydrogel to neotissue.

Functionalized Carbon Nanotube and Graphene Oxide Embedded Electrically Conductive Hydrogel Synergistically Stimulates Nerve Cell Differentiation

Nerve regeneration after injury is a critical medical issue. In previous work, we have developed an oligo(poly(ethylene glycol) fumarate) (OPF) hydrogel incorporated with positive charges as a promising nerve conduit. In this study, we introduced cross-linkable bonds to graphene oxide and carbon nanotube to obtain the functionalized graphene oxide acrylate (GOa) and carbon nanotube poly(ethylene glycol) acrylate (CNTpega). An electrically conductive hydrogel was then fabricated by covalently embedding GOa and CNTpega within OPF hydrogel through chemical cross-linking followed by in situ reduction of GOa in l-ascorbic acid solution. Positive charges were incorporated by 2-(methacryloyloxy)ethyltrimethylammonium chloride (MTAC) to obtain rGOaCNTpega-OPF-MTAC composite hydrogel with both surface charge and electrical conductivity. The distribution of CNTpega and GOa in the hydrogels was substantiated by transmission electron microscopy (TEM), and strengthened electrical conductivities were determined. Excellent biocompatibility was demonstrated for the carbon embedded composite hydrogels. Biological evaluation showed enhanced proliferation and spreading of PC12 cells on the conductive hydrogels. After induced differentiation using nerve growth factor (NGF), cells on the conductive hydrogels were effectively stimulated to have robust neurite development as observed by confocal microscope. A synergistic effect of electrical conductivity and positive charges on nerve cells was also observed in this study. Using a glass mold method, the composite hydrogel was successfully fabricated into conductive nerve conduits with surficial positive charges. These results suggest that rGOa-CNTpega-OPF-MTAC composite hydrogel holds great potential as conduits for neural tissue engineering.

A New Vertebral Body Replacement Strategy Using Expandable Polymeric Cages

We have developed a novel polymeric expandable cage that can be delivered via a posterior-only surgical approach for the treatment of noncontained vertebral defects. This approach is less invasive than an anterior-only or combined approach and much more cost-effective than currently used expandable metal cages. The polymeric expandable cage is composed of oligo poly(ethylene glycol) fumarate (OPF), a hydrogel that has been previously shown to have excellent nerve and bone tissue biocompatibility. OPF hydrogel cages can expand to twice their original diameter and length within a surgical time frame following hydration. Modulation of parameters such as polymeric network crosslink density or the introduction of charge to the network allowed for precise expansion kinetics. To meet specific requirements due to size variations in patient vertebral bodies, we fabricated a series of molds with varied diameters and explored the expansion kinetics of the OPF cages. Results showed a stable expansion ratio of approximately twofold to the original size within 20 min, regardless of the absolute value of the cage size. Following implantation of a dried OPF cage into a noncontained vertebral defect and its in situ expansion with normal saline, other augmentation biomaterials, such as poly(propylene fumarate) (PPF), can be injected to the lumen of the OPF cage and allowed to crosslink in situ. The OPF/PPF composite scaffold can provide the necessary rigidity and stability to the augmented spine.

Covalent crosslinking of graphene oxide and carbon nanotube into hydrogels enhances nerve cell responses

Healing of nerve injuries is a critical medical issue. Biodegradable polymeric conduits are a promising therapeutic solution to provide guidance for axon growth in a given space, thus helping nerve heal. Extensive studies in the past decade reported that conductive materials could effectively increase neurite and axon extension in vitro and nerve regeneration in vivo. In this study, graphene oxide and carbon nanotubes were covalently functionalized with double bonds to obtain crosslinkable graphene oxide acrylate (GOa) sheets and carbon nanotube poly(ethylene glycol) acrylate (CNTpega). An electrically conductive reduced GOa-CNTpega-oligo(polyethylene glycol fumarate) (OPF) hydrogel (rGOa-CNTpega-OPF) was successfully fabricated by chemically crosslinking GOa sheets and CNTpega with OPF chains followed by in situ chemical reduction in l-ascorbic acid solution. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) imaging showed homogenous distribution of GOa/CNTpega carbon content in the rGOa-CNTpega-OPF composite hydrogel, resulting in a significant increase of electrical conductivity compared with neutral OPF without carbon content. Cell studies showed excellent biocompatibility and distinguished PC12 cell proliferation and spreading on the rGOa-CNTpega-OPF composite hydrogel. Fluorescent microscopy imaging demonstrated robustly stimulated neurite development in these cells on a conductive rGOa-CNTpega-OPF composite hydrogel compared with that on neutral OPF hydrogels. These results illustrated a promising potential for the rGOa-CNTpega-OPF composite hydrogel to serve as conduits for neural tissue engineering.

Positively Charged Oligo[Poly(Ethylene Glycol) Fumarate] Scaffold Implantation Results in a Permissive Lesion Environment after Spinal Cord Injury in Rat

Positively charged oligo[poly(ethylene glycol) fumarate] (OPF+) scaffolds loaded with Schwann cells bridge spinal cord injury (SCI) lesions and support axonal regeneration in rat. The regeneration achieved is not sufficient for inducing functional recovery. Attempts to increase regeneration would benefit from understanding the effects of the scaffold and transplanted cells on lesion environment. We conducted morphometric and stereological analysis of lesions in rats implanted with OPF+ scaffolds with or without loaded Schwann cells 1, 2, 3, 4, and 8 weeks after thoracic spinal cord transection. No differences were found in collagen scarring, cyst formation, astrocyte reactivity, myelin debris, or chondroitin sulfate proteoglycan (CSPG) accumulation. However, when scaffold-implanted animals were compared with animals with transection injuries only, these barriers to regeneration were significantly reduced, accompanied by increased activated macrophages/microglia. This distinctive and regeneration permissive tissue reaction to scaffold implantation was independent of Schwann cell transplantation. Although the tissue reaction was beneficial in the short term, we observed a chronic fibrotic host response, resulting in scaffolds surrounded by collagen at 8 weeks. This study demonstrates that an appropriate biomaterial scaffold improves the environment for regeneration. Future targeting of the host fibrotic response may allow increased axonal regeneration and functional recovery.

Hydrolytically degradable, dendritic polyglycerol sulfate based injectable hydrogels using strain promoted azide–alkyne cycloaddition reaction

An anionic degradable hydrogel based on a heparin mimetic polymer was prepared using PEG-PCL-DIC as a crosslinker.

Control of Mesh Size and Modulus by Kinetically Dependent Cross-Linking in Hydrogels

Kinetically controlled cross-linking processes produce mechanically distinguishable hydrogels using identical precursor chemistry. The oxime ligation demonstrates tunable reaction kinetics with pH and buffer strength, which induce changes in the structural features of hydrogels and determine their mechanical properties. Small-angle neutron scattering and swelling studies provide an insight into how structural properties correlate with mechanical properties for this hydrogel system.

Comparison of cellular architecture, axonal growth, and blood vessel formation through cell-loaded polymer scaffolds in the transected rat spinal cord

The use of multichannel polymer scaffolds in a complete spinal cord transection injury serves as a deconstructed model that allows for control of individual variables and direct observation of their effects on regeneration. In this study, scaffolds fabricated from positively charged oligo[poly(ethylene glycol)fumarate] (OPF(+)) hydrogel were implanted into rat spinal cords following T9 complete transection. OPF(+) scaffold channels were loaded with either syngeneic Schwann cells or mesenchymal stem cells derived from enhanced green fluorescent protein transgenic rats (eGFP-MSCs). Control scaffolds contained extracellular matrix only. The capacity of each scaffold type to influence the architecture of regenerated tissue after 4 weeks was examined by detailed immunohistochemistry and stereology. Astrocytosis was observed in a circumferential peripheral channel compartment. A structurally separate channel core contained scattered astrocytes, eGFP-MSCs, blood vessels, and regenerating axons. Cells double-staining with glial fibrillary acid protein (GFAP) and S-100 antibodies populated each scaffold type, demonstrating migration of an immature cell phenotype into the scaffold from the animal. eGFP-MSCs were distributed in close association with blood vessels. Axon regeneration was augmented by Schwann cell implantation, while eGFP-MSCs did not support axon growth. Methods of unbiased stereology provided physiologic estimates of blood vessel volume, length and surface area, mean vessel diameter, and cross-sectional area in each scaffold type. Schwann cell scaffolds had high numbers of small, densely packed vessels within the channels. eGFP-MSC scaffolds contained fewer, larger vessels. There was a positive linear correlation between axon counts and vessel length density, surface density, and volume fraction. Increased axon number also correlated with decreasing vessel diameter, implicating the importance of blood flow rate. Radial diffusion distances in vessels significantly correlated to axon number as a hyperbolic function, showing a need to engineer high numbers of small vessels in parallel to improving axonal densities. In conclusion, Schwann cells and eGFP-MSCs influenced the regenerating microenvironment with lasting effect on axonal and blood vessel growth. OPF(+) scaffolds in a complete transection model allowed for a detailed comparative, histologic analysis of the cellular architecture in response to each cell type and provided insight into physiologic characteristics that may support axon regeneration.

Multi-scale mechanical and transport properties of a hydrogel

In this paper, molecular dynamic simulation was used to study the effect of water on the equilibrated structure and mechanical properties of cross-linked hydrogel at multiple scales. The hydrogel consisted of Polyethylene glycol diglycidyl ether (PEGDGE) as epoxy and the Jeffamine, poly-oxy-alkylene-amines, as curing agent. The results for systems with various water contents indicated that the cross-links were more hydrophilic within the hydrogel structure. Effects of cross-linking on the transport properties were also investigated by computing diffusion coefficients of water molecules. A new Coarse-Grained (CG) scheme for hydrogels is proposed, and validated by comparing the transport properties with the all-atom method, demonstrating the capability of the model to capture the correct dynamic evolution of the system. The all-atom model of the hydrogel was mapped to the CG model using the MARTINI force field. This method resulted in a more realistic representation of the stiffness of the system, compared to the previous experimental studies in the literature. The variation of the stiffness of the hydrogel as a function of the water content showed that 40% water content is the optimal value for mechanical performance of the hydrogel.

Controlled release of doxorubicin from pH-responsive microgels

Stimuli-responsive hydrogels have enormous potential in drug delivery applications. They can be used for site-specific drug delivery due to environmental variables in the body such as pH and temperature. In this study, we have developed pH-responsive microgels for the delivery of doxorubicin (DOX) in order to optimize its anti-tumor activity while minimizing its systemic toxicity. We used a copolymer of oligo(polyethylene glycol) fumarate (OPF) and sodium methacrylate (SMA) to fabricate the pH-responsive microgels. We demonstrated that the microgels were negatively charged, and the amounts of charge on the microgels were correlated with the SMA concentration in their formulation. The resulting microgels exhibited sensitivity to the pH and ionic strength of the surrounding environment. We demonstrated that DOX was efficiently loaded into the microgels and released in a controlled fashion via an ion-exchange mechanism. Our data revealed that the DOX release was influenced by the pH and ionic strength of the solution. Moreover, we designed a phenomenological mathematical model, based on a stretched exponential function, to quantitatively analyze the cumulative release of DOX. We found a linear correlation between the maximum release of DOX calculated from the model and the SMA concentration in the microgel formulation. The anti-tumor activity of the released DOX was assessed using a human chordoma cell line. Our data revealed that OPF-SMA microgels prolonged the cell killing effect of DOX.

Building bridges: leveraging interdisciplinary collaborations in the development of biomaterials to meet clinical needs

Our laboratory at Rice University has forged numerous collaborations with clinicians and basic scientists over the years to advance the development of novel biomaterials and the modification of existing materials to meet clinical needs. This review highlights collaborative advances in biomaterials research from our laboratory in the areas of scaffold development, drug delivery, and gene therapy, especially as related to applications in bone and cartilage tissue engineering.

Foamed oligo(poly(ethylene glycol)fumarate) hydrogels as versatile prefabricated scaffolds for tissue engineering

Radically cross-linked hydrogels are frequently used as cell carriers due to their excellent biocompatibility and their tissue-like mechanical properties. Through frequent investigation, PEG-based polymers such as oligo(poly(ethylene glycol)fumarate [OPF] have proven to be especially suitable as cell carriers by encapsulating cells during hydrogel formation. In some cases, NaCl or biodegradable gelatin microparticles were added prior to cross-linking in order to provide space for the proliferating cells, which would otherwise stay embedded in the hydrogel matrix. However, all of these immediate cross-linking procedures involve time consuming sample preparation and sterilization directly before cell culture and often show notable swelling after their preparation. In this study, ready to use OPF-hydrogel scaffolds were prepared by gas foaming, freeze drying, individual packing into bags and subsequent γ-sterilization. The scaffolds could be stored and used "off-the-shelf" without any need for further processing prior to cell culture. Thus the handling was simplified and the sterility of the cell carrier was assured. Further improvement of the gel system was achieved using a two component injectable system, which may be used for homogenous injection molding in order to create individually shaped three dimensional scaffolds. In order to evaluate the suitability of the scaffolds for tissue engineering, constructs were seeded with juvenile bovine chondrocytes and cultured for 28 days. Cross-sections of the respective constructs showed an intense and homogenous red staining of GAG with safranin O, indicating a homogenous cell distribution within the scaffolds and the production of substantial amounts of GAG-rich matrix.

Synthesis of oligo(poly(ethylene glycol) fumarate)

This protocol describes the synthesis of oligo(poly(ethylene glycol) fumarate) (OPF; 1-35 kDa; a polymer useful for tissue engineering applications) by a one-pot reaction of poly(ethylene glycol) (PEG) and fumaryl chloride. The procedure involves three parts: dichloromethane and PEG are first dried; the reaction step follows, in which fumaryl chloride and triethylamine are added dropwise to a solution of PEG in dichloromethane; and finally, the product solution is filtered to remove by-product salt, and the OPF product is twice crystallized, washed and dried under vacuum. The reaction is affected by the molecular weight of PEG and reactant molar ratio. The OPF product is cross-linked by radical polymerization by either a thermally induced or ultraviolet-induced radical initiator, and the physical properties of the OPF oligomer and resulting cross-linked hydrogel are easily tailored by varying PEG molecular weight. OPF hydrogels are injectable, they polymerize in situ and they undergo biodegradation by hydrolysis of ester bonds. The expected time required to complete this protocol is 6 d.

Incorporation of phosphate group modulates bone cell attachment and differentiation on oligo(polyethylene glycol) fumarate hydrogel

In this work, we have investigated the development of a synthetic hydrogel that contains a negatively charged phosphate group for use as a substrate for bone cell attachment and differentiation in culture. The photoreactive, phosphate-containing molecule, bis(2-(methacryloyloxy)ethyl)phosphate (BP), was incorporated into oligo(polyethylene glycol) fumarate hydrogel and the mechanical, rheological and thermal properties of the resulting hydrogels were characterized. Our results showed changes in hydrogel compression and storage moduli with incorporation of BP. The modification also resulted in decreased crystallinity as recorded by differential scanning calorimetry. Our data revealed that incorporation of BP improved attachment and differentiation of human fetal osteoblast (hFOB) cells in a dose-dependent manner. A change in surface chemistry and mineralization of the phosphate-containing surfaces verified by scanning electron microscopy and energy dispersive X-ray analysis was found to be important for hFOB cell attachment and differentiation. We also demonstrated that phosphate-containing hydrogels support attachment and differentiation of primary bone marrow stromal cells. These findings suggest that BP-modified hydrogels are capable of sustaining attachment and differentiation of both bone marrow stromal cells and osteoblasts that are critical for bone regeneration.

Modulation of mesenchymal stem cell chondrogenesis in a tunable hyaluronic acid hydrogel microenvironment

An injectable and biodegradable hydrogel system comprising hyaluronic acid-tyramine (HA-Tyr) conjugates can safely undergo covalent cross-linking in vivo by the addition of small amounts of peroxidase and hydrogen peroxide (H(2)O(2)), with the independent tuning of the gelation rate and degree of cross-linking. Such hydrogel networks with tunable mechanical and degradation properties may provide the additional level of control needed to enhance chondrogenesis and overall cartilage tissue formation in vitro and in vivo. In this study, HA-Tyr hydrogels were explored as biomimetic matrices for caprine mesenchymal stem cells (MSCs) in cartilage tissue engineering. The compressive modulus, equilibrium swelling and degradation rate could be controlled by varying the concentration of H(2)O(2) as the oxidant in the oxidative coupling reaction. Cellular condensation reflected by the increase in effective number density of rounded cells in lacunae was greater in softer hydrogel matrices with lower cross-linking that displayed enhanced scaffold contracture. Conversely, within higher cross-linked matrices, cells adopted a more elongated morphology, with a reduced degree of cellular condensation. Furthermore, the degree of hydrogel cross-linking also modulated matrix biosynthesis and cartilage tissue histogenesis. Lower cross-linked matrix enhanced chondrogenesis with increases in the percentage of cells with chondrocytic morphology; biosynthetic rates of glycosaminoglycan and type II collagen; and hyaline cartilage tissue formation. With increasing cross-linking degree and matrix stiffness, a shift in MSC differentiation toward fibrous phenotypes with the formation of fibrocartilage and fibrous tissues was observed. These findings suggest that the tunable three-dimensional microenvironment of the HA-Tyr hydrogels modulates cellular condensation during chondrogenesis and has a dramatic impact on spatial organization of cells, matrix biosynthesis, and overall cartilage tissue histogenesis.

Comparison of polymer scaffolds in rat spinal cord: a step toward quantitative assessment of combinatorial approaches to spinal cord repair

The transected rat thoracic (T(9/10)) spinal cord model is a platform for quantitatively comparing biodegradable polymer scaffolds. Schwann cell-loaded scaffolds constructed from poly (lactic co-glycolic acid) (PLGA), poly(ɛ-caprolactone fumarate) (PCLF), oligo(polyethylene glycol) fumarate (OPF) hydrogel or positively charged OPF (OPF+) hydrogel were implanted into the model. We demonstrated that the mechanical properties (3-point bending and stiffness) of OPF and OPF + hydrogels closely resembled rat spinal cord. After one month, tissues were harvested and analyzed by morphometry of neurofilament-stained sections at rostral, midlevel, and caudal scaffold. All polymers supported axonal growth. Significantly higher numbers of axons were found in PCLF (P < 0.01) and OPF+ (P < 0.05) groups, compared to that of the PLGA group. OPF + polymers showed more centrally distributed axonal regeneration within the channels while other polymers (PLGA, PCLF and OPF) tended to show more evenly dispersed axons within the channels. The centralized distribution was associated with significantly more axons regenerating (P < 0.05). Volume of scar and cyst rostral and caudal to the implanted scaffold was measured and compared. There were significantly smaller cyst volumes in PLGA compared to PCLF groups. The model provides a quantitative basis for assessing individual and combined tissue engineering strategies.

The effects of fixed electrical charge on chondrocyte behavior

In this study we have compared the effects of negative and positive fixed charges on chondrocyte behavior in vitro. Electrical charges have been incorporated into oligo(poly(ethylene glycol) fumarate) (OPF) using small charged monomers such as sodium methacrylate (SMA) and (2-(methacryloyloxy) ethyl)-trimethyl ammonium chloride (MAETAC) to produce negatively and positively charged hydrogels, respectively. The physical and electrical properties of the hydrogels were characterized by measuring and calculating the swelling ratio and zeta potential, respectively. Our results revealed that the properties of these OPF modified hydrogels varied according to the concentration of charged monomers. Zeta potential measurements demonstrated that the electrical properties of the OPF hydrogel surfaces changed on incorporation of SMA and MAETAC and that these changes in electrical properties were dose-dependent. Attenuated total reflectance Fourier transform infrared spectroscopy was used to determine the hydrogel surface composition. To assess the effects of surface properties on chondrocyte behavior primary chondrocytes isolated from rabbit ears were seeded as a monolayer on top of the hydrogels. We demonstrated that the cells remained viable over 7 days and began to proliferate while seeded on top of the hydrogels. Collagen type II staining was positive in all samples, however, the staining intensity was higher on negatively charged hydrogels. Similarly, glycosaminoglycan production was significantly higher on negatively charged hydrogels compared with a neutral hydrogel. Reverse transcriptase polymerase chain reaction showed up-regulation of collagen type II and down-regulation of collagen type I on the negatively charged hydrogels. These findings indicate that charge plays an important role in establishing an appropriate environment for chondrocytes and, hence, in the engineering of cartilage. Thus, further investigations into charged hydrogels for cartilage tissue engineering is merited.