Three-Dimensional Microgel Platform for the Production of Cell Factories Tailored for the Nucleus Pulposus

New Hope for Treating Intervertebral Disc Degeneration: Microsphere-Based Delivery System

Intervertebral disc (IVD) degeneration (IVDD) has been considered the dominant factor in low back pain (LBP), and its etiological mechanisms are complex and not yet fully elucidated. To date, the treatment of IVDD has mainly focused on relieving clinical symptoms and cannot fundamentally solve the problem. Recently, a novel microsphere-based therapeutic strategy has held promise for IVD regeneration and has yielded encouraging results with in vitro experiments and animal models. With excellent injectability, biocompatibility, and biodegradability, this microsphere carrier allows for targeted delivery and controlled release of drugs, gene regulatory sequences, and other bioactive substances and supports cell implantation and directed differentiation, aiming to improve the disease state of IVD at the source. This review discusses the possible mechanisms of IVDD and the limitations of current therapies, focusing on the application of microsphere delivery systems in IVDD, including targeted delivery of active substances and drugs, cellular therapy, and gene therapy, and attempts to provide a new understanding for the treatment of IVDD.

Considerations in the Development of Small-Diameter Vascular Graft as an Alternative for Bypass and Reconstructive Surgeries: A Review

BACKGROUND

Current design strategies for small diameter vascular grafts (< 6 mm internal diameter; ID) are focused on mimicking native vascular tissue because the commercially available grafts still fail at small diameters, notably due to development of intimal hyperplasia and thrombosis. To overcome these challenges, various design approaches, material selection, and surface modification strategies have been employed to improve the patency of small-diameter grafts.

REVIEW

The purpose of this review is to outline various considerations in the development of small-diameter vascular grafts, including material choice, surface modifications to enhance biocompatibility/endothelialization, and mechanical properties of the graft, that are currently being implanted. Additionally, we have taken into account the general vascular physiology, tissue engineering approaches, and collective achievements of the authors in this area. We reviewed both commercially available synthetic grafts (e-PTFE and PET), elastic polymers such as polyurethane and biodegradable and bioresorbable materials. We included naturally occurring materials by focusing on their potential application in the development of future vascular alternatives.

CONCLUSION

Until now, there are few comprehensive reviews regarding considerations in the design of small-diameter vascular grafts in the literature. Here-in, we have discussed in-depth the various strategies employed to generate engineered vascular graft due to their high demand for vascular surgeries. While some TEVG design strategies have shown greater potential in contrast to autologous or synthetic ePTFE conduits, many are still hindered by high production cost which prevents their widespread adoption. Nonetheless, as tissue engineers continue to develop on their strategies and procedures for improved TEVGs, soon, a reliable engineered graft will be available in the market. Hence, we anticipate a viable TEVG with resorbable property, fabricated via electrospinning approach to hold a greater potential that can overcome the challenges observed in both autologous and allogenic grafts. This is because they can be mechanically tuned, incorporated/surface-functionalized with bioactive molecules and mass-manufactured in a reproducible manner. It is also found that most of the success in engineered vascular graft approaching commercialization is for large vessels rather than small-diameter grafts used as cardiovascular bypass grafts. Consequently, the field of vascular engineering is still available for future innovators that can take up the challenge to create a functional arterial substitute.

Designing Microgels for Cell Culture and Controlled Assembly of Tissue Microenvironments

Micron-sized hydrogels, termed microgels, are emerging as multifunctional platforms that can recapitulate tissue heterogeneity in engineered cell microenvironments. The microgels can function as either individual cell culture units or can be assembled into larger scaffolds. In this manner, individual microgels can be customized for single or multi-cell co-culture applications, or heterogeneous populations can be used as building blocks to create microporous assembled scaffolds that more closely mimic tissue heterogeneities. The inherent versatility of these materials allows user-defined control of the microenvironments, from the order of singly encapsulated cells to entire three-dimensional cell scaffolds. These hydrogel scaffolds are promising for moving towards personalized medicine approaches and recapitulating the multifaceted microenvironments that exist in vivo.

An in situ collagen-HA hydrogel system promotes survival and preserves the proangiogenic secretion of hiPSC-derived vascular smooth muscle cells

Human-induced pluripotent stem cell-derived vascular smooth muscle cells (hiPSC-VSMCs) with proangiogenic properties have huge therapeutic potential. While hiPSC-VSMCs have already been utilized for wound healing using a biomimetic collagen scaffold, an in situ forming hydrogel mimicking the native environment of skin offers the promise of hiPSC-VSMC mediated repair and regeneration. Herein, the impact of a collagen type-I-hyaluronic acid (HA) in situ hydrogel cross-linked using a polyethylene glycol-based cross-linker on hiPSC-VSMCs viability and proangiogenic paracrine secretion was investigated. Our study demonstrated increases in cell viability, maintenance of phenotype and proangiogenic growth factor secretion, and proangiogenic activity in response to the conditioned medium. The optimally cross-linked and functionalized collagen type-I/HA hydrogel system developed in this study shows promise as an in situ hiPSC-VSMC carrier system for wound regeneration.

Modulating inflammation in a cutaneous chronic wound model by IL-10 released from collagen-silica nanocomposites via gene delivery

Cutaneous chronic wounds remain a major clinical challenge which requires the development of novel wound dressings. Previously, we showed that collagen-silica nanocomposites consisting of polyethyleneimine (PEI)-DNA complexes associated with silica nanoparticles (SiNP), collagen hydrogel and 3T3 fibroblasts, can work as a local "cell factory". Indeed, the "in-gel" transfection leads to a sustained production and release of biomolecules. Herein, we further explored the possibility for nanocomposites to deliver interleukin-10 (IL-10), a potent anti-inflammatory cytokine, which favors tissue repair. Its anti-inflammatory effect was evaluated in an in vitro inflammation model carried out by LPS (lipopolysaccharide) activation of macrophages embedded in collagen gel. The IL-10 synthesis from nanocomposites was detected over one week in the range of 200-400 pg mL^-1 and reached a maximum at day 5 without any observed cytotoxic effects. PEI₁₀-SiNP outperformed free PEI₁₀ and PEI₂₅-SiNP, implying that the introduction of SiNP improved the transfection efficiency of low M_w of PEI. In addition, the structure and mechanical properties of collagen-silica nanocomposites were stable over one week. Subsequently, the ability of nanocomposites to modulate inflammation was tested in a 3D model of inflammation. The decrease of TNF-α and IL-1β gene expression by 20-80% indicated successful inhibition of inflammation by IL-10 released from nanocomposites. Taken together, the nanocomposites are capable of producing effective doses of IL-10 which inhibit the synthesis of pro-inflammatory cytokines and favor the expression of wound healing cytokines. Therefore, the as-constructed 3D gene delivery system represents a promising strategy for the controlled release of therapeutic biomolecules favoring cutaneous wound healing.

Fabrication and characterization of microspheres encapsulating astrocytes for neural regeneration

Astrocytes play a critical role in supporting the normal physiological function of neurons. Recent studies have revealed that astrocyte transplantation can promote axonal regeneration and functional recovery after spinal cord injury. Biomaterial can be designed as a growth-permissive substrate and serve as a carrier for astrocyte transplantation into injured spinal cord. In this study, we developed a method to generate collagen microspheres encapsulating astrocytes by injecting a mixture of collagen and astrocytes into a cell culture medium with a syringe controlled by a syringe pump. The collagen microspheres were crosslinked with poly(ethylene glycol) ether tetrasuccinimidyl glutarate (4S-StarPEG) to reduce the degradation rate. The viability of cells in the crosslinked microspheres was higher than 90%. Astrocytes were transfected with plasmids encoding nerve growth factor (NGF)-ires-enhanced green fluorescent protein (EGFP) genes by electroporation and encapsulated in crosslinked microspheres. The level of NGF released into the cell culture medium was higher than that remaining in the microspheres or astrocytes. When microspheres encapsulating astrocytes transfected with plasmids encoding NGF-ires-EGFP genes were added into the cultured rat dorsal root ganglion, the axonal growth was significantly enhanced. This study shows that the microspheres can be potentially used as a carrier of astrocytes to promote nerve regeneration in injured neural tissue.

Stem Cell Approaches to Intervertebral Disc Regeneration: Obstacles from the Disc Microenvironment

Intervertebral disc (IVD) degeneration results in segmental instability and irritates neural compressive symptoms, such as low back pain and motor deficiency. The transplanting of stem cell into degenerative discs has attracted increasing clinical attention, as a new and proven approach to alleviating disc degeneration and to relieving discogenic pains. Aside from supplementation with stem cells, the IVD itself already contains a pool of stem and progenitor cells. Since the resident disc stem cells are incapable of reversing the pathologic changes that occur during aging and disc degeneration, it has been debated as to whether transplanted stem cells are capable of providing an efficient and durable therapeutic effect, even though there have been positive outcomes in both animal models and in clinical trials. This review aims to decipher the interactions between the stem cell and the disc microenvironment. Within their new niches in the IVD, the exogenous stem cell shows metabolic adaptation to the low-glucose supply, hypoxia, and compressive loadings, but demonstrates little tolerance to the disc-like acidity and hypertonicity. Similarly, the survival of endogenous stem cells is threatened as well by the harsh disc microenvironment, which may exhaust the stem cell resources and restrict the self-repair capacity of a degenerating IVD. To eliminate the intrinsic obstacles within the stressful disc niches, stem cells should be delivered with an injectable scaffold that provides both survival and mechanical support. Quick healing or concretion of the injection injuries, which minimizes stem cell leakage and disturbance to disc homeostasis, is of equal importance toward achieving efficient stem cell-based disc regeneration.

Surface modification and endothelialization of biomaterials as potential scaffolds for vascular tissue engineering applications

This review highlights the recent developments of surface modification and endothelialization of biomaterials in vascular tissue engineering applications.