A Minireview of Microfluidic Scaffold Materials in Tissue Engineering

In 2020, nearly 107,000 people in the U.S needed a lifesaving organ transplant, but due to a limited number of donors, only ∼35% of them have actually received it. Thus, successful bio-manufacturing of artificial tissues and organs is central to satisfying the ever-growing demand for transplants. However, despite decades of tremendous investments in regenerative medicine research and development conventional scaffold technologies have failed to yield viable tissues and organs. Luckily, microfluidic scaffolds hold the promise of overcoming the major challenges associated with generating complex 3D cultures: 1) cell death due to poor metabolite distribution/clearing of waste in thick cultures; 2) sacrificial analysis due to inability to sample the culture non-invasively; 3) product variability due to lack of control over the cell action post-seeding, and 4) adoption barriers associated with having to learn a different culturing protocol for each new product. Namely, their active pore networks provide the ability to perform automated fluid and cell manipulations (e.g., seeding, feeding, probing, clearing waste, delivering drugs, etc.) at targeted locations in-situ. However, challenges remain in developing a biomaterial that would have the appropriate characteristics for such scaffolds. Specifically, it should ideally be: 1) biocompatible—to support cell attachment and growth, 2) biodegradable—to give way to newly formed tissue, 3) flexible—to create microfluidic valves, 4) photo-crosslinkable—to manufacture using light-based 3D printing and 5) transparent—for optical microscopy validation. To that end, this minireview summarizes the latest progress of the biomaterial design, and of the corresponding fabrication method development, for making the microfluidic scaffolds.

Syndecan-4 Functionalization of Tissue Regeneration Scaffolds Improves Interaction with Endothelial Progenitor Cells

Key to most implanted cell free scaffolds for tissue regeneration is the ability to sequester and retain undifferentiated mesenchymal stem cells at the repair site. In this report, syndecan-4, a heparan sulfate containing proteoglycan, was investigated as a unique molecule for use in scaffold functionalization. An electrospun hybrid scaffold comprised of poly (glycerol) sebacate (PGS), silk fibroin and type I collagen (PFC) was used as a model scaffold to develop a procedure and test the hypothesis that functionalization would result in increased scaffold binding of endothelial progenitor cells (EPCs). For these studies both Syndecan-4 and stromal derived factor-1α (SDF-1α) were used in functionalization PFC. Syndecan-4 functionalized PFC bound 4.8 fold more SDF-1α compared to nonfunctionalized PFC. Binding was specific as determined by heparin displacement studies. After culture for 7 days, significantly, more EPCs were detected on PFC scaffolds having both syndecan-4 and SDF-1α compared to scaffolds of PFC with only syndecan-4, or PFC adsorbed with SDF-1α, or PFC alone. Taken together, this study demonstrates that EPCs can be bound to and significantly expanded on PFC material through syndecan-4 mediated growth factor binding. Syndecan-4 with a multiplicity of binding sites has the potential to functionalize and expand stem cells on a variety of scaffold materials for use in tissue regeneration.

Aspects of In Vitro Biodegradation of Hybrid Fibrin–Collagen Scaffolds

The success of the regenerative process resulting from the implantation of a scaffold or a tissue-engineered structure into damaged tissues depends on a series of factors, including, crucially, the biodegradability of the implanted materials. The selection of a scaffold with appropriate biodegradation characteristics allows for synchronization of the degradation of the construct with the processes involved in new tissue formation. Thus, it is extremely important to characterize the biodegradation properties of potential scaffold materials at the stage of in vitro studies. We have analyzed the biodegradation of hybrid fibrin–collagen scaffolds in both PBS solution and in trypsin solution and this has enabled us to describe the processes of both their passive and enzymatic degradation. It was found that the specific origin of the collagen used to form part of the hybrid scaffolds could have a significant effect on the nature of the biodegradation process. It was also established, during comparative studies of acellular scaffolds and scaffolds containing stem cells, that the cells, too, make a significant contribution to changes in the biodegradation and structural properties of such scaffolds. The study results also provided evidence indicating the dependency between the pre-cultivation period for the cellular scaffolds and the speed and extent of their subsequent biodegradation. Our discussion of results includes an attempt to explain the mechanisms of the changes found. We hope that the said results will make a significant contribution to the understanding of the processes affecting the differences in the biodegradation properties of hybrid, biopolymer, and hydrogel scaffolds.

Effect of Polymeric Matrix Stiffness on Osteogenic Differentiation of Mesenchymal Stem/Progenitor Cells: Concise Review

Mesenchymal stem/progenitor cells (MSCs) have a multi-differentiation potential into specialized cell types, with remarkable regenerative and therapeutic results. Several factors could trigger the differentiation of MSCs into specific lineages, among them the biophysical and chemical characteristics of the extracellular matrix (ECM), including its stiffness, composition, topography, and mechanical properties. MSCs can sense and assess the stiffness of extracellular substrates through the process of mechanotransduction. Through this process, the extracellular matrix can govern and direct MSCs’ lineage commitment through complex intracellular pathways. Hence, various biomimetic natural and synthetic polymeric matrices of tunable stiffness were developed and further investigated to mimic the MSCs’ native tissues. Customizing scaffold materials to mimic cells’ natural environment is of utmost importance during the process of tissue engineering. This review aims to highlight the regulatory role of matrix stiffness in directing the osteogenic differentiation of MSCs, addressing how MSCs sense and respond to their ECM, in addition to listing different polymeric biomaterials and methods used to alter their stiffness to dictate MSCs’ differentiation towards the osteogenic lineage.

The Adipose-Derived Stem Cell and Endothelial Cell Coculture System—Role of Growth Factors?

Adequate vascularization is a fundamental prerequisite for bone regeneration, formation and tissue engineering applications. Endothelialization of scaffold materials is a promising strategy to support neovascularization and bone tissue formation. Besides oxygen and nutrition supply, the endothelial network plays an important role concerning osteogenic differentiation of osteoprogenitor cells and consecutive bone formation. In this study we aimed to enhance the growth stimulating, proangiogenic and osteogenic features of the ADSC and HUVEC coculture system by means of VEGFA165 and BMP2 application. We were able to show that sprouting phenomena and osteogenic differentiation were enhanced in the ADSC/HUVEC coculture. Furthermore, apoptosis was unidirectionally decreased in HUVECs, but these effects were not further enhanced upon VEGFA165 or BMP2 application. In summary, the ADSC/HUVEC coculture system per se is a powerful tool for bone tissue engineering applications.

Recent Advancements in 3D Printing of Polysaccharide Hydrogels in Cartilage Tissue Engineering

The application of hydrogels coupled with 3-dimensional (3D) printing technologies represents a modern concept in scaffold development in cartilage tissue engineering (CTE). Hydrogels based on natural biomaterials are extensively used for this purpose. This is mainly due to their excellent biocompatibility, inherent bioactivity, and special microstructure that supports tissue regeneration. The use of natural biomaterials, especially polysaccharides and proteins, represents an attractive strategy towards scaffold formation as they mimic the structure of extracellular matrix (ECM) and guide cell growth, proliferation, and phenotype preservation. Polysaccharide-based hydrogels, such as alginate, agarose, chitosan, cellulose, hyaluronan, and dextran, are distinctive scaffold materials with advantageous properties, low cytotoxicity, and tunable functionality. These superior properties can be further complemented with various proteins (e.g., collagen, gelatin, fibroin), forming novel base formulations termed “proteo-saccharides” to improve the scaffold’s physiological signaling and mechanical strength. This review highlights the significance of 3D bioprinted scaffolds of natural-based hydrogels used in CTE. Further, the printability and bioink formation of the proteo-saccharides-based hydrogels have also been discussed, including the possible clinical translation of such materials.

Characterization of Cellulose Acetate Based Scaffolds Derived from Kapok Fiber (Ceiba pentandra (L) Gaertn)

Utilization of natural biopolymers has shown potential in generating innovations for tissue engineering applications. This study aims to fabricate scaffolds from cellulose acetate derived from kapok fiber. Cellulose is extracted from raw kapok fibers by alkali treatment and delignification then synthesized into cellulose acetate. Kapok cellulose acetate (KCA) is dissolved in dimethyl sulfoxide to fabricate the scaffold. Materials were characterized using Attenuated Total Reflectance – Fourier Transform Infrared (ATR-FTIR) spectrometer, X-ray diffractometer (XRD) and Differential Scanning Calorimeter (DSC). FTIR analysis has shown that cellulose was extracted from kapok and cellulose acetate was successfully synthesized. XRD analysis also confirmed the presence of cellulose acetate. Results have also shown that synthesized KCA seems to have higher crystallinity than commercially available cellulose acetate (CCA). The degree of substitution (DS) of KCA was found to be 2.85 which is close to the DS value of tri-substituted cellulose acetate. DSC analysis has shown lower glass transition temperature of 52.15°C but higher degradation temperature of 300.43°C than the CCA. Moreover, the values for the enthalpy of fusion for two endotherms of KCA (44.0556 J/g and 18.6946 J/g) are higher than the values for CCA by 344% and 261%, respectively; thus, indicating the higher degree of crystallinity for synthesized KCA samples.

Tissue Engineering Scaffold Materials to Repair Sports Cartilage Injury

With the continued development of sports in China, sports sometimes cause cartilage damage. The purpose of this research is to study the tissue engineering scaffold material for sports cartilage damage repair. In this study, mesenchymal rat bone marrow stem cells (to put it simply, stem cells are a type of cell with unlimited or immortal self-renewal capacity, capable of producing at least one type of highly differentiated progeny cells) were obtained by the total bone wash method. The cells were inoculated into the cell culture bottle. When the primary cultured cells proliferated to about 80% of the culture bottle area, the cells were digested with trypsin to open the cell link, then the medium containing 10% serum was added to terminate the cell digestion, and then the passage expansion was carried out according to the cell density. PLGA/NHA and PLGA were heated to 65°C under ultrasonic vibration until uniform PLGA/NHA and PLGA solutions were obtained. Then, the samples were added to the tube mold and then heated and cooled to obtain the composite porous scaffold of mesenchymal stem cells. 10 μl MSCs cell suspension was extracted with a microinjector, and the needle was injected from the inside of the scaffold, and the cell suspension was added outside the scaffold to ensure that there were composite cells inside and outside the scaffold. The subcutaneous tissue of the skin was cut along the medial side of the knee joint and the capsule of the switch segment was cut. The scaffold materials were filled into the osteochondral defect to observe the cartilage healing. The mechanical strength of 0.5% PLGA-MSCs composite porous scaffold was increased to 1.1 MPa, and the cell density was high. The repair of cartilage in rats was the best. The results showed that the porous scaffolds designed in this study have good compatibility and are beneficial to repair sports cartilage injury.