Investigation of the changes of biophysical/mechanical characteristics of differentiating preosteoblasts in vitro

Engineered Nanofibrillar Collagen with Tunable Biophysical Properties for Myogenic, Endothelial, and Osteogenic Cell Guidance

A goal of regenerative engineering is the rational design of materials to restore the structure-function relationships that drive reparative programs in damaged tissues. Despite the widespread use of extracellular matrices for engineering tissues, their application has been limited by a narrow range of tunable features. The primary objective of this study is to develop a versatile platform for evaluating tissue-specific cellular interactions using Type I collagen scaffolds with highly tunable biophysical properties. The kinetics of collagen fibrillogenesis were modulated through a combination of varied shear rate and pH, during neutralization, to achieve a broad range of fibril anisotropy, porosity, diameter, and storage modulus. The role that each of these properties play in guiding muscle, bone, and vascular cell types was comprehensively identified and informed the in vitro generation of three distinct musculoskeletal engineered constructs. Myogenesis was highly regulated by smaller fibrils and larger storage moduli, endothelial inflammatory phenotype was predominantly guided by fibril anisotropy, and osteogenesis was enhanced by highly porous collagen with larger fibrils. This study introduces a novel approach for dynamically modulating Type I collagen materials and provides a robust platform for investigating cell-material interactions, offering insights for future rational design of tissue-specific regenerative biomaterials. STATEMENT OF SIGNIFICANCE: The biophysical properties of regenerative materials facilitate key cell-substrate interactions that can guide the morphology, phenotype, and biological response of cells. In this study, we describe the fabrication of an engineered collagen hydrogel that can be modified to exhibit control over a wide range of biophysical features including fibril organization and size, nanoscale porosity, and mechanics. We identified the unique combination of collagen features that optimally promote regenerative muscle, bone, and vascular cell types while also delineating the properties that hinder these same cellular responses. This study presents a highly accessible method to control the biophysical properties of collagen hydrogels that can be adapted for a broad range of tissue engineering and regenerative applications.

The role of cellular traction forces in deciphering nuclear mechanics

Cellular forces exerted on the extracellular matrix (ECM) during adhesion and migration under physiological and pathological conditions regulate not only the overall cell morphology but also nuclear deformation. Nuclear deformation can alter gene expression, integrity of the nuclear envelope, nucleus-cytoskeletal connection, chromatin architecture, and, in some cases, DNA damage responses. Although nuclear deformation is caused by the transfer of forces from the ECM to the nucleus, the role of intracellular organelles in force transfer remains unclear and a challenging area of study. To elucidate nuclear mechanics, various factors such as appropriate biomaterial properties, processing route, cellular force measurement technique, and micromanipulation of nuclear forces must be understood. In the initial phase of this review, we focused on various engineered biomaterials (natural and synthetic extracellular matrices) and their manufacturing routes along with the properties required to mimic the tumor microenvironment. Furthermore, we discussed the principle of tools used to measure the cellular traction force generated during cell adhesion and migration, followed by recently developed techniques to gauge nuclear mechanics. In the last phase of this review, we outlined the principle of traction force microscopy (TFM), challenges in the remodeling of traction forces, microbead displacement tracking algorithm, data transformation from bead movement, and extension of 2-dimensional TFM to multiscale TFM.

Fluorine-incorporated TiO2 nanotopography enhances adhesion and differentiation through ERK/CREB pathway

This study compared the topography of different titanium surface structures (TiO₂ nanotube and grain) with similar elemental compositions (TiO2 and fluorine [F]) on the Ti surface. High magnification indicated that the surfaces of the control and etching groups were similar to each other in a flat, smooth form. The group anodized for 1 h was observed with TiO₂ nanotubes organized very neatly and regularly. In the group anodized for 30 min after etching, uneven wave and nanopore structures were observed. In addition, MTT assay showed that the F of the surface did not adversely affect cell viability, and the initial cell adhesion was increased in the 2.8% F‐incorporated TiO₂ nanograin. At the edge of adherent cells, filopodia were observed in spreading form on the surfaces of the anodizing and two‐step processing groups, and they were observed in a branch shape in the control and etching groups. Moreover, cell adhesion molecule and osteogenesis marker expression was increased at the F‐incorporated TiO₂ nanostructure. In addition, it was found that the expression of p‐extracellular signal‐regulated kinase (ERK) and p‐cAMP response element‐binding protein (CREB) increased in the TiO₂ nanograin with the nanopore surface compared to the micro rough and nanotube surfaces relative to the osteogenic‐related gene expression patterns. As a result, this study confirmed that the topographic structure of the surface is more affected by osteogenic differentiation than the pore size and that differentiation by specific surface composition components is by CREB. Thus, the synergy effect of osteogenic differentiation was confirmed by the simultaneous activation of CREB/ERK.

Alteration of Young's modulus in mesenchymal stromal cells during osteogenesis measured by atomic force microscopy

Mechanical properties of biological tissues are increasingly recognized as an important parameter for the indication of disease states as well as tissue homeostasis and regeneration. Multipotent mesenchymal stromal/stem cells (MSCs), which play important roles in bone formation and remodeling, are potential cell sources for regenerative medicine. However, the cellular mechanical properties of differentiating MSCs corresponding to the substrate stiffness has not been sufficiently studied. In this study, we used Atomic Force Microscopy (AFM) to measure changes of stiffness of human MSCs cultured in rigid Petri dish and on polyacrylamide (PA) substrates during osteogenic differentiation. The results showed that the Young's modulus of MSC cytoplasmic outer region increased over time during osteogenesis. There is a strong linear correlation between the osteogenic induction time and the Young's modulus of the cells cultured in rigid Petri dishes in the first 15 days after the induction; the Young's modulus approaches to a plateau after day 15. On the other hand, the Young's moduli of MSCs cultured on PA gels with stiffness of 7 kPa and 42 kPa also increase over time during osteogenic differentiation, but the inclination of such increase is much smaller than that of MSCs differentiating in rigid dishes. Herein, we established a protocol of AFM measurement to evaluate the maturation of stem cell osteogenic differentiation at the single cell level and could encourage further AFM applications in tissue engineering related to mechanobiology.

Synthesis and Biocompatibility Characterizations of in Situ Chondroitin Sulfate-Gelatin Hydrogel for Tissue Engineering

Novel hydrogel composed of both chondroitin sulfate (CS) and gelatin was developed for better cellular interaction through two step double crosslinking of N-(3-diethylpropyl)-N-ethylcarbodiimide hydrochloride (EDC) chemistries and then click chemistry. EDC chemistry was proceeded during grafting of amino acid dihydrazide (ADH) to carboxylic groups in CS and gelatin network in separate reactions, thus obtaining CS-ADH and gelatin-ADH, respectively. CS-acrylate and gelatin-TCEP was obtained through a second EDC chemistry of the unreacted free amines of CS-ADH and gelatin-ADH with acrylic acid and tri(carboxyethyl)phosphine (TCEP), respectively. In situ CS-gelatin hydrogel was obtained via click chemistry by simple mixing of aqueous solutions of both CS-acrylate and gelatin-TCEP. ATR-FTIR spectroscopy showed formation of the new chemical bonds between CS and gelatin in CS-gelatin hydrogel network. SEM demonstrated microporous structure of the hydrogel. Within serial precursor concentrations of the CS-gelatin hydrogels studied, they showed trends of the reaction rates of gelation, where the higher concentration, the quicker the gelation occurred. In vitro studies, including assessment of cell viability (live and dead assay), cytotoxicity, biocompatibility via direct contacts of the hydrogels with cells, as well as measurement of inflammatory responses, showed their excellent biocompatibility. Eventually, the test results verified a promising potency for further application of CS-gelatin hydrogel in many biomedical fields, including drug delivery and tissue engineering by mimicking extracellular matrix components of tissues such as collagen and CS in cartilage.

Competing Roles of Substrate Composition, Microstructure, and Sustained Strontium Release in Directing Osteogenic Differentiation of hMSCs

Strontium releasing bioactive ceramics constitute an important class of biomaterials for osteoporosis treatment. In the present study, we evaluated the synthesis, phase assemblage, and magnetic properties of strontium hexaferrite, SrFe₁₂O₁₉, (SrFe) nanoparticles. On the biocompatibility front, the size- and dose-dependent cytotoxicity of SrFe against human mesenchymal stem cells (hMSCs) were investigated. After establishing their non-toxic nature, we used the strontium hexaferrite nanoparticles (SrFeNPs) in varying amount (x = 0, 10, and 20 wt %) to consolidate bioactive composites with hydroxyapatite (HA) by multi-stage spark plasma sintering (SPS). Rietveld refinement of these spark plasma sintered composites revealed a near complete decomposition of SrFe₁₂O₁₉ to magnetite (Fe₃O₄) along with a marked increase in the unit cell volume of HA, commensurate with strontium-doped HA. The cytocompatibility of SrHA-Fe composites with hMSCs was assessed using qualitative and quantitative morphological analysis along with phenotypic and genotypic expression for stem cell differentiation. A marked decrease in the stemness of hMSCs, indicated by reduced vimentin expression and acquisition of osteogenic phenotype, evinced by alkaline phosphatase (ALP) and collagen deposition was recorded on SrHA-Fe composites in osteoinductive culture. A significant upregulation of osteogenic marker genes (Runx2, ALP and OPN) was detected in case of the SrHA-Fe composites, whereas OCN and Col IA expression were similarly high for baseline HA. However, matrix mineralization was elevated on SrHA-Fe composites in commensurate with the release of Sr²⁺ and Fe²⁺. Summarizing, the current work is the first report of strontium hexaferrite as a non-toxic nanobiomaterial. Also, SrHA-based iron oxide composites can potentially better facilitate bone formation, when compared to pristine HA.

The effect of equiaxial stretching on the osteogenic differentiation and mechanical properties of human adipose stem cells

Although mechanical cues are known to affect stem cell fate and mechanobiology, the significance of such stimuli on the osteogenic differentiation of human adipose stem cells (hASCs) remains unclear. In this study, we investigated the effect of long-term mechanical stimulation on the attachment, osteogenic differentiation and mechanical properties of hASCs. Tailor-made, pneumatic cell stretching devices were used to expose hASCs to cyclic equiaxial stretching in osteogenic medium. Cell attachment and focal adhesions were visualised using immunocytochemical vinculin staining on days 3 and 6, and the proliferation and alkaline phosphatase activity, as a sign of early osteogenic differentiation, were analysed on days 0, 6 and 10. Furthermore, the mechanical properties of hASCs, in terms of apparent Young's modulus and normalised contractility, were obtained using a combination of atomic force microscopy based indentation and computational approaches. Our results indicated that cyclic equiaxial stretching delayed proliferation and promoted osteogenic differentiation of hASCs. Stretching also reduced cell size and intensified focal adhesions and actin cytoskeleton. Moreover, cell stiffening was observed during osteogenic differentiation and especially under mechanical stimulation. These results suggest that cyclic equiaxial stretching modifies cell morphology, focal adhesion formation and mechanical properties of hASCs. This could be exploited to enhance osteogenic differentiation.

Lipid-based surface engineering of PLGA nanoparticles for drug and gene delivery applications

The use of poly(lactic-co-glycolic acid) (PLGA)-based nanocarriers presents several major challenges, including their synthetic hydrophobic surface, low transfection efficiency, short circulation half-life, and nonspecific tissue distribution. Numerous engineering strategies have been employed to overcome these problems, with lipid-based surface functionalization of PLGA nanoparticles (NPs) showing promising results in the development of PLGA-based clinical nanomedicines. Surface engineering with different lipids enhances the target specificity of the carrier and improves its physicochemical properties as well as NP-cell associations, such as cellular membrane permeability, immune responses, and long circulation half-life in vivo. This review focuses on recent advances in the lipid-based surface engineering of PLGA NPs for drug and gene delivery applications.

Research trends in biomimetic medical materials for tissue engineering: 3D bioprinting, surface modification, nano/micro-technology and clinical aspects in tissue engineering of cartilage and bone

This review discusses about biomimetic medical materials for tissue engineering of bone and cartilage, after previous scientific commentary of the invitation-based, Korea-China joint symposium on biomimetic medical materials, which was held in Seoul, Korea, from October 22 to 26, 2015. The contents of this review were evolved from the presentations of that symposium. Four topics of biomimetic medical materials were discussed from different research groups here: 1) 3D bioprinting medical materials, 2) nano/micro-technology, 3) surface modification of biomaterials for their interactions with cells and 4) clinical aspects of biomaterials for cartilage focusing on cells, scaffolds and cytokines.