Delivering Mechanical Stimulation to Cells: State of the Art in Materials and Devices Design

Klotho senses mechanical stimuli and modulates tension-induced osteogenesis

Delicate external mechanosensing, efficient intracellular mechanotransduction and effective alveolar bone remodeling lay the foundation of orthodontic tooth movement (OTM). Periodontal ligament stem cells (PDLSCs) are thought to be the primary cells that withstand mechanical stimuli and respond to biomechanical signals during orthodontic treatment. Nevertheless, the cellular and molecular mechanisms of orthodontic force-induced mechanosignaling and osteogenesis in PDLSCs still remain unclear. In the present study, we hypothesize that the ageing suppressor, Klotho, is correlated with orthodontic force-triggered mechanical signaling cascades, further contributing to alveolar bone remodeling. This study reveals that Klotho expression is notably upregulated via cytoskeletal-nuclei-mediated epigenetic modifications, consistent with osteogenic differentiation on the tension side during OTM. Additionally, Klotho deficiency undermines tensile force-induced new bone formation in NFκB- and PI3K/Akt-dependent manners. Notably, RNA sequencing (RNA-seq) results and targeted force application experiments unveil that Klotho not only functions as a downstream effector of external stress but also acts as an upstream regulator in mechanical signaling for the first time. In summary, we identify the indispensable role of Klotho in mechanotransduction and alveolar bone formation, which provide a latent target of linking cell senescence to mechanical force in future studies and offer novel insights into orthodontic force-induced tooth movement and bone remodeling.

From Mechanoelectric Conversion to Tissue Regeneration: Translational Progress in Piezoelectric Materials

Piezoelectric materials, capable of converting mechanical stimuli into electrical signals, have emerged as promising tools in regenerative medicine due to their potential to stimulate tissue repair. Despite a surge in research on piezoelectric biomaterials, systematic insights to direct their translational optimization remain limited. This review addresses the current landscape by bridging fundamental principles with clinical potential. The biomimetic basis of piezoelectricity, key molecular pathways involved in the synergy between mechanical and electrical stimulation for enhanced tissue regeneration, and critical considerations for material optimization, structural design, and biosafety is discussed. More importantly, the current status and translational quagmire of mechanisms and applications in recent years are explored. A mechanism-driven strategy is proposed for the therapeutic application of piezoelectric biomaterials for tissue repair and identify future directions for accelerated clinical applications.

Adhesive hydrogel barriers synergistically promote bone regeneration by self-constructing microstress and mineralization microenvironment

Mechanical loading is a key factor in bone growth and regeneration. In bone defect repair, combining micro-stress stimulation with an excellent inorganic microenvironment offers a more effective strategy for promoting bone regeneration. In this study, guided by the strategy to create both micro-stress and a mineralization microenvironment in the bone defect area, a membrane-like hydrogel barrier (PN-GEL@BP-PE) was designed. The hydrogel barrier adheres tightly to the bone surface via polyethyleneimine/polyacrylic acid (PEI/PAA) and generates micro-stress through the volume deformation of poly(N-isopropylacrylamide) at body temperature. Meanwhile, the inorganic microenvironment that promotes bone mineralization is induced by the calcium recruitment properties of black phosphorus nanosheets (BPNs). This membrane activates the cellular micro-stress response in mesenchymal cells, working synergistically with the calcium recruitment effect of BPNs to enhance osteogenic mineralization. In vivo, the bone regeneration effect of the hydrogel membrane is approximately 50% higher than that of conventional treatments, indicating that PN-GEL@BP-PE exhibits strong osteogenic efficacy. This synergistic strategy, combining osteogenic physical and chemical microenvironments, represents a promising direction for future research.

Massage-Mimicking Nanosheets Mechanically Reorganize Inter-organelle Contacts to Restore Mitochondrial Functions in Parkinson's Disease

Parkinson's disease (PD) is exacerbated by dysfunction of inter-organelle contact, which depends on cellular responses to the mechanical microenvironment and can be regulated by external mechanical forces. Delivering dynamic mechanical forces to neural cells proves challenging due to the skull. Inspired by the effects of massage; here PEGylated black phosphorus nanosheets (PEG-BPNS), known for their excellent biocompatibility, biodegradability, specific surface area, mechanical strength, and flexibility, are introduced, which are capable of adhering to neural cell membrane and generating mechanical stimulation with their lateral size of 200 nm, exhibiting therapeutic potential in a 1-methyl-4-phenyl-1,2,3,6-te-trahydropyridine-induced PD mouse model by regulating inter-organelle contacts. Specifically, it is found that 200 nm PEG-BPNS, acting as "NanoMassage," significantly increase  plasma membrane tension, as evidenced by fluorescent lipid tension reporter fluorescence lifetime analysis. This mechanical force modulates actin reorganization, subsequently regulating the contacts between actin, mitochondria, and endoplasmic reticulum, further controlling mitochondrial fission and mitigating mitochondrial dysfunction in PD, exhibiting therapeutic efficacy via intranasal administration. These findings provide a noninvasive strategy for applying mechanical stimulation to deep brain areas and elucidate the mechanism of NanoMassage mediating inter-organelle contacts, suggesting the rational design of "NanoMassage" to remodel inter-organelle communications in neurodegenerative disease treatment.

Scaffold geometries designed to promote bone ingrowth by enhancing mechanobiological stimulation and biotransportation - A multiobjective optimisation approach

In a tissue-engineered bone scaffold implant, the process of neo-tissue ingrowth and remodelling into hard lamellar bone occurs slowly; it typically requires a period of several months to a year (or more) to complete. This research considers the design optimisation of a scaffold's unit cell geometry for the purpose of accelerating the rate at which neo-tissue forms in the porous network of the scaffold (ingrowth), and hence, reduce the length of time to complete the bone ingrowth process. In this study, the basic structure of the scaffold is the Schwarz Primitive (P) surface unit cell, selected for its compelling biomechanical and permeability characteristics. The geometry of the scaffold is varied using two parameters (namely iso-value, k, and spatial period, a) within the surface equation defining the Schwarz P-surface unit cell. In total, sixteen different unit cell geometries are considered here with the porosity ranging from 50% to 82%. The design objectives for the scaffold are to (i) enhance mechanobiological stimulus conditions conducive to bone apposition and (ii) enhance permeability to improve the transport of nutrients/oxygen and metabolities to and from the sites of neo-tissue formation throughout the porous scaffold. The independent design variables (k and a) of the periodic unit cell geometry are optimised to best satisfy the design objectives of appositional mechanobiological stimulus and biotransporting permeability. First, a reconstructed sheep mandible computed tomographic (CT)-based finite element (FE) analysis model is used to relate the strain energy density and mechanobiological stimulus to the design variables. Next, a computational fluid dynamics (CFD) model of a 5 × 5 × 5 unit cell scaffold is developed to relate the distributions of pressure and fluid velocity to the design variables. Then, surrogate modelling is undertaken in which bivariate cubic polynomial response surfaces are fitted to the FE and CFD analysis output data to form mathematical functions of each objective with respect to the two design variables. Finally, a multiobjective optimisation algorithm is invoked to determine the best trade-off between the competing design objectives of mechanobiological stimulus and biofluidic permeability. The novel design of the scaffold structure is anticipated to provide a better biomechanical and biotransport environment for tissue regeneration.

Mechanical interaction between a hydrogel and an embedded cell in biomicrofluidic applications

Thanks to their softness, biocompatibility, porosity, and ready availability, hydrogels are commonly used in microfluidic assays and organ-on-chip devices as a matrix for cells. They not only provide a supporting scaffold for the differentiating cells and the developing organoids, but also serve as the medium for transmitting oxygen, nutrients, various chemical factors, and mechanical stimuli to the cells. From a bioengineering viewpoint, the transmission of forces from fluid perfusion to the cells through the hydrogel is critical to the proper function and development of the cell colony. In this paper, we develop a poroelastic model to represent the fluid flow through a hydrogel containing a biological cell modeled as a hyperelastic inclusion. In geometries representing shear and normal flows that occur frequently in microfluidic experiments, we use finite-element simulations to examine how the perfusion engenders interstitial flow in the gel and displaces and deforms the embedded cell. The results show that pressure is the most important stress component in moving and deforming the cell, and the model predicts the velocity in the gel and stress transmitted to the cell that is comparable to in vitro and in vivo data. This work provides a computational tool to design the geometry and flow conditions to achieve optimal flow and stress fields inside the hydrogels and around the cell.

Influence of viscosity on adipogenic and osteogenic differentiation of mesenchymal stem cells during 2D culture

Accumulatively, cellular behaviours triggered by biochemical cues have been widely explored and the focus of research is gradually shifting to biophysical cues. Compared to physical parameters such as stiffness, substrate morphology and viscoelasticity, the influence of viscosity on cellular behaviours is relatively unexplored and overlooked. Thus, in this study, the influence of viscosity on the adipogenic and osteogenic differentiation of human mesenchymal stem cells (hMSCs) was investigated by adjusting the viscosity of the culture medium. Viscosity exhibited different effects on adipogenic and osteogenic differentiation of hMSCs during two-dimensional (2D) culture. High viscosity facilitated osteogenic while inhibiting adipogenic differentiation. During adipogenic differentiation, the effect of viscosity on cell proliferation was negligible. However, during osteogenic differentiation, high viscosity decreased cell proliferation. The different influence of viscosity could be explained by the activation of mechanotransduction regulators of Yes-associated protein (YAP) and β-catenin. High viscosity could promote YAP and β-catenin nuclear translocation during osteogenic differentiation, which was responsible for the increased osteogenesis. High viscosity inhibited adipogenesis through promoting YAP nuclear translocation. This study could broaden the understanding of how viscosity can affect stem cell differentiation during 2D culture, which is valuable for tissue engineering.

Multimodal analysis of traction forces and the temperature dynamics of living cells with a diamond-embedded substrate

Cells and tissues are constantly exposed to chemical and physical signals that regulate physiological and pathological processes. This study explores the integration of two biophysical methods: traction force microscopy (TFM) and optically detected magnetic resonance (ODMR) to concurrently assess cellular traction forces and the local relative temperature. We present a novel elastic substrate with embedded nitrogen-vacancy microdiamonds that facilitate ODMR-TFM measurements. Optimization efforts focused on minimizing sample illumination and experiment duration to mitigate biological perturbations. Our hybrid ODMR-TFM technique yields TFM maps and achieves approximately 1 K precision in relative temperature measurements. Our setup employs a simple wide-field fluorescence microscope with standard components, demonstrating the feasibility of the proposed technique in life science laboratories. By elucidating the physical aspects of cellular behavior beyond the existing methods, this approach opens avenues for a deeper understanding of cellular processes and may inspire the development of diverse biomedical applications.

Physical cues of scaffolds promote peripheral nerve regeneration

The effective treatment of long-gap peripheral nerve injury (PNI) remains a challenge in clinical settings. The autograft, the gold standard for the long-gap PNI therapy, has several limitations, including a limited supply of donor nerve, size mismatch between the donor and recipient sites, functional loss at the donor site, neuroma formation, and the requirement for two operations. With the increasing abundance of biocompatible materials with adjustable structures and properties, tissue engineering provides a promising avenue for bridging peripheral nerve gaps and addressing the above issues of autograft. The physical cues provided by tissue engineering scaffolds, essential for regulating the neural cell fate and microenvironments, have received considerable research attention. This review elaborates on three major physical cues of tissue engineering scaffolds for peripheral nerve regeneration: topological structure, mechanical support, and electrical stimulation. These three aspects are analogs to Lego bricks, wherein different combinations result in diverse functions. Innovative and more effective bricks, along with multi-level and all-around integration, are expected to provide new advances in tissue engineering for peripheral nerve generation.

From animal testing to in vitro systems: advancing standardization in microphysiological systems

Limitations with cell cultures and experimental animal-based studies have had the scientific and industrial communities searching for new approaches that can provide reliable human models for applications such as drug development, toxicological assessment, and in vitro pre-clinical evaluation. This has resulted in the development of microfluidic-based cultures that may better represent organs and organ systems in vivo than conventional monolayer cell cultures. Although there is considerable interest from industry and regulatory bodies in this technology, several challenges need to be addressed for it to reach its full potential. Among those is a lack of guidelines and standards. Therefore, a multidisciplinary team of stakeholders was formed, with members from the US Food and Drug Administration (FDA), the National Institute of Standards and Technology (NIST), European Union, academia, and industry, to provide a framework for future development of guidelines/standards governing engineering concepts of organ-on-a-chip models. The result of this work is presented here for interested parties, stakeholders, and other standards development organizations (SDOs) to foster further discussion and enhance the impact and benefits of these efforts.

Organismal Function Enhancement through Biomaterial Intervention

Living organisms in nature, such as magnetotactic bacteria and eggs, generate various organic-inorganic hybrid materials, providing unique functionalities. Inspired by such natural hybrid materials, researchers can reasonably integrate biomaterials with living organisms either internally or externally to enhance their inherent capabilities and generate new functionalities. Currently, the approaches to enhancing organismal function through biomaterial intervention have undergone rapid development, progressing from the cellular level to the subcellular or multicellular level. In this review, we will concentrate on three key strategies related to biomaterial-guided bioenhancement, including biointerface engineering, artificial organelles, and 3D multicellular immune niches. For biointerface engineering, excess of amino acid residues on the surfaces of cells or viruses enables the assembly of materials to form versatile artificial shells, facilitating vaccine engineering and biological camouflage. Artificial organelles refer to artificial subcellular reactors made of biomaterials that persist in the cytoplasm, which imparts cells with on-demand regulatory ability. Moreover, macroscale biomaterials with spatiotemporal regulation characters enable the local recruitment and aggregation of cells, denoting multicellular niche to enhance crosstalk between cells and antigens. Collectively, harnessing the programmable chemical and biological attributes of biomaterials for organismal function enhancement shows significant potential in forthcoming biomedical applications.

Electroactive 4D Porous Scaffold Based on Conducting Polymer as a Responsive and Dynamic In Vitro Cell Culture Platform

In vivo, cells reside in a 3D porous and dynamic microenvironment. It provides biochemical and biophysical cues that regulate cell behavior in physiological and pathological processes. In the context of fundamental cell biology research, tissue engineering, and cell-based drug screening systems, a challenge is to develop relevant in vitro models that could integrate the dynamic properties of the cell microenvironment. Taking advantage of the promising high internal phase emulsion templating, we here designed a polyHIPE scaffold with a wide interconnected porosity and functionalized its internal 3D surface with a thin layer of electroactive conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) to turn it into a 4D electroresponsive scaffold. The resulting scaffold was cytocompatible with fibroblasts, supported cellular infiltration, and hosted cells, which display a 3D spreading morphology. It demonstrated robust actuation in ion- and protein-rich complex culture media, and its electroresponsiveness was not altered by fibroblast colonization. Thanks to customized electrochemical stimulation setups, the electromechanical response of the polyHIPE/PEDOT scaffolds was characterized in situ under a confocal microscope and showed 10% reversible volume variations. Finally, the setups were used to monitor in real time and in situ fibroblasts cultured into the polyHIPE/PEDOT scaffold during several cycles of electromechanical stimuli. Thus, we demonstrated the proof of concept of this tunable scaffold as a tool for future 4D cell culture and mechanobiology studies.

3D culture of bovine articular chondrocytes in viscous medium encapsulated in agarose hydrogels for investigation of viscosity influence on cell functions

The mechanical properties of an extracellular microenvironment can affect cell functions. The effects of elasticity and viscoelasticity on cell functions have been extensively studied with hydrogels of tunable mechanical properties. However, investigation of the viscosity effect on cell functions is still very limited and it can be tricky to explore how viscosity affects cells in three-dimensional (3D) culture due to the lack of appropriate tools. In this study, agarose hydrogel containers were prepared and used to encapsulate viscous media for 3D cell culture to investigate the viscosity effect on the functions of bovine articular chondrocytes (BACs). Polyethylene glycol of different molecular weights was used to adjust culture medium viscosity in a large range (72.8-679.2 mPa s). The viscosity affected gene expression and secretion of cartilagenious matrices, while it did not affect BAC proliferation. The BACs cultured in the lower viscosity medium (72.8 mPa s) showed a higher level of cartilaginous gene expression and matrix secretion.

Tight Regulation of Mechanotransducer Proteins Distinguishes the Response of Adult Multipotent Mesenchymal Cells on PBCE-Derivative Polymer Films with Different Hydrophilicity and Stiffness

Mechanotransduction is a molecular process by which cells translate physical stimuli exerted by the external environment into biochemical pathways to orchestrate the cellular shape and function. Even with the advancements in the field, the molecular events leading to the signal cascade are still unclear. The current biotechnology of tissue engineering offers the opportunity to study in vitro the effect of the physical stimuli exerted by biomaterial on stem cells and the mechanotransduction pathway involved in the process. Here, we cultured multipotent human mesenchymal/stromal cells (hMSCs) isolated from bone marrow (hBM-MSCs) and adipose tissue (hASCs) on films of poly(butylene 1,4-cyclohexane dicarboxylate) (PBCE) and a PBCE-based copolymer containing 50 mol% of butylene diglycolate co-units (BDG50), to intentionally tune the surface hydrophilicity and the stiffness (PBCE = 560 Mpa; BDG50 = 94 MPa). We demonstrated the activated distinctive mechanotransduction pathways, resulting in the acquisition of an elongated shape in hBM-MSCs on the BDG50 film and in maintaining the canonical morphology on the PBCE film. Notably, hASCs acquired a new, elongated morphology on both the PBCE and BDG50 films. We found that these events were mainly due to the differences in the expression of Cofilin1, Vimentin, Filamin A, and Talin, which established highly sensitive machinery by which, rather than hASCs, hBM-MSCs distinguished PBCE from BDG50 films.

Viscous Microcapsules as Microbioreactors to Study Mesenchymal Stem/Stromal Cells Osteolineage Commitment

It is essential to design a multifunctional well-controlled platform to transfer mechanical cues to the cells in different magnitudes. This study introduces a platform, a miniaturized bioreactor, which enables to study the effect of shear stress in microsized compartmentalized structures. In this system, the well-established cell encapsulation system of liquefied capsules (LCs) is used as microbioreactors in which the encapsulated cells are exposed to variable core viscosities to experience different mechanical forces under a 3D dynamic culture. The LC technology is joined with electrospraying to produce such microbioreactors at high rates, thus allowing the application of microcapsules for high-throughput screening. Using this platform for osteogenic differentiation as an example, shows that microbioreactors with higher core viscosity which produce higher shear stress lead to significantly higher osteogenic characteristics. Moreover, in this system the forces experienced by cells in each LC are simulated by computational modeling. The maximum wall shear stress applied to the cells inside the bioreactor with low, and high core viscosity environment is estimated to be 297 and 1367 mPa, respectively, for the experimental setup employed. This work outlines the potential of LC microbioreactors as a reliable in vitro customizable platform with a wide range of applications.

Wetting of Cell Aggregates on Microdisk Topography Structures Achieved by Maskless Optical Projection Lithography

Cell aggregates as a 3D culture model can effectively mimic the physiological processes such as embryonic development, immune response, and tissue renewal in vivo. Researches show that the topography of biomaterials plays an important role in regulating cell proliferation, adhesion, and differentiation. It is of great significance to understand how cell aggregates respond to surface topography. Herein, microdisk array structures with the optimized size are used to investigate the wetting of cell aggregates. Cell aggregates exhibit complete wetting with distinct wetting velocities on the microdisk array structures of different diameters. The wetting velocity of cell aggregates reaches a maximum of 293 µm h^-1 on microdisk structures with a diameter of 2 µm and is a minimum of 247 µm h^-1 on microdisk structures of 20 µm diameter, which suggests that the cell-substrates adhesion energy on the latter is smaller. Actin stress fibers, focal adhesions (FAs), and cell morphology are analyzed to reveal the mechanisms of variation of wetting velocity. Furthermore, it is demonstrated that cell aggregates adopt climb and detour wetting modes on small and large-sized microdisk structures, respectively. This work reveals the response of cell aggregates to micro-scale topography, providing guidance for better understanding of tissue infiltration.

Alginate sulfate/ECM composite hydrogel containing electrospun nanofiber with encapsulated human adipose-derived stem cells for cartilage tissue engineering

Stem cell therapy is a promising strategy for cartilage tissue engineering, and cell transplantation using polymeric scaffolds has recently gained attention. Herein, we encapsulated human adipose-derived stem cells (hASCs) within the alginate sulfate hydrogel and then added them to polycaprolactone/gelatin electrospun nanofibers and extracellular matrix (ECM) powders to mimic the cartilage structure and characteristic. The composite hydrogel scaffolds were developed to evaluate the relevant factors and conditions in mechanical properties, cell proliferation, and differentiation to enhance cartilage regeneration. For this purpose, different concentrations (1-5 % w/v) of ECM powder were initially loaded within an alginate sulfate solution to optimize the best composition for encapsulated hASCs viability. Adding 4 % w/v of ECM resulted in optimal mechanical and rheological properties and better cell viability. In the next step, electrospun nanofibrous layers were added to the alginate sulfate/ECM composite to prepare different layered hydrogel-nanofiber (2, 3, and 5-layer) structures with the ability to mimic the cartilage structure and function. The 3-layer structure was selected as the optimum layered composite scaffold, considering cell viability, mechanical properties, swelling, and biodegradation behavior; moreover, the chondrogenesis potential was assessed, and the results showed promising features for cartilage tissue engineering application.

Progress in biomechanical stimuli on the cell-encapsulated hydrogels for cartilage tissue regeneration

BACKGROUND

Worldwide, many people suffer from knee injuries and articular cartilage damage every year, which causes pain and reduces productivity, life quality, and daily routines. Medication is currently primarily used to relieve symptoms and not to ameliorate cartilage degeneration. As the natural healing capacity of cartilage damage is limited due to a lack of vascularization, common surgical methods are used to repair cartilage tissue, but they cannot prevent massive damage followed by injury.

MAIN BODY

Functional tissue engineering has recently attracted attention for the repair of cartilage damage using a combination of cells, scaffolds (constructs), biochemical factors, and biomechanical stimuli. As cyclic biomechanical loading is the key factor in maintaining the chondrocyte phenotype, many studies have evaluated the effect of biomechanical stimulation on chondrogenesis. The characteristics of hydrogels, such as their mechanical properties, water content, and cell encapsulation, make them ideal for tissue-engineered scaffolds. Induced cell signaling (biochemical and biomechanical factors) and encapsulation of cells in hydrogels as a construct are discussed for biomechanical stimulation-based tissue regeneration, and several notable studies on the effect of biomechanical stimulation on encapsulated cells within hydrogels are discussed for cartilage regeneration.

CONCLUSION

Induction of biochemical and biomechanical signaling on the encapsulated cells in hydrogels are important factors for biomechanical stimulation-based cartilage regeneration.