Figure 1. Schematic illustration of matrix mechanical cues that regulate cell behaviours. In general, the matrix mechanical cues include elasticity, viscoelasticity, topography, fibre stiffness, and external stimuli, all of which can regulate many cellular behaviours, including cell adhesion, spreading, proliferation, migration and differentiation. However, a single mechanical stimulus is generally insufficient to induce stem cell differentiation and achieve tissue regeneration; instead, multimodal mechanical factors including elasticity, viscoelasticity, topography and external mechanical stimuli would have a synergistic effect in guiding cell behaviours to promote tissue regeneration.

Figure 2. (A, B) Distinct moduli of human tissues suggesting tissue-specific stiffness (A) and substrate elasticity (B) used to direct stem cell differentiation toward the cell phenotypes of various tissues. Figure 2A was adapted from Handorf et al.8 by using some of its data in combination with other data source13-15 and Figure 2B was reprinted from Han et al.18 Copyright © Royal Society of Chemistry.

Figure 3. Stages in the development of research showing how matrix elasticity regulates cell behaviours. (A) The pioneering study demonstrates that human BMSCs were effectively induced to differentiate into neuronal, muscle or bone lineages when they were cultured on soft, medium, or stiff substrates. Stiffness was then considered as one of the most important mechanical cues in tissue engineering. Reprint from Engler et al.20 Copyright 2006, with permission from Elsevier. (B) In the following years, numerous biomaterials have been developed to explore cell behaviours including stem cell fate affected by substrate elasticity, and its underlying molecular mechanism. (C) In recent years, many studies began to focus on the interplay between substrate elasticity and other cues (such as topography, geometry, growth factors, etc.) and its effect on cell behaviours. In addition, to simulate dynamic changes in stiffness in vivo, biomaterials with dynamic elasticity in situ have been designed to explore mechanobiological pathways that may differ from those under static cell culture. Further, new mechanosensitive proteins have been found to be involved in the cellular responses toward matrix elasticity, including YAP, piezo, caveolin-1, etc. BMSCs: bone marrow mesenchymal stem cells; FAK: focal adhesion kinase; MAPK: mitogen-activated protein kinase; ROCK: RHO-related protein kinase 1; YAP: yes-associated protein.

Figure 4. Schematic diagram showing the molecular mechanism of the changes undergone by viscoelastic hydrogels when subjected to an external force. (A) Polyacrylamide-based hydrogels with different loss moduli varied through the movement of loose ends of polymer chains, or the loosing of entangled linear polyacrylamide. (B) Physically cross-linked hydrogels with varying viscoelasticity through the breaking of ionic interactions, hydrogen bonding, guest-host interactions, etc. In particular, for ionically cross-linked hydrogels, the viscoelasticity can also be tuned by incorporating covalent cross-linkers and polymer spacers. (C) Chemically-dynamic cross-linked hydrogels which change through the dissociation of chemical covalent bonds.

Figure 5. Substrate stress-relaxation regulates scaffold remodelling and bone formation in vivo. (A) Young’s modulus and stress relaxation of slow- and fast-relaxing alginate hydrogels. (B) Representative micro-computed tomography renderings of rat calvaria 3 months post-injury. (C) Masson’s trichrome staining of the defect site in fast-relaxing and slow-relaxing gel conditions. Scale bars: 1 cm in B and 2 mm in C. Data are expressed as mean ± SD (n = 8–10) and were analysed by Student’s t-test. τ1/2: stress relaxation rate. Reproduced with the permission of Darnell et al.94 Copyright Wiley-VCH Verlag GmbH & Co. KGaA.

Figure 6. YAP/TAZ mediated pathway through mechano-transduction. A cell probes its ECM mechanical environment via membrane receptors (e.g. integrins) to transmit force which regulates the stability of focal adhesion complexes containing focal adhesion kinase (FAK). FAK phosphorylates and activates mechanoresponsive signalling elements, such as mitogen-activated protein kinase (MAPK) and transforming protein Ras homolog gene family member A (RHOA). Simultaneously, the intracellular force regulates the nuclear translocation of transcription regulator such as yes-associated protein (YAP)/transcriptional co-activator with PDZ-binding motif (TAZ).