Table 3.

Summary of representative studies that report the effects of matrix rigidity on in vivo phenomena mainly related with fibrosis, cancer, aging, and inflammation.

In vivo phenomena	Cells	Engineered matrix	Findings/comments	Refs
Fibrosis (liver, cardiac, lung, vessel, brain)	Rat hepatic stellate cell	Collagen-/fibronectin-/PLL-coated polyacrylamide gel	Hepatic stellate cells differentiate into myofibroblasts on stiff substrates, and the degree of differentiation increases along with stiffness	Olsen et al. [64]
	Rat hepatocyte, rat hepatic stellate cell, rat liver sinusoidal endothelial cell	Collagen-coated polyacrylamide gel	High level of stiffness induces intracellular tension through cytoskeleton, inducing nuclear deformation of liver cells. Disruption of cytoskeleton connection attenuates the effect	Guixé-Muntet et al. [65]
	Human liver sinusoidal endothelial cell (hLSEC), human hepatic stellate cell (hHSC), mouse cells (mLSEC and mHSC) for in vivo	Polyethylene glycol gel, collagen type 1 hydrogel	Study with in vitro liver model reveals that angiogenesis is important in early-stage model, while late-stage model is more concerned with collagen deposition. Findings offer idea for stage-specific therapeutic targets	Liu et al. [66]
	Rat cardiac fibroblast	Collagen type 1 hydrogel	Reduced serum amount and increased matrix stiffness promote the myofibroblast phenotype in the myocardium	Galie et al. [67]
	Porcine valve interstitial cell (VIC)	Collagen-coated polyacrylamide gel	Matrix stiffness modulates myofibroblast differentiation of VIC by rendering the cells to be more responsive to TGF-β stimulation	Chen et al. [68]
	Porcine valvular endothelial cell (VEC)	Collagen-coated polydimethyl siloxane gel	TGF-β selectively activates endothelial-to-mesenchymal transition of VECs on stiffer substrates	Zhong et al. [69]
	Mouse lung fibroblast	Collagen-coated polyacrylamide gel	MKL-1 activation as a result of actin cytoskeletal remodeling leads to the promotion of myofibroblast differentiation	Huang et al. [70]
	Human lung fibroblast	Gelatin methacryloylated hydrogel (GelMA)	Both isotypes of ROCK are involved in myofibroblast differentiation	Htwe et al. [71]
	Rat brain tissue	N/A	CNS tissue softens when injured, unlike other mammalian tissues	Moeendarbary et al. [74]
	Bovine aortic endothelial cell, human umbilical vein endothelial cell	Collagen-coated polyacrylamide gel	Stiffening of intima ECM alters cell contractility and leads to endothelial leukocyte extravasation, which is a critical step in atherosclerosis	Huynh et al. [99]
Tumor (onset, progression, metastasis)	Glioblastoma, adenocarcinoma, fibrosarcoma cell line (87-MG, T98G, MDA-MB-231, and HT1080)	Fibronectin-coated polyacrylamide hydrogel	Cancer cells go through durotaxis, the degree of which depends highly on the local stiffness gradient	Duchez et al. [87]
	Human mammary epithelial cell (MCF-10A)	Interpenetrating network of alginate and basement membrane matrix	High stiffness of ECM induces malignant phenotype for mammary epithelial cells, which can be counteracted by altering ECM composition	Chaudhuri et al. [89]
	MCF10A, HEK293	Collagen-coated polyacrylamide gel, hyaluronan and gelatin-based 3D hydrogel	Increase in ECM rigidity can transform normal cells into tumor precursors, a process bolstered by RTK-Ras oncogenes	Panciera et al. [91]
	Epithelial cell and cancer cell (MCF10A, hTERT-HME1, MCF7, MDA-MB-231)	Interpenetrating network of alginate and basement membrane matrix, polyacrylamide hydrogel	Increased stiffness leads to wrinkled nuclei and more accessible chromatin sites for Sp1-mediated tumorigenicity	Stowers et al. [92]
	Human gastric cell line (MKN74, KATO3)	Interpenetrating network of alginate and collagen type 1	Physical properties of microenvironment epigenetically reprogram gastric tumor cells	Jang et al. [93]
	Human umbilical vein endothelial cells, human hepatic sinusoid endothelial cells	Fibronectin-coated polyacrylamide gel	Reduction of rigidity at metastatic site improves colorectal cancer response to anti-tumor drug	Shen et al. [96]
Aging (brain)	Oligodendrocyte progenitor cell (OPC)	Laminin-coated polyacrylamide gel	Less stiff scaffold, mimicking the microenvironment of young brain, rejuvenates OPC	Segel et al. [101]
Inflammation	Mouse fibroblast	Polydimethyl siloxane disc	Soft silicone coating around the stiff implant reduces inflammatory reaction and ECM deposition in vivo	Noskovicova et al. [78]
	Monocyte (primary, THP-1, U937)	Fibrin hydrogel, collagen gel, PEG diacrylate hydrogel, fibrinogen-coated polyacrylamide gel	Pro-inflammatory reaction of MSC is modulated by adjustment of substrate rigidity	Meli et al. [112]
	Human mesenchymal stem cell	Alginate-based hydrogel	MSC primed by soft substrates reacts more profoundly to TNFα stimulation	Wong et al. [116]