Table 1.

Stiffness measurements of myocardium and cardiac ECM

Paper	Sample Tested	Measurement	Analysis	Reported stiffness	Key Findings
Demer 1983 [82]	Canine LV	Uniaxial and Biaxial tensile	Exponential stress-strain function	A = 4.3±3.6kPa (fiber direction)	Myocardial stiffness was anisotropic between fiber and cross-fiber
				A = 3.9±2.7kPa (cross-fiber direction)	directions.
Halperin 1987 [91]	Canine IVS	Triaxial; Tensile in-plane with transverse compression	Slope of indentation peak stress-strain curve	E = 14.8kPa (Transverse Compressive)	In-plane stress and strain indices positively correlated with transverse stiffness.
Przyklenk 1987 [92]	Canine LVFW	Uniaxial tensile	TM at 2g/mm2	TM = 2–4kPa	Tensile strength and stiffness correlated positively with hydroxyproline content. Epicardium and visceral pericardium were the stiffest and most collagenous.
Yin 1987 [93]	Canine LV	Biaxial tensile	Fit three strain-energy functions	See paper for functions and parameters
Humphrey 1990 [94]	Canine LV, RV walls	Biaxial tensile		Stress strain curves but no moduli reported
Sacks & Chuong 1993 [83]	Canine RVFW	Biaxial tensile	Reported the maximum TM of generated stress-strain curves	TM = ~75kPa (RVFW, fiber)	RVFW had qualitatively similar mechanics to the LVFW but with more anisotropy.
				TM = ~30–50kPa (RVFW, cross-fiber)
				TM = ~50kPa (LVFW, both)
Novak 1994 [84]	Canine IVS and LVFW	Biaxial tensile	Five parameter pseudostrain-energy function	Several fitted parameters	Inner and outer regions of the LVFW were stiffer than the middle sections tested.
Kag 1996 [95]	Bovine endo, myo-, epicardium	Biaxial tensile	Pseudostrain -energy functions	Several fitted parameters	Endocardium was stiffer at low strains.
Lieber 2004 [96]	Rat isolated myocytes	AFM indentation	Classical Infinitesimal Strain Theory – Conical Geometry	E = 35.1±0.7kPa (4 months age)	Myocytes stiffened with age, and intracellular stiffness was independent of myocyte dimensions.
				E = 42.5±1kPa (30 months age)
Berry 2006 [75]	Rat LV; infarcted and control	AFM indentation	Hertz Model	E = 18±2kPa (healthy)	Infarcted rat hearts were stiffer than control/healthy tissues.
				E = 55±15kPa (infarct)
Engler 2008 [81]	Quail, embryonic	AFM indentation	Hertz model	E = 11 kPa
Fomovsky, Holmes 2010 [76]	Rat LV; infarcted	Biaxial tensile	Strain-energy function quadratic fit	C1 = 400kPa stiffening to ~1mPa	Infarcted rat hearts increased in collagen content, collagen crosslinking, and biaxial tensile strength over time from 3 to 6 weeks after Ml. Infarcts were mechanically isotropic rather than anisotropic.
Jacot 2010 [77]	Murine epicardial surface E13.5 to P14	AFM indentation	Hertz Model	E = 12±4 kPa (E13.5)
				E = 39±7kPa (P14)
Gershlak	Rat LV ECM	Uniaxial	Young’s	E = 10kPa (fetal)
2013 [97]		tensile	modulus (60–70% strain; linear region)	E = 20kPa (adult)
Kichula 2014 [98]	Explanted Ovine LV	Biaxial tensile	Constitutive strain-energy function from	C = 0.679±0.16 kPa from curve fitting.	Longitudinal stiffness was significantly higher than circumferential stiffness in passive measurements of control heart tissue, highlighting the importance of the method choice for reporting overall myocardial stiffness.
			Guccione et	At 0.1–0.15:
			al, 1991	E=9.9kPa
			E at strains 0.05–0.1; 0.1–0.15; 0.15–0.2	(longitudinal)
				E = 70kPa (circumferential)
Bhana 2014 [78]	Rat myocardium	Micropipette Aspiration	Homogeneous half-space model[99]	E = 6.8±2.8kPa (neonatal)	Polyacrylamide gels of myocardial-like stiffness were optimal for the In vitro culture of rat cardiomyocytes.
				E = 25.6±15.9kPa (adult)
Sommer 2015 [100]	Human heart transplant biopsies	Biaxial tensile, Triaxial shear		E~52±30kPa (fiber)
				E~26±12kPa (cross-fiber) (5% E calculated from table 10) LVFW: 54kPa Septum: 48kPa RVFW: 44kPa
Perea Gil 2015 [79]	Porcine LV, intact vs. dECM	AFM indentation	Hertz Model	Native tissue: 26.1 ±3.6kPa,	Decellularization caused no significant changes to E. There were no significant differences in stiffness between heart layers.
Quinn 2016 [101]	Rat LV ECM	Uniaxial tensile	Microstructur al fiber recruitment model: E at full fiber recruitment and stretch of 1.25	Emax = 345 ±73kPa (healthy)	ECM elastic modulus did not differ between circumferential and longitudinal stretching. In the postinfarct scar, collagen content increased but tensile elastic modulus decreased. The non-linear toe regions of the stress-strain curves were elongated following Ml.
				Emax = 152 ±104kPa (8wks post Ml)
				E1.25 = 87 ±44kPa (healthy)
				E1.25 = 38 ±26kPa (8wks post Ml)
Ramadan 2017 [102]	Ovine, Porcine LV	Uniaxial tensile	Young’s modulus with assumptions	Tensile moduli: E 47±23kPa (ovine)	Linear frequency dependence of storage modulus also measured by DMA, tan delta around soft rubber
			from Chen et al, 1996[103]	E = 63±23kPa (porcine)	(PDMS)
Gluck 2017 [104]	Porcine dECM LV and SAN	AFM indentation	Hertz Model	E 5.35±0.14kPa (LV)	SAN ECM was stiffer than LV ECM.
				E = 16.69 ±0.32kPa (SAN)
Spreeuwel 2017 [80]	Murine LV and RV, mdx and TAC models	Microindentati on, 2mm probe	Indentation model for planar anisotropic soft tissue from Cox et al 2006 [105]	ELV = 12.4±4.8 kPa (healthy)	Mdx mutation significantly lowered myocardial LV modulus but not RV modulus. TAC did not have a significant impact on overall LV modulus.
				ERV = 11.1±3.9 kPa (healthy)
Notari 2018 [106]	Murine neonatal dECM LV, P1/P2	AFM indentation	Hertz Model	E~15kPa (P1)
				E~35–45kPa (P2)
Fujita 2018 [107]	Goat dECM ventricle	Uniaxial compression	Nonlinear Kelvin Model	K1=0.2kPa
				K2=0.1kPa

Heart Anatomy Abbreviations: LV = left ventricle; IVS = interventricular septum; LVFW = left ventricular free wall; RVFW = right ventricular free wall; SAN = sinoatrial node; dECM = decellularized extracellular matrix; MI = myocardial infarction; TAC = trans-aortic constriction

Mechanics Abbreviations: A = stress-strain amplitude parameter; TM = tangent modulus; E = Young’s/elastic modulus; K1, K2 = spring stiffness elements; C1 = Cauchy-Green fitted coefficient; DMA = dynamic mechanical analysis