Hsiao-Ying Shadow Huang, Siyao Huang, Kaitlyn Barbour
A heart valve leaflet flexes tens of millions of times each year inside a fluid carrying active collagenases, yet its collagen network holds for decades before disease unravels it within months. This review argues the resolving principle is mechanical. Tensile strain reshapes the collagen triple helix and folds its scissile bond away from matrix metalloproteinases (MMPs), a protective behavior termed strain-stabilization. The mechanism is traced from reconstituted collagen and tendon to native semilunar valve leaflets under physiologically realistic biaxial loading. The organizing framework is a V-shaped force-degradation curve: enzymatic attack is rapid at zero strain, falls to a minimum under moderate physiological tension, and rises under pathological overload, where single-molecule measurements record a 100-fold increase in MMP-1 cleavage rate at ~10 pN. Two biaxial studies anchor the argument. Equibiaxial stretching of porcine leaflets accelerated collagenolysis, whereas loading matched to diastolic strain ratios suppressed it almost entirely. Crimp, fiber rotation, and heterogeneous recruitment govern how tissue-level strain reaches individual molecules and which arm of the V-curve a leaflet occupies. Glutaraldehyde-fixed bioprosthetic valves fail by this logic inverted: fixation freezes the helix, disables the protective cycle, and strips glycosaminoglycans from the interfibrillary space before implantation. Conditioning tissue-engineered scaffolds under physiological biaxial strain produces collagen anisotropy matching native leaflets and tunes valve interstitial cell biosynthesis toward a homeostatic phenotype. Loading is a design variable, and a valve can be engineered to keep its collagen in the protected regime.