A key yet often overlooked factor in the unpredictable failure of degradable systems is “acquired” heterogeneity — a spatially and temporally evolving unevenness of degradation that emerges and aggravates during service, rather than originating from manufacturing defects. Controlling this inherent randomness is particularly critical for biodegradable implants, where corrodible metals offer the necessary strength but are susceptible to such unpredictable localized weakness.
Inspired by the Poisson raindrop question in mathematics, Professor Jiandong Ding at Fudan University derived a set of equations for the spatiotemporal evolution of random degradation. The research team employed these equations to quantify the acquired corrosion heterogeneity of biodegradable metals with and without polymer coatings.
They found that coating iron with polylactide (PLA) increases the corrosion rate by 3-fold but reduces the inhomogeneity by 4,000-fold in a blood-mimetic Hank’s solution, surprisingly leading to later device fracture despite faster degradation. The mechanism involves two synergistic effects: PLA hydrolysis creates an acidic microenvironment (pH~5.6) at the PLA-Fe interface, and the coating inhibits the deposition of a passive calcium-phosphorus layer on the iron surface. Together, these factors lower the energy barrier for pit nucleation, transforming corrosion from “few and deep” pits to “many and shallow” pits across the substrate.
Mechanical tests revealed that despite greater corrosion mass loss, PLA-coated iron wires exhibited significantly higher ultimate strength and elongation at fracture than bare iron after 30 days of immersion. In cyclic fatigue tests, bare iron wires fractured at approximately 19,000 cycles, while PLA-coated wires withstood up to 40,000 cycles without fracture.
Based on these fundamental findings, Ding’s research team at Fudan University in Shanghai collaborated with Fuwai Hospital in Beijing and Biotyx Medical (a subsidiary of Lifetech Scientific) in Shenzhen to develop a biodegradable metal–polymer composite stent with struts as thin as a strand of hair (< 70 μm total thickness). The stent’s safety and efficacy were validated through preclinical studies in porcine models, clinical trials involving 1,108 patients across multiple cohorts, and 5-year follow-up of all 45 first-in-human (FIH) cases. Core clinical data show: 0 cardiac deaths, 0 target vessel myocardial infarctions, 5 clinically indicated target lesion revascularizations, a target lesion failure rate of 11.1%, and 0% stent thrombosis — outcomes that compare favorably with both the discontinued Absorb BVS and the gold-standard Xience stent.
The theoretical framework and polymer coating strategy established in this study extend far beyond vascular stents. Any scenario where “acquired heterogeneity” arises from random spatiotemporal variability and affects system reliability — from ships and hydraulic structures to biological evolution and big data analytics — stands to benefit from the insights provided.
Science China Materials
Experimental study