4D-Printed Shape Memory Alloy Bone Scaffolds for Dynamic Adaptation in Defect Repair

Abstract

This study developed 4D printed temperature-responsive NiTi scaffolds for dynamic bone defect repair. Medical-grade NiTi (55.8 wt% Ni) was laser powder bed fused into triply periodic minimal surface structures with gradient porosity (65-80%). Scaffolds exhibited phase transformation temperatures (Af=37.2°C, Ms=24.8°C) enabling physiological deployment. Elastic modulus transitioned from 2.8±0.3 GPa at 20°C to 0.85±0.12 GPa at 45°C, matching cortical-to-cancellous bone properties. Scaffolds achieved 98.2% shape recovery within 45 seconds, generating 12.5±2.3 MPa recovery stress. In vitro results showed 4.2-fold RUNX2 upregulation and 8.9-fold increased proliferation versus static controls. Calvarial defect models demonstrated superior regeneration with 85.3±6.8% new bone volume at 12 weeks versus 68.5±7.2% for static scaffolds. Real-time strain mapping revealed 40% stress concentration reduction during adaptation. These findings establish 4D printed NiTi scaffolds as transformative for minimally invasive bone treatment, offering personalized adaptation through synchronized mechanical-biological stimulation.