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<p>This study investigates dissolution-induced decoupling of strength and stiffness in uncalcined hydroxyapatite (ucHA)/poly-L-lactic acid (PLLA) biodegradable composites by correlating dissolution kinetics with microstructural reorganization and mechanical evolution. ucHA particle size was controlled from 9.50 to 2.29 <i>µ</i>m through jet mill and incorporated into PLLA at 0–20 wt% via twin-screw extrusion to fabricate composition-controlled composites. Dissolution behavior was evaluated for up to 30 days in a citric acid buffer according to ISO 10993-14. During the dissolution, the calcium and phosphorus ion release increased nonlinearly with ucHA content, exhibiting a pronounced acceleration above 10 wt% ucHA. In addition, the concentration of calcium and phosphorus ions released into the solution increased with dissolution time, but slowly decreased and remained constant after 20 days, suggesting a mineralization reaction by dissolution. These dissolution processes were accompanied by marked surface reorganization, including cavity formation and the development of densely distributed nanoscale mineralized particles with characteristic sizes of approximately 40–55&#xa0;nm. Mechanical behavior evolved non-proportionally during dissolution. Prior to dissolution, flexural strength reached a maximum of 99.09&#xa0;MPa at 10 wt% ucHA. After 30 days of dissolution, polymer-rich composites exhibited pronounced strength degradation, whereas composites containing ≥ 10 wt% ucHA preserved or slightly increased flexural strength, reaching up to 101.92&#xa0;MPa despite ongoing material loss. In contrast, flexural modulus increased substantially with dissolution time, accompanied by a transition in stress-strain response from progressive, matrix-dominated deformation to stiffness-controlled fracture behavior. These results demonstrate that dissolution does not simply degrade ucHA/PLLA composites but induces microstructural reorganization that progressively shifts load-bearing contributions from the polymer matrix to the inorganic phase, thereby decoupling strength retention from stiffness evolution. This dissolution-induced mechanical transition provides a quantitative materials-design strategy for tailoring long-term performance of bioresorbable calcium phosphate-polymer composites through controlled inorganic content and dissolution kinetics.</p>

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Dissolution-induced decoupling of strength and stiffness in uncalcined HA/PLLA biodegradable composites

  • 期刊论文

  • 发表日期：

    2026年5月27日

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  • Woo Young Jang,
  • Jeong Ho Chang

摘要

This study investigates dissolution-induced decoupling of strength and stiffness in uncalcined hydroxyapatite (ucHA)/poly-L-lactic acid (PLLA) biodegradable composites by correlating dissolution kinetics with microstructural reorganization and mechanical evolution. ucHA particle size was controlled from 9.50 to 2.29 µm through jet mill and incorporated into PLLA at 0–20 wt% via twin-screw extrusion to fabricate composition-controlled composites. Dissolution behavior was evaluated for up to 30 days in a citric acid buffer according to ISO 10993-14. During the dissolution, the calcium and phosphorus ion release increased nonlinearly with ucHA content, exhibiting a pronounced acceleration above 10 wt% ucHA. In addition, the concentration of calcium and phosphorus ions released into the solution increased with dissolution time, but slowly decreased and remained constant after 20 days, suggesting a mineralization reaction by dissolution. These dissolution processes were accompanied by marked surface reorganization, including cavity formation and the development of densely distributed nanoscale mineralized particles with characteristic sizes of approximately 40–55 nm. Mechanical behavior evolved non-proportionally during dissolution. Prior to dissolution, flexural strength reached a maximum of 99.09 MPa at 10 wt% ucHA. After 30 days of dissolution, polymer-rich composites exhibited pronounced strength degradation, whereas composites containing ≥ 10 wt% ucHA preserved or slightly increased flexural strength, reaching up to 101.92 MPa despite ongoing material loss. In contrast, flexural modulus increased substantially with dissolution time, accompanied by a transition in stress-strain response from progressive, matrix-dominated deformation to stiffness-controlled fracture behavior. These results demonstrate that dissolution does not simply degrade ucHA/PLLA composites but induces microstructural reorganization that progressively shifts load-bearing contributions from the polymer matrix to the inorganic phase, thereby decoupling strength retention from stiffness evolution. This dissolution-induced mechanical transition provides a quantitative materials-design strategy for tailoring long-term performance of bioresorbable calcium phosphate-polymer composites through controlled inorganic content and dissolution kinetics.