Poly(lactic acid) (PLA) has attracted much attention as a material for bioabsorbable bone fixation devices, however degradation rate of PLA is very low. Surface treatment of PLA has been investigated to increase degradation rate due to improvement in hydrophilicity. In this study, effects of ion-implantation on degradation rate and mechanical properties were investigated. Argon gas which is chemically stable was used as ion source, and argon ion was implanted in the surface of PLA. The contact angle and the surface roughness of the ion-implanted PLA were measured to evaluate hydrophilicity, and tensile tests and micro-indentation tests were conducted to evaluate mechanical properties. The difference in surface morphology after <i>in vitro</i> degradation test between ion-implanted and un-implanted region was observed to investigate degradation properties of ion-implanted PLA. As a result, tensile strength increased by 8.2 % compared to un-implanted specimen and Vickers hardness increased by 9.5%. The ion-implantation might affect mechanical properties in the overall specimen rather than those in the surface. The result of <i>in vitro</i> degradation test showed that an initial degradation rate was accelerated by ion-implantation. This result is consistent with the increasing hydrophilicity of ion-implanted PLA. Those results suggested that ion-implantation accelerates degradation of PLA without decreasing mechanical properties.

Keywords

Poly(lactic acid), Bioabsorbable, Ion-implantation, Degradation rate, Mechanical properties

Paper information

[Advance Publication] (Proper information for citation will be announced after formal publication)

I<span style="caret-color: rgb(69, 69, 69); color: rgb(69, 69, 69); font-family: robotoregular, Meiryo; font-size: 13px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; display: inline !important; float: none;">n order to improve bioactivity of hydroxyapatite (HA), the surface of HA was modified with calcium or magnesium ion irradiation. Calcium ion irradiation improved bioactivity of HA up to ion dose 10</span><sup style="box-sizing: border-box; line-height: inherit; position: relative; vertical-align: baseline; top: -0.5em; margin: 0px; padding: 0px; border: 0px; font-family: robotoregular, Meiryo; font-size: inherit; font-style: normal; font-variant-caps: normal; font-weight: normal; font-stretch: inherit; caret-color: rgb(69, 69, 69); color: rgb(69, 69, 69); letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none;">14</sup><span style="caret-color: rgb(69, 69, 69); color: rgb(69, 69, 69); font-family: robotoregular, Meiryo; font-size: 13px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; display: inline !important; float: none;"><span class="Apple-converted-space">&nbsp;</span>ions/cm</span><sup style="box-sizing: border-box; line-height: inherit; position: relative; vertical-align: baseline; top: -0.5em; margin: 0px; padding: 0px; border: 0px; font-family: robotoregular, Meiryo; font-size: inherit; font-style: normal; font-variant-caps: normal; font-weight: normal; font-stretch: inherit; caret-color: rgb(69, 69, 69); color: rgb(69, 69, 69); letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none;">2</sup><span style="caret-color: rgb(69, 69, 69); color: rgb(69, 69, 69); font-family: robotoregular, Meiryo; font-size: 13px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; display: inline !important; float: none;">, whereas magnesium ion irradiation did not affect bone-like apatite formation. It is found that this different phenomena were attributed to surface electrical potential. Ion irradiation generally induces surface nano-scopic damage, however the strength of HA was not significantly affected by ion irradiation. From cell proliferation, no toxicity of HA irradiated with calcium ions was confirmed. From these results, the effectiveness of calcium ion irradiation on the treatment of bone defect using HA is confirmed.</span>

Keywords

Hydroxyapatite, Ion irradiation, Strength, Bone-like apatite formation, Cell proliferation

Paper information

[Advance Publication] (Proper information for citation will be announced after formal publication)

In this study, effects of strain rate on the mechanical properties of injection-molded poly(L-lactide) (PLLA) were investigated experimentally. The effect of crystallinity on the strain rate dependency of mechanical properties of PLLA was also examined by annealing the specimens at 70 and 130 °C for 24 hours. In order to characterize the mechanical properties, tensile and compressive tests were conducted. The results of tensile tests indicate that the Young's modulus kept constant up to strain rate of 10-1. On the other hand, tensile strength of non-annealing, 70°C-24h and 130°C-24 specimens increased with increasing strain rates up to 10-1, 10-2 and 10-3, respectively, and decreased or kept constant because of decrease in the fracture strain with increasing strain rate. The effect of strain rate became lower with increasing crystallinity, which means the strain rate dependency of the PLLA under tensile loading is more effective in the amorphous region. The results of compressive tests indicate that the compressive Young's modulus kept constant up to strain rate of 10-1. On the other hand, 0.2 % proof stress increased with increasing strain rate. This tendency was similar to the tensile test.

Keywords

Poly(L-lactide), Strain Rate, Crystallinity, Amorphous

Paper information

Shusaku YAMADI and Satoshi KOBAYASHI, “Effect of Strain Rate on the Mechanical Properties of Crystallized Poly(L-lactide)”, Journal of Biomechanical Science and Engineering, Vol. 3, No. 4 (2008), pp.453-460 . doi:10.1299/jbse.3.453