Development of Tissue-Safe, Shape Memory Polymer Scaffolds

Thermoresponsive shape memory polymers (SMPs) were developed as self-fitting scaffolds to heal cranial bone defects. Exposure to temperatures above the melt transition temperature (T_m) causes scaffolds to undergo shape recovery, driving its expansion to the tissue perimeter for improved osseointegration and healing. SMP scaffolds were originally prepared from semi-crystalline, biodegradable <i>linear</i>-poly(ε-caprolactone)-diacrylate (PCL-DA; 10k g mol^-1). However, a major limitation of scaffolds based on <i>linear</i>-PCL-DA is a T_m of ~55 °C. Such a scaffold could be submerged into saline at this temperature and pressed into a bone defect for self-fitting. However, to extend the working time, post-implantation irrigation with ~55 °C saline would induce thermal damage to the surrounding tissues. Ideally, a SMP self-fitting bone scaffold would have a reduced T_m, but still be &gt; 37 °C in order to ultimately return to its rigid state for mechanical support. Other devices prepared from SMP scaffolds could also be envisioned if the T_m was reduced to 37 °C, affording shape recovery (expansion) upon implantation. For instance, this would enable the development of self-expanding gynecological stents. Thus, in this work, a T_m of ~39 - 45 °C (bone scaffolds) and ~37 °C (stents) was targeted for tissue safety and functionality. We sought to lower the T_mof PCL by altering the architecture, creating <i>star</i>-PCL-tetracrylate (<i>star</i>-PCL-TA), and by also decreasing molecular weight (M_n). This yielded scaffolds with T_m of ~45 °C (10k g mol^-1), ~40 °C (7.5k g mol^-1), and ~29 °C (5k g mol^-1). Owing to reduced crystallinity, scaffolds based on <i>star</i>-PCL-TA degraded faster than <i>linear</i>-PCL-DA of the same M_n, and also exhibited decreased compressive moduli. Scaffold degradation rates increased as the M_n of <i>star</i>-PCL-TA was decreased. Overall, by altering the architecture of the PCL macromer from a <i>linear</i> to a<i> star</i> architecture and reducing the M_n, more tissue-safe SMP scaffolds were created. Specifically, scaffolds with T_m values ideal for self-fitting bone scaffolds and self-expanding stents were obtained.