High performance shape memory polymer networks based on rigid nanoparticle cores

Abstract

Smart materials that can respond to external stimuli are of widespread interest in biomedical science. Thermal-responsive shape memory polymers, a class of intelligent materials that can be fixed at a temporary shape below their transition temperature (T(trans)) and thermally triggered to resume their original shapes on demand, hold great potential as minimally invasive self-fitting tissue scaffolds or implants. The intrinsic mechanism for shape memory behavior of polymers is the freezing and activation of the long-range motion of polymer chain segments below and above T(trans), respectively. Both T(trans) and the extent of polymer chain participation in effective elastic deformation and recovery are determined by the network composition and structure, which are also defining factors for their mechanical properties, degradability, and bioactivities. Such complexity has made it extremely challenging to achieve the ideal combination of a T(trans) slightly above physiological temperature, rapid and complete recovery, and suitable mechanical and biological properties for clinical applications. Here we report a shape memory polymer network constructed from a polyhedral oligomeric silsesquioxane nanoparticle core functionalized with eight polyester arms. The cross-linked networks comprising this macromer possessed a gigapascal-storage modulus at body temperature and a T(trans) between 42 and 48 degrees C. The materials could stably hold their temporary shapes for > 1 year at room temperature and achieve full shape recovery <or= 51 degrees C in a matter of seconds. Their versatile structures allowed for tunable biodegradability and biofunctionalizability. These materials have tremendous promise for tissue engineering applications.

Scheme 1.

Depiction of a nanostructured SMP network.

Fig. 1.

Preparation and thermal-mechanical properties of SMPs containing POSS (POSS-SMP) versus organic (Org-SMP) cores: (A) synthesis and cross-linking of macromers; (B) storage modulus (E′)-temperature and loss angle (Tan δ)-temperature (denoted by black arrows) curves of POSS-SMP-20 versus Org-SMP-20; (C) recovery rates of POSS-SMP-20 (red arrows) versus Org-SMP-20 (blue arrows) from an identical rolled-up temporary shape (Left) to fully extended rectangle (30.0 mm × 6.0 mm × 0.5 mm) in water at 51 °C.

Fig. 2.

Thermal-mechanical properties of POSS-SMPs with varying PLA arm lengths: (A) DSC heat flow (ΔH)-temperature curves and the transparent appearance (Inset); (B) storage modulus (E′)-temperature and loss angle (Tanδ)-temperature curves (denoted by black arrows); (C) one-way shape memory cycles. Starting with 0% tensile stain at 85 °C, all specimens were subjected to consecutive cycles of tensile deformation (1), cooling (2), unloading of tensile stress (3), and recovering (4). The first 4 cycles of each specimen are representatively shown.

Fig. 3.

Chemical modification of POSS-SMP with a bioactive peptide: (A) Synthetic scheme illustrating the introduction of azido groups during the covalent cross-linking of POSS-(PLA₂₀)₈ and subsequent conjugation of fluorescently labeled integrin-binding peptide to POSS-SMP via “click” chemistry. (1) 100 ppm DBTDL, CH₂Cl₂, argon, r.t., 12 h; 75 °C, argon, 24 h; 75 °C under vacuum, 48 h. (2) Aqueous solution of CuSO₄ (2.5 mM) and L(+)-ascorbic acid sodium salt (7.5 mM), r.t., 24 h. (B) Storage modulus (E′;)-temperature curves and loss angle (Tan δ)-temperature curves (denoted by black arrows) of POSS-SMP-20, POSS-SMP-20-Az, and POSS-SMP-20-Peptide. (C) Differential interference contrast (DIC) and fluorescent (Fl) micrographs confirming the covalent conjugation of the fluorescently labeled peptide via click chemistry. In the negative control (Left), POSS-SMP-20-Az was exposed to the fluorescently labeled peptide in the absence of ascorbic acid under otherwise identical reaction conditions.