Introduction: Electrospinning is a common processing technique employed to produce microscale or nanoscale polymeric fibers from a polymer solution using a high voltage differential. This technique has been used to fabricate shape memory materials – materials that have the ability to fix a temporary shape and return to the original shape upon application of an external stimulus, such as heat[1]. Dual-electrospinning[2], or the process of electrospinning two polymeric solutions simultaneously, is an easy fabrication method to make composites. In the present work, biodegradable shape memory elastomeric composites (SMECs) were fabricated by dual-electrospinning a synthesized thermoplastic, biodegradable, elastomeric polyurethane with linear polycaprolactone (PCL).
Materials and Methods: Elastomeric polyurethanes were synthesized with short-chain PCL or caprolactone-containing soft segments and polyhedral oligomeric silsesquioxane (POSS), a nanoscale organic/inorganic hybrid material that has a well-defined, low melting point (~ 120ºC), as a hard segment.
Figure 1: Dual-electrospinning a biodegradable elastomer and PCL.
Once synthesized, this material was dual-electrospun with linear PCL to fabricate shape memory elastomeric composites. After electrospinning, films were made by compression molding the films above PCL’s melting point, but below the melting point of POSS, creating a condensed film of biodegradable elastomeric fibers bonded by PCL. The composition of the material was controlled by varying the flow rates of the two materials and was confirmed by differential scanning calorimetry.
Results and Discussion:
Figure 2: Scanning Electron Micrographs (SEMs) of electrospun fibers of the polyurethane, dual-electrospun fibrous web, and PCL.
The elastomers were designed to have a biodegradable soft segment provide elasticity and control the degradation rate, while the biocompatible hard segment physically cross-linked the polymer via crystallization. These synthesized thermoplastic polyurethane elastomers have high molecular weights (> 100 kDa), low modulus (< 15 MPa), and are elastomeric with greater than 70% recovery upon stretching. The compacted dual-electrospun materials have high strain to failure (>1000% strain), composition dependent modulus, and shape memory properties with high fixing and recovery ratios with as little as 7% PCL. In this system, PCL fixes the temporary shape of the material while the elastomer drives recovery to the permanent shape. Further, the materials degrade via hydrolysis and are durable, having greater than 400% strain-to-failure after a month of degradation.
Figure 3: Shape memory cycle of a shape memory elastomeric composite, with >90% shape fixing of the strained state and >90% recovery back to the original strain.
Conclusion: In conclusion, biodegradable SMECs have been fabricated by dual-electrospinning. The properties of these composites – soft, elastomeric, biodegradable, and shape memory – allow them to be versatile biomaterials for soft applications, such as sutures, stents, or tissue engineering scaffolds for soft tissues.
Gerry Fredrickson of Boston Scientific Corporation; Boston Scientific Corporation
References:
[1] P.T. Mather, X. Luo, and I.A. Rousseau. Shape Memory Polymer Research, Annu. Rev. Mater. Res., 2009, 39, 445-471
[2] J.M. Robertson, H.B. Nejad, and P.T. Mather. Dual-Spun Shape Memory Elastomeric Composites, ACS Macro Lett., 2015, 4, 436-440