Melt electrowriting of a biocompatible photo-crosslinkable poly(D,L-lactic acid)/poly(epsilon-caprolactone)-based material with tunable mechanical and functionalization properties

The use of polymeric biomaterials to create tissue scaffolds using additive manufacturing techniques is a well-established practice, owing to the incredible rapidity and complexity in design that modern 3D printing methods can provide. One frontier approach is melt electrowriting (MEW), a technique that takes advantage of electrohydrodynamic phenomena to produce fibers on the scale of 10's of microns with designs capable of high resolution and accuracy. Poly(e-caprolactone) (PCL) is a material that is commonly used in MEW due to its favorable thermal properties, high stability, and biocompatibility. However, one of the drawbacks of this material is that it lacks the necessary chemical groups which allow covalent crosslinking of additional elements onto its structure. Attempts to functionalise PCL structures therefore often rely on the functional units to be applied externally via coatings or integrally mixed elements. Both can be extremely useful depending on their applications, but can add extra difficulties into the use of the resulting structures. Coatings require careful design and application to prevent rapid degradation, while elements mixed into the polymer melt must deal with the possibilities of phase separation and changes to MEW properties of the unadulterated polymer. With this in mind, this study sought to imbibe functionality to MEW-printed scaffolds using the approach of adding functional units directly, via covalent bonding of functional groups to the polymer itself. To this end, this study employs a recently developed class of polymers called acrylate-endcapped urethane-based polymers (AUPs). The polymer backbone of the specific AUP used consists of a poly(D,L-lactic acid) (PDLLA)/PCL copolymer chain, which is functionalized with 6 acrylate groups, 3 at either end. Through blending of the AUP with PCL, various concentrations of this mixture were used with MEW to produce scaffolds that possessed acrylate groups on their surface. Using UV crosslinking, these groups were tagged with Fluorescein-o-Acrylate to verify that PDLLA/PCL AUP/PCL blends facilitate the direct covalent bonding of external agents directly onto the MEW material. Blending of the AUP with PCL increases the scaffold's stiffness and ultimate strength. Finally, blends were proven to be highly biocompatible, with cells attaching and proliferating readily at day 3 and 7 post seeding. Through this work, PDLLA/PCL AUP/PCL blends clearly demonstrated as a biocompatible material that can be processed using MEW to create functionalised tissue scaffolds.