Printability and mechanical performance of biomedical PDMS-PEEK composites developed for material extrusion

Polydimethylsiloxane (PDMS) materials are widely adopted in the manufacture of facial prostheses, lab-on-chip devices and scaffolds for soft-tissue engineering applications; however, their processing by additive manufacturing (AM) has proved challenging. Liquid silicone rubbers (LSRs) are favoured for their high shape fidelity when cast, but their low viscosity and surface tension often prevent self-support, post-extrusion. Poly(ether) ether ketone (PEEK) particle reinforcement through interfacial bonding has proven to enhance key properties of PDMS, expanding their end-use functionality. Still, the impact of such particles on the printability of LSR-PDMS is not explored. In this study, for the first time, solvent-free biocompatible PDMS-PEEK composites (up to 30 wt% PEEK) were successfully characterised for material extrusion (ME) printing. Rheological analysis confirmed shear-thinning of all PDMS-PEEK composites under applied load (within the tolerances of the printer) and dominant storage moduli at rest (i.e. prints can self-support), considered highly desirable for ME-based printing. Attained rheological datasets were used to guide initial printability studies, which revealed finer track fidelity with rising fractional content of PEEK, at comparable print speed and displacement values. Composites with higher PEEK content demonstrated significant increases in Shore A hardness and stiffness (in tension and compression) in bulk form. Last but not least, enhanced shape fidelity (thanks to PEEK reinforcement) and geometrical autonomy further expanded the manufacturing freedom of PDMS, whereby infill density could be controlled in order to increase the range of mechanical performance, previously unachievable with conventional casting fabrication. Fundamentally, this could lead to the manufacture of bespoke spatially graded multi-material structures and devices that could be used to replicate the heterogenous properties of soft human tissues and in other advanced material applications.