Novel biomedical PDMS-PEEK composites developed for material extrusion: characterisation, printability and performance

Cardiovascular diseases such as diabetes mellitus and atherosclerosis are responsible for 72-90% of lower limb amputations across the U.K. and U.S. Although surgeons go to great lengths to produce healthy, well-formed stumps with the capacity to load pain free, this can be unachievable. Soft-polymer liners are currently state-of-the art structures situated between the residual limb and hard prosthetic socket, acting as an interface. Presently, these aren’t tailormade but rather adopt a one size fits all approach. As consequence of ill-fit and mismatching mechanical performance (between limb and liner), residual appendage/s can become susceptible to pain, requiring clinical intervention without a guaranteed fix.

To overcome complications, functionally graded materials could be used in conjunction with Additive Manufacturing (AM) technologies to manufacture bespoke prosthetic liners (and other biomedical implants, devices, prosthesis), tailormade to the patient. Despite accessibility to cheap, desktop machines (including Direct Ink Writing), there is a lack of ‘off-the-shelf’ functionally gradable materials (FGM’s) characterised for such technologies. Hence, there is both opportunity and requirement to develop functionally enhanced polymeric biomaterials that can mimic the heterogenous nature of natural human tissues (soft-to-hard) and be processed by Direct Ink Writing (DIW) technology.

Silicone-based materials such as polydimethylsiloxane (PDMS) are favoured in the construction of prosthetic liners, as their mechanical properties resemble those of natural soft-tissues including low stiffness, high extensibility, and flexibility. However, processability of PDMS by AM has proved challenging, due to its low surface tension (leading to distortion and collapse). By contrast, poly(ether) ether ketone (PEEK), another ‘state-of-the-art’ of polymer, is regularly adopted in the construction of hard-tissue prosthesis and has the capacity to retain its shape when processed by AM technologies respectively. Presently, the compositing and functional grading of such materials has received limited attention and their ‘real world-performance’ remains unknown. By compositing PDMS and PEEK, a series of novel polymers could be produced with an enhanced range (soft-to-hard) of mechanical properties, better mimicking the tissues of the body.

In this body of work, composites fabricated from two different PDMS (medical grade and non-medical grade) and PEEK were explored for the first time. PEEK particle fractions were successfully introduced into PDMS’s matrix up to 30 wt.% without the use of toxic solvents, considered an advantage when developing materials that interact with the body.

Extensive chemical (FTIR and EDX) and topological analysis (SEM) was then performed to profile composition and macroscopic structure. Thermal profiling confirmed that all materials can successfully function at body (37°C) and sterilisation temperatures (121°C) and retain high levels of mass up to 300°C before the onset of thermal decay, well within the tolerance’s prosthetic liners and implantable devices experience.

Under rheological assessment, all PDMS-PEEK composites were shear thinning in nature, which decreased with rising PEEK content and hence suited to DIW printing style. Furthermore, viscosity rises were seen as PEEK loading increased adopting the characteristics of the solid filler. All PDMS-PEEK composites were thixotropic in nature, requiring time to rebuild their structure after the application of shear rate. Notably rebuild times increased with PEEK particle loading. Amplitude sweeps were undertaken to determine the composites ability to self-support and retain shape after deposition, which was successfully achieved.

It was determined that each PDMS-PEEK composite required distinct input variables to be printed by material extrusion technologies. As viscosity rose with PEEK particle content , deposition pressure climbed (100 - 300 P/µl) and print speed declined (5.00 - 1.25 mm/s) for the PDMS matrix to adhere (to the build platform) and build. Resultingly, a range of bulk and spatially graded mechanical test pieces and components were fabricated adopting all composite materials show casing print feasibility and versatility.

Mechanical testing of bulk PDMS-PEEK composites revealed increases in stiffness with rising PEEK particle volumes in tension (0.6 - 1.50 MPa) and Shore A hardness (39 - 59). Furthermore, in compression, elastic moduli (0.7 - 5.4 MPa) values could be further tailored through variable infill (20 - 100%). It was determined that rising material volume and PEEK content significantly improved the compressive performance of PDMS. Additionally, preliminary investigations confirmed that PDMS-PEEK composites could be successfully functionally graded between materials with irreversible chemical adhesion between material interfaces, supporting the robustness of the composites. Presently, composite mechanical performance is far closer to that of soft tissue, rather ranging up to the properties of cortical bone. Further investigation and material modification is required to increase mechanical stiffness range, whilst maintaining irreversible chemical adhesion. In turn, expanding the composites appeal and adoption. The ability to further tailor the mechanical performance of PDMS-PEEK composites could lead to the manufacture of better implants, devices and prosthetics that mimic the heterogenous properties of the human body or alternatively be adopted for other industrial end use applications.