July 2019 3DPrint.com
In ‘3D Printing of Flexible Electrodes for Clinical Applications,’ Laura Blanco Peña of the University of Wollongong presents her thesis on the benefits of 3D printing in medicine, and specifically in devices like cochlear implants. Usually presented in the shape of a tiny seashell, the cochlear implant encourages hearing as the auditory nerve is stimulated. In this research, Peña 3D printed electrode arrays to offer more complex stimulation, experimenting with both inkjet printing and 3D printing of conductive rLCGO/PDMS coaxial fibres.
An electrode array consists of platinum/iridium (90/10) wires in a silicon carrier with 22 platinum electrode contacts on the distal end, with each wire providing a channel for stimulation to the patient’s ear. “Since the device is implanted within the head of the patient, the materials used for its fabrication must ensure its safety and long-term functionality, and therefore, be biocompatible, resistant to mechanical forces, and stable over time. The materials used for the implant fabrication that are in contact with the patient’s tissues– silicon, Pt, titanium, and ceramics – show the required biocompatibility, corrosion-resistance, low reactiveness, and mechanical resistance while ensuring the conductivity and flexibility of the electrode,” states Peña in her research.
Components in thermal and piezoelectric (acoustic) inkjet printers
While silver offers the best conductivity in terms of metals, it also poses serious health issues due to reactivity and cytotoxicity; platinum (Pt), however, is suitable for medical applications due to high biocompatibility and good conductivity. “To advance on the inkjet printing of Pt-precursor ink, we aimed to optimise the printing parameters in order to be able to print continuous, straight lines in different directions,” states Peña. “Before this, an evaluation of the effect of air plasma and polydopamine coating on PDMS wettability over time was also performed in order to understand the most suitable surface treatment method for printing.”
Although the research team attempted to print the Pt-precursors via inkjet printing for suitable conductivity in situ, they were not able to create the desired conductive patterns. They encountered significant challenges due to lack of conductivity—and although the approach has potential, Peña stated that further study of the patterns would be required to find a better solution.
Next, they explored the use of coaxial structures with graphene fibres as the core, and polydimethylsiloxane (PDMS) for the outer layer. Graphene has become popular in 3D metal printing and in creating composites due to its good mechanical properties, conductivity, and biocompatibility too. Graphene fibres require a carrier material, however, with PDMS being a good option. Even better, the fibres can be coated with platinum for even better conductivity and biocompatibility.
“Mimicking electric wires, the rLCGO/PDMS coaxial fibre would have a conductive core (rLCGO fiber) surrounded by an outer layer of insulating PDMS,” stated Peña. The extrusion of such fibres next to each other and in multiple layers, where rLCGO fibres are arranged in parallel within a PDMS structure would allow to fabricate a flexible, solid, conductive constructs with a multidimensional electrode array having the desired number of conductive channels.”
Potential 3D printed coaxial construct for the CI. Coaxial fibres have a rLCGO fibre as conductive core and PDMS as insulating, outer layer. 3D printing these fibres would allow fabricating a flexible, solid construct with multiple parallel rLCGO fibres acting as an electrode array surrounded by PDMS.
The researchers customised a 3D printing setup for the coaxial conductive fibres, optimising the process for manufacturing flexible, conductive structures. There was a significant challenge, however, when 3D printing structures with a solid, rLCGO fibre. This caused ‘dragging’ toward the centre of the structure, and due to the time constrictions of the study, the research team was not able to find a quick solution.
“Although printing layers in a single run needs more optimisation, a prototype construct having two layers with four parallel rLCGO fibres each was created and used to show how bending it does not affect its electrical properties. The fibres used for the development of the printing process were not highly conductive, although their conductivity could be dramatically increased by platinization, as shown in this work. Nevertheless, other rLCGO fibres exhibiting higher conductivity could be used instead. “The data shown in this work is still very preliminary, but promising. Optimisation of the 3D printing process must be the next step towards the development of this technology for the CI,” concluded Peña.