Factors affecting biocompatible 3D printing photosensitive resins used for medical applications

3D printing has been increasingly used to manufacture medical devices in the last twenty years (Tack et al. 2016; Kermavnar et al. 2021). The traditional production of medical devices is carried out in line with strict regulations put in place to protect those involved in the supply chain (Kramer et al. 2020). The development of regulations for 3D printed medical devices are yet to be fully established due to difficulties in defining their parameters (Ricles et al. 2018). Currently, existing regulations are applied to 3D printing where possible (Di Prima et al. 2016). This has raised concerns regarding the future adoption of 3D printing into key industries (Horst 2020). Of particular focus on this topic, is the use of biocompatible 3D printing photosensitive resins (Alifui-Segbaya et al. 2017; Lupuleasa et al. 2018; González et al. 2020). These materials require specific post-processing to achieve their intended material properties (Jindal et al. 2020; Kim et al. 2020). Post-processing generally consists of washing parts in an alcohol solution such as isopropyl alcohol, drying the parts, and then post-curing with ultra-violet light and sometimes heat for a prescribed amount of time. Post-processing information is provided by material manufacturers generically with the caveat that post-processing should be extended for ‘large’ or more ‘complex’ geometries but do not define these parameters (3DSystems 2020c; Formlabs 2022).

The initial research of this thesis explores how 3D printing is utilised to benefit the production of medical devices. Firstly, its use to rapidly replenish PPE and other devices during the COVID-19 pandemic, whilst highlighting issues arising from the use of a decentralised 3D printing supply chain. Secondly, a review to assess how the ability to produce bespoke geometries is used to create patient-specific devices for palliative medicine. As palliative medicine often requires a rapid and bespoke solution for patients, 3D printing is often used at the point-of-care. This review aimed to gain a better insight into the literature, and systematically identify recent 3D printed developments within the field. The review identified no correlation between the device being produced and the machine/material used to make it. This would suggest that education and availability of 3D printing systems at the point-of-care needs improving. Research is then directed towards the efficacy of 3D printings application by investigating the information supplied with biocompatible materials. A review of the grey literature identified 99 rigid, and 31 flexible biocompatible 3D printing resins. The information supplied with those materials varied in quantity, quality and terminology used. From this, two experiments were performed to test the outcomes of extending post-curing times on simulated ‘large’ and ‘complex’ geometries using commercially available iniocompatible 3D printing resins. In chapter 6, the cure depth of ‘large’ geometries are tested. The results of this experiment showed that materials containing opaque pigments were unable to cure to the full depth of the test model even when exposed to 500% of the recommended post-curing treatment. The second experiment tested further post-curing times on ‘complex’ geometries, and was quantified by testing the materials mechanical properties. The results showed that extending the post-curing time was insufficient in curing opaque pigmented resins. In one case, specimens in the outer exposed layer showed a tensile strength of 58MPa, whereas specimens from the inner layer only showed 19MPa.

The outcomes of this research suggest that standardisation needs to be implemented concerning the information provided by material manufacturers, and that the success of post-curing photosensitive resins is largely dependent on the pigmentation of the material.