BIOCOMPATIBILITY OF 3D PRINTED METHACRYLATE FOR HEARING AIDS AND INNER EAR DEVICES

The capacity of 3D printing (3DP) technologies to initiate speedy polymerization of solvent free resins accounts for their utility in the manufacturing of medical devices. Nonetheless, independent biological evaluation of 3D printed materials is recommended due to the unique parameters of the manufacturing process, which can influence their physical, chemical, and biological properties. In this study, E-Shell 450 material indicated for 3DP of hearing aid shells and inner ear devices was examined for biological safety using zebrafish bioassays adapted to OECD fish embryo test. In addition, the proprietary material was characterized for composition using headspace gas-chromatography mass spectrometry (GC-MS). To initiate test, newly fertilized zebrafish eggs were cultured on non-treated and ethanol-treated materials in glass petri dishes with ultrapure water, incubated at 28.5°C and assessed for developmental endpoints of toxicity at 24h interval until 96h. Data confirmed non-treated material was extremely toxic in bioassays within 24h whereas ethanol-treated material showed a relative lower toxicity possibly due to ethanoic-aqueous interactions as observed by GC-MS. With the current influx of 3D printing materials, users are urged to exercise caution. Operators must also take cognizance of the potential toxicity of the chemicals used in 3DP and implement safety measures to limit their exposure.


INTRODUCTION
The recent hype surrounding 3D printing (3DP) attests to its growing popularity in almost every manufacturing sector including medicine, architecture, sports, aerospace and automotive engineering and contemporary arts [1].The different technologies in 3DP offer a spectrum of capabilities for the manufacturing of polymeric medical devices such as hearing aid shells and inner ear devices.The digital manufacturing process simply involves feeding a virtual model (usually 'STL' file) into a designated 3D printer to build parts in successive layers and the desired 3D part is completed.The capacity of the technologies to initiate speedy polymerization of solvent free resins primarily accounts for their utility in 3DP.Nonetheless, independent biological evaluation of the devices is highly recommended [2,3] due to the unique parameters of the manufacturing process, which can influence their physical, chemical, and biological properties [4].In this study, E-Shell 450 material [5] indicated for hearing aid shells and inner ear devices is examined for biological safety using zebrafish bioassays adapted to the Organization for Economic Cooperation and Development (OECD) fish embryo test [6].
Representative materials were built with Digital Light Processing (DLP) technology.DLP is similar to stereolithography (SL) in that both are vat photopolymerization processes that require washing built parts in organic solvents to remove any wet resin remnants, followed by postcuring to harden them.However, DLP uses a more conventional light source such as an arc lamp, with a liquid crystal display panel or a deformable mirror device, which is applied to the entire surface of the vat of resin in a single pass, relatively making it faster than SL [7].To extrapolate toxicity effects to residual monomer and degradation products that may be present in the proprietary material, it was characterized for composition using headspace gas-chromatography mass spectrometry (GC-MS).

RESULTS
After 24h, non-treated materials induced ≈70% embryo death or lethality while surviving embryos (Fig. 2) were largely unhealthy, hence test was discontinued.Although ethanol-treated materials recorded only 5% mortality after 24h, additional 50% with increased hypopigmentation, pericardial edema, yolk sac resorption delay and hypoactive behavior were observed in surviving fish by 96h (Fig. 3).Average growth length in surviving fish after 96h was 3241.30µmcompared to 3590.33 µm in controls.At the end of the test, fish were euthanized in 0.4 % anaesthetic tricaine mesylate solution.

DISCUSSION
Experimental results in this study relate specifically to the photopolymer examined based on composition, manufacturing parameters, postprocessing and the test protocols used.
Toxicological data indicate non-treated E-Shell 450 was extremely toxic in zebrafish bioassay whereas ethanol-treated E-Shell 450 showed a relative lower toxicity but also, severe cumulative sublethal and teratogenic effects in surviving fish.The improved biological performance in treated materials is likely due to induced swelling in polymeric chains, which allowed insoluble substances, in this case, chemical compounds to diffuse in the water used to rinsed them [10].
As per standards definitions [11], E-Shell 450 is a surface device hence does not require stringent biological evaluation compared to methacrylates for intraoral devices .Nonetheless, it is worth emphasising that uncured methacrylate monomers can be absorbed through the skin [12] and cause allergic reactions (e.g.dermatitis) [13,14].In addition, the toxicity of methacrylate esters is theorized to involve alkylation of critical cellular nucleophiles via Michael addition [15].Some of the developmental endpoints observed in toxic bioassays are comparable to those reported in animal studies that linked methacrylic esters to embryonic fetal toxicity, teratogenicity [12] and cardiovascular function [16,17].Furthermore, some of chemical compounds observed are used in industrial applications and can be toxic if present in threshold dose.For instance, cyclomethicone is used in cosmetic and personal products [18] imperative that the biological performance of 3D printed materials is not overlooked.Users are advised to exercise caution and if necessary demand approved certification for these materials.
Since 3DP is not a "one-stop", manufacturing process operators must therefore take cognizance of the potential toxicity of the chemicals used and implement safety measures to limit their exposure.

ACKNOWLEDGEMENTS
The author thanks EnvisionTec GmbH, Germany for kindly supplying E-Shell 450 material for the study, GJL for supporting the study with zebrafish resources and expertise, and AW and JB for the analytical chromatography.

CONFLICT OF INTEREST STATEMENT
No competing financial interests exist.

Table 2
shows 16 chemical compounds that reduced to 5 with the applied ethanol treatment.For reliability, only chemical compounds observed in ≥ 75% or n=3/4 of the photocured samples are reported.

Table 2 .
Chemical composition of non-treated and ethanol-treated photocured materials

preprints.org) | NOT PEER-REVIEWED | Posted: 17 July 2018 doi:10.20944/preprints201807.0315.v1
some 'closed' 3DP systems do not work with third-party resins [23], hence desired functional properties may not be guaranteed.With the current influx of 3D printers and materials, it is Preprints (www.