3D-printing of Biomedical Grade Silicones

Introduction

The production process of Fused Deposition Modeling (FDM) has been successfully established in many branches of the industry.
Nowadays it is not only used to produce concepts and prototypes but also for functional parts. [1]

The market for the FDM-printer has grown strongly in the last years. One of the main reasons is the rise of the open-source-community which made the access to these systems affordable and the system better [2][3].

Almost every day we can read about new developments in the field of the FDM-technology [2] [4][7]. A big part of the innovation is due to research and development of new printable materials [5]. Besides the standard filaments like Acrylnitril-Butadien-Styrol (ABS) and Polylactic-Acid (PLA) there are already exotic materials available for printing, e.g. materials which are similar to wood or sandstone. Also flexible materials like thermoplastic polyurethan (TPU) or thermoplastic elastomer (TPE) are possible to print [6].

One of this material innovations has been under development by the German company stamos+braun prothesenwerk gmbh in cooperation with the Technische Universität (TU) Dresden. Stamos & Braun offer different kinds of prosthetic devices, some of them are made out of silicone with a realistic optic (Figure 1).

The group from the „Institute of Electromechanical and Electronic Design“ of the TU Dresden has been working on special solutions for additive technologies for several years [8][9][10]. The results of the group have been the base for the cooperation with stamos+braun prothesen-werk gmbh with the goal of creating complex and complicated prosthetic parts out of silicone.

Research

It was necessary to do a lot of research and development on the behavior of the material and in the construction field to be able to 3D-print a heat cured high consistency biomedical grade silicone rubber.
A prototype of a 3D-printer made it possible to make first prints. After further tests and developments it was possible to print more complex shapes (Figure 2).

One of the big advantages of heat cured high consistency biomedical grade silicone rubber compared to other materials is that you can apply multiple layers on top of each other without any connection problems between the layers.
Only because of this feature the behavior of the material is about the same if it is printed or produced in the traditional way.

Based on these first results further studies tried to push the limits of the technically feasible. In a first step, different variations of the inner structures of printed silicone parts in connection with the behavior of the material under load were investigated. To this effect some different lattice structures combined with different materials have been tested (Figure 3).

One special case was the problem of the weight and the missing ability of the material to take over sheare forces in silicone foot prosthetics. Lattice structures, as known from traditional 3D printing, could be a first idea to reduce the weight of the prosthesis. For this case a special pad was constructed for a silicone foot prosthesis (Figure 4) and then printed in 3D.

Closed structures with an infill of less than 40% have been possible.
Compared with the original parts out of full silicone the weight of the pad could be reduced with the lattice structure by up to 70%.

Even though the weight has been reduced significantly, the silicone pad is still highly rigid in the Z-direction. To avoid this it is possible to place the pad in the printer in a different position. Thus it becomes possible to change the orientation of the lattice structure.

A second option to reduce the weight is the integration of light materials like synthetics. In the following we consider the case of a hand prosthesis. The synthetic pieces are mimicking the bone structure and thus improve the haptic and rigidness. Overall, this provides for a more realistic feel of the joints and soft tissue.
In addition to the improved haptic the weight of the prosthesis can be reduced significantly (Figure 6).

First tests of a combined printing of silicone and synthetic materials have been successful. It will soon become possible to 3D-print artificial hands, finger and partial hand prosthetics combined with bone structures in one process.

Future prospects

The use of heat cured high consistency silicone 3D-printing will not be restricted to the medical field. It can also be used in the car- and airplane industry or in the field of make-up artists.

The goal now is to optimize the lattice structures for more stiffness and more flexibilty. Silicone foot-, hand-, and partial hand prosthetics will be printed in 3D out of biomedical grade silicone in full color.

Another next step will be optimizing the dual printing of synthetic material combined with biomedical grade silicon. Once this is optimized it will be possible to print a light bone structure with joints together with the silicone cover in one step.

Another branch of development right now is the printing of complex parts like heart, kidney or stomach models for animations and training purposes for students. Partial face epitheses will be another target for the 3D-printing of silicone. A full color print with heat cured biomedical grade silicone is foreseen for the near future.
[1] Gebhard, A.: Generative Fertigungsverfahren. 4. Aufl., München: Carl Hanser Verlag 2013

[2] www.trinckle.com/blog/marktstudie-fur-3d-druck/, download, 19.06.2015

[3] reprap.org/, download, 19.06.2015

[4] www.3ders.org/index.html, download, 19.06.2015

[5] 3druck.com/visionen-prognosen/markt-fuer-3d-druck-materialien-soll-bis-2019-jaehrlich-um-204-wachsen-0228316/, download, 19.06.2015

[6] 3druck.com/3d-druck-material-liste/, download, 19.06.2015

[7] 3druck.com/drucker-und-produkte/3d-hubs-veroeffentlicht-trendreport-fuer-november-5826656/, download, 19.06.2015

[8] Böhme, M.: Optimierung der mechanischen Eigenschaften von mittels 3D-Druck erzeugten Objekten. Diplomarbeit TU Dresden 2015.

[9] Günther, L.:Mechanikkonstruktion eines 3D-Druckers. Diplomarbeit TU Dresden 2014.

[10] Entwicklung eines 3D-Druckers für gelartige Pasten. IFTE 2014.

[11] www.prothesenwerk.com, download, 19.06.2015