An Emulsion Approach to Resolve the Paradox of 3D Printing of Very Soft Silicones

Abstract

3D printing of silicone has been a challenge for medical applications for several years. The main property of medical silicone (mostly polydimethylsiloxane) is its mechanical similitude with human tissues. The very soft tissues as prostate, skin, liver, etc, are of Young's modulus lower than 100 kPa. 3D printing for such soft materials is not as obvious as printing thermoplastic polymers. In this work, the goal is to elaborate formulations of room‐temperature‐vulcanizing (RTV) silicone leading to final materials of Young's modulus down to 25 kPa while being processable by liquid deposition modeling, a layer by layer deposit of extruded filament. The main challenge is keeping the material with a sufficient firmness during the process to maintain the shape of the object but without increasing its final rigidity. Therefore, a RTV silicone is mixed with a hydrophilic liquid to create an emulsion of high yield stress behavior. Different emulsions are tested to optimize the composition on the basis of rheological measurements carried out to determine the yield stress variation and compare it to an emulsion model. Then, after printing and curing, the hydrophilic liquid is removed to create a material of fine porosity and consequently of low Young's modulus.

Phase‐Change‐Enabled, Rapid, High‐Resolution Direct Ink Writing of Soft Silicone

“…Rheological Measurements: The rheological properties of the PDMSbased ink were measured using a rotational rheometer (MCR501, Anton Paar GmbH) equipped with a parallel plate (25 mm) fixture, a gap distance of 1 mm was selected. To measure the storage modulus G′ and loss modulus G″ at different temperatures (20,45, and 60 °C), frequency sweep tests (100-0.01 rad s −1 ) were conducted in the linear viscoelastic region at a strain of 0.05%. For the yield stress τ y , stress ramps were applied covering the shear stress range of 10-50 000 Pa.…”

“…Unfortunately, for commercial silicone formulations, the most suitable way is polymerization, which suffers from slow kinetics and a corresponding slow increase in viscosity, leading to material flow, poor resolution, and long build time. [ 18 ] Although, many new printing strategies for soft silicone have been invented, including capillary‐suspension‐type [ 15,19 ] and emulsion‐type inks, [ 20 ] as well as tailored printing concepts such as embedded printing, [ 21,22 ] multinozzle 3D printing, [ 13 ] or other customized printing methods. [ 11,18 ] Nevertheless, the rapid 3D printing of silicone elastomers with high resolution is still difficult to achieve, limiting their applications, especially in flexible electronics [ 6,23 ] and smart soft devices.…”

“…Unfortunately, for commercial silicone formulations, the most suitable way is polymerization, which suffers from slow kinetics and a corresponding slow increase in viscosity, leading to material flow, poor resolution, and long build time. [18] Although, many new printing strategies for soft silicone have been invented, including capillary-suspension-type [15,19] and emulsion-type inks, [20] as well as tailored printing concepts such as embedded Soft silicone is an ideal flexible material for application, e.g., in soft robotics, flexible electronics, bionics, or implantable biomedical devices. However, gravity-driven sagging, filament stretching, and deformation can cause inevitable defects during rapid manufacturing, making it hard to obtain complex, high-resolution 3D silicone structures with direct ink writing (DIW) technology.…”

Enhancing the Yield Stress in Liquid Polydimethylsiloxane to Allow Its 3D Printing: Hydrogels as Removable Fillers

“…For a low‐filled PDMS, it has been proposed that an emulsion can be made which will enhance the yield stress of the system, and allow the printing of such formulations. [ 20 ] It should be mentioned that emulsion formulations have already been used for 3D printing, but with rigid materials, such as photocurable polycaprolactone [ 21 ] or acrylate polymer. [ 22,23 ] The aim was essentially to obtain a very porous final rigid scaffold.…”

A silicone-based support material eliminates interfacial instabilities in 3D silicone printing

“…1, A and B) ( 9 , 15 ). Modifying silicone inks with additives can stabilize 3D printed structures ( 16 , 17 ), yet a versatile approach to additive manufacturing with unmodified silicone inks remains elusive. One route to achieving high-quality 3D silicone printing without ink modification is to eliminate the disruptive role of interfacial tension by using support materials that are chemically similar to the printed inks they stabilize (Fig.…”