Byungseok Yoo,† David Bowen,‡ * Nathan Lazarus,∫ and Darryll Pines†
†Department of Aerospace Engineering, University of Maryland, College Park, MD 20742
‡Laboratory for Physical Sciences, University of Maryland, College Park, MD 20740
∫US Army Research Laboratory, Adelphi, MD 20783, USA
This research had the following objectives.
- Demonstrate the laser direct structuring (LDS) process was successful using superflexible, biocompatible (softer-than-skin) additive cure silicone doped with copper chromite.
- Determine process bounds such as minimum laser power and doping level, and explore process robustness.
- Perform preliminary mechanical evaluations of LDS-on-silicone circuits, such as flexibility, adhesion, breakage strain, and strain-cycle longevity.
- Demonstrate/evaluate LDS-on-silicone inductors and 3D hemispherical spirals.
Figure 1: Schematic illustration of LDS on silicone process. 1. Mixing and casting. 2. Laser structuring. 3. Electroless copper plating. 4. Resulting robust elastomeric circuit; Source: 
Elastomeric substrates for electronics are promising materials for mechanically-tunable or strain sensors, biocompatible sensors, and wearable electronics. A challenge with most elastomeric materials is their low-energy surfaces, causing poor adhesion in conductive direct ink writing . Conductive liquid metal can also be injected into hollow channels that have been cast or printed in an elastomeric material [2, 3], but both of these methods have lower circuit conductivity and limited circuit complexity.
With the research presented here, we sought to expand the LDS materials catalog further to include supersoft biocompatible silicone, and perform initial evaluations of free-form silicone parts integrated with LDS circuits. It was thought that the LDS laser roughening and the electroless growth of copper, rather than the deposit-cure-adhere process of direct ink writing, would give LDS traces exceptional adhesion; this was found to be true. Also, LDS would create solderable, low-resistance copper traces of the highest quality achievable on an elastomer.
Samples were fabricated using Smooth-On Ecoflex 00-30 (30 Shore 00) additive cure silicone, doped with copper chromite powder. The copper chromite doping threshold was found to be 2 – 4 w/w% for successful LDS at 2.5W laser power. The elastic modulus of the silicone increased with doping level, from about 80 kPa at 10% w/w up to about 130 kPa at 50% w/w. Thus, within the demonstrated range of successful LDS, the doping level can be used to control the material stiffness. Laser power was also explored using the LPKF ProtoLaser 3D IR laser structuring system and 30 w/w% doped silicone, with successful practical structuring achieved above a minimum between 0.5W and 0.75W average laser power.
LDS horseshoe circuit trace samples were fabricated for preliminary mechanical testing. The horseshoe traces were electroplated up to a thickness of about 32 µm, then overmolded with undoped silicone, with breakage strains measured to range from 83% up to 173%. Samples were also cyclically stretched up to 60% strain to test longevity, with failure occurring at around 78 cycles. On other samples of varying dopant levels, 3mm wide copper strips were fabricated to evaluate adhesion; adhesive peel strength increased with silicone stiffness, from 1 kN/m at 10% w/w doping to 5 kN/m at 50% w/w doping. The separation of the copper strip from the silicone was determined to be the result of the surface silicone tearing away from the bulk, rather than the copper de-adhering from the silicone.
Functional planar meander and Hilbert inductors were fabricated and their inductance was measured before and after a 100% strain was applied. Pre-strain measured and predicted inductances were in agreement, with post-strain measurements showing up to a 20% increase from the initial values. An additional 3D hemispherical spiral helix was successfully fabricated on a silicone bowl, which demonstrated trace self-healing after breakage, becoming electrically continuous when returned to the unstrained state.
Perhaps more promising than free-form silicone objects integrated with LDS circuits, is the possibility of using this silicone material as a coating for textiles. Preliminary testing of spiral circuit traces on denim fabric impregnated with LDS silicone have been successful; silicone textile coatings create robust circuits while leaving fabric soft and flexible. This is an area of future research.
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 Yoo, B., Bowen, D., Lazarus, N., Pines, D., Laser Direct Structured 3D Circuits on Silicone. ACS Applied Materials & Interfaces 2022 14 (16), 18854-18865, DOI: 10.1021/acsami.2c01029