Stretchable Arduinos embedded in soft robots

TL;DR

This work presents a generalized method to translate any complex two-layer circuit into a soft, stretchable form, which enabled the creation of stretchable single-board microcontrollers and other commercial circuits, without design simplifications.

Abstract

To achieve real-world functionality, robots must have the ability to carry out decision-making computations. However, soft robots stretch and therefore need a solution other than rigid computers. Examples of embedding computing capacity into soft robots currently include appending rigid printed circuit boards (PCBs) to the robot, integrating soft logic gates, and exploiting material responses for material-embedded computation. Although promising, these approaches introduce limitations such as rigidity, tethers, or low logic gate density. The field of stretchable electronics has sought to solve these challenges, but a complete pipeline for direct integration of single-board computers, microcontrollers, and other complex circuitry into soft robots has remained elusive. We present a generalized method to translate any complex two-layer circuit into a soft, stretchable form. This enabled the creation of stretchable single-board microcontrollers (including Arduinos) and other commercial circuits (including Sparkfun circuits), without design simplifications. As demonstrations of the method's utility, we embed highly stretchable (>300% strain) Arduino Pro Minis into the bodies of multiple soft robots. This makes use of otherwise inert structural material, fulfilling the promise of the stretchable electronics field to integrate state-of-the-art computational power into robust, stretchable systems during active use.

Figures (19)

• Figure 1: End-to-end method to translate complex commercial circuit boards into stretchable forms. The method is uniquely enabled by biphasic conductors and tackified silicones, enabling circuit integration into fully functioning wearables and soft robots during use.
• Figure 2: OGaIn characterization and substrate compatibility. (A) Normalized resistance change vs. strain for 5 OGaIn samples, plotted alongside the theoretical values for a bulk conductor and previous results from our biphasic gallium-indium alloy (BGaIn, Sanchez-Botero2022). Shaded region represents one standard deviation. (B) Representative electrical resistance vs. cycle number for an OGaIn trace stretched to 150% strain over 1000 cycles. (C) Representative electrical resistance vs. cycle number for an OGaIn trace with a 0 $\Omega$ resistor stretched to 150% strain over 1000 cycles. (D) OGaIn adhesion vs. ASTM D6195-22 tack measurement of various stretchable substrates. Error bars indicate one standard deviation from the mean over 5 trials.
• Figure 3: Circuit manufacturing. (A) Original, open-source Arduino Pro Mini file in Autodesk Eagle. (B) Modified circuit design ready for laser cutting. (C) Circuit substrate sandwiched between sticker paper. (D) Laser cut board outline and VIAs (vertical interconnect accesses) in sticker paper with CO$_2$ laser. (E) Cut top layer of circuit traces using UV laser. (F) Cut bottom layer of circuit traces using UV laser. (G) Paint bottom traces and fill VIAs with OGaIn. (H) Remove bottom mask and test trace conductivity. (I) Encapsulate with material of choice. (J) Paint top traces with OGaIn and remove mask. Test trace conductivity. (K) Place components and wires. (L) Encapsulate top. Add Sil-Poxy around microprocessor.
• Figure 4: Circuit characterization. All circuits in this figure used VHB tape as a substrate. (A) Circuit schematic showing top and bottom traces in red and cyan, respectively, and vertical interconnect accesses (VIAs) in black. (B) Identifying key circuit components and materials. Note that the top and bottom interconnect layers can be seen, since VHB tape is translucent. In addition, the VIAs can be seen as round dots at various locations on the interconnects. Scale bar 5 mm. Trace widths in insets are 0.25 mm. (C) Image of circuit before strain testing on the materials testing system (Instron 3345) and just before serial disconnect at 404% strain. Scale bars 18.8 mm. (D) Force vs. strain curves for each sample when strained until serial disconnect, noting when serial disconnect occurred for each sample. (E) Cycle number when serial disconnect occurred for each sample, when the circuit was repeatedly strained to 100%. (F) Comparing cyclic behavior of neat VHB tape (circuit substrate material) and the circuit at 1, 10 and 200 cycles. Solid lines are means over 5 samples after the number of strain cycles indicated in the legend. Shaded area indicates one standard deviation.
• Figure 5: Additional circuits. (A) Arduino Lilypad in rigid and soft form at 0% strain, and the soft form at 415% strain. Scale bar 7 mm. (B) Sparkfun Sound Detector in rigid and soft form at 0% strain, and the soft form at 258% strain. Scale bar 9 mm. (C) Sparkfun RGB and Gesture Sensor in rigid and soft form at 0% strain, and the soft form at 442% strain. Scale bar 9 mm.