Soft Digital Logic

We developed information-processing soft machines by taking advantage of material instabilities, specifically, the buckling of membranes. Similar to a pop-up toy, a bistable membrane is able to flip when a pressure is applied. We used such membranes for the kinking of tubing, which allowed the development of a complete set of logic gates (NOT, AND, OR gates), which we assembled to functional blocks including shift registers, set-reset latches, and volatile memory. We used these functional blocks to demonstrate a soft gripper that could switch between two states (closed and open gripper), being operated from a single soft button.


Figure 1. Soft bistable valves configured as logic gates. OR and AND logic gates are depicted symbolically, along with truth tables summarizing their two binary inputs and resulting binary output values (A and D); because the two inputs are binary, four distinct states exist for each gate. The pneumatic connections for the OR and AND gates are shown in B and E, respectively, with schematic representations of the valves in each of their four possible states. We experimentally characterized these logic gates over each of their four possible states by varying the two inputs independently (C and F).


Soft Ring Oscillator

We paid special attention to NOT gates and their assembly to ring oscillators, as they produce oscillating signals from a single, constant pressure supply. These oscillatory output pressures enable several applications including undulating and rolling motions in soft robots, size-based particle separation, pneumatic mechanotherapy (medical application), and metering fluids.

  • A Soft Ring Oscillator
    D.J. Preston, H.J. Jiang, V. Sanchez, P. Rothemund, J. Rawson, M.P. Nemitz, W.-K. Lee, Z. Suo, C.J. Walsh, G.M. Whitesides
    Science Robotics, 4(31), 2019


Figure 2. Soft ring oscillator. The ring oscillator always contains either two adjacent unactuated inverters (A), in which case one of the inverters inflates, or two adjacent actuated inverters (B), in which case one of the inverters deflates. (C) The three-inverter ring oscillator generates three temporally coordinated output pressures, shown here as PA, PB, and PC, when a constant supply pressure, Psupp, is applied.

Soft Memory

Our most recent work entails a non-volatile memory device that permanently stores information. By changing the properties of the membrane, it is able to rest in the most-recently flipped state. Pneumatic signals are used to flip the membrane from one state to the other. Memory devices are important to push research towards increasingly complex robot behaviors. Much of this work draws parallels to computer engineering, whereas we just passed the point of having built the first transistor.

  • Soft Non-Volatile Memory for Non-Electronic Information Storage in Soft Robots
    M. P. Nemitz, C. K. Abrahamsson, L. Wille, D. J. Preston, A. A. Stokes, G. M. Whitesides
    IEEE Soft Robotics Conference, New Haven, 2020


Figure 3. Soft non-volatile memory. (A) Crossection of a soft valve with a bistable membrane; the membrane remains in its position unless flipped via external pneumatic actuation. (B) The device is configured as a set-reset latch for the storage of information. The S and R lines set and reset the latch; if the latch is set, it outputs supply pressure, and if it is reset, it supplies atmospheric pressure. (C) Characterization: it requires a higher pressure to flip the membrane to the top side (S = 8 kPa) than to the bottom side (R = 1 kPa), which can be explained by the membrane design and material characteristics. (D) A power outage does not affect the storage of information, and hence the soft valve with a bistable membrane configured as an S-R latch acts as a non-volatile memory device.

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