Valveless microliter combustion for densely packed arrays of powerful soft actuators

Existing tactile stimulation technologies powered by small actuators offer low-resolution stimuli compared to the enormous mechanoreceptor density of human skin. Arrays of soft pneumatic actuators initially show promise as small-resolution (1- to 3-mm diameter), highly conformable tactile display strategies yet ultimately fail because of their need for valves bulkier than the actuators themselves. In this paper, we demonstrate an array of individually addressable, soft fluidic actuators that operate without electromechanical valves. We achieve this by using microscale combustion and localized thermal flame quenching. Precisely, liquid metal electrodes produce sparks to ignite fuel lean methane–oxygen mixtures in a 5-mm diameter, 2-mm tall silicone cylinder. The exothermic reaction quickly pressurizes the cylinder, displacing a silicone membrane up to 6 mm in under 1 ms. This device has an estimated free-inflation instantaneous stroke power of 3 W. The maximum reported operational frequency of these cylinders is 1.2 kHz with average displacements of ∼100 µm. We demonstrate that, at these small scales, the wall-quenching flame behavior also allows operation of a 3 × 3 array of 3-mm diameter cylinders with 4-mm pitch. Though we primarily present our device as a tactile display technology, it is a platform microactuator technology with application beyond this one.

Through the senses, human beings constantly gain rich information about the external world; ultimately, everything one knows comes from what one first learns through one’s sense powers (1). Though sight is generally considered to be our strongest sensorial asset, touch (pressure, pain, vibration, temperature, etc.) intimately connects us with our nearby environment and our own bodies. Touch is, perhaps, more necessary for survival than any other sense (2–8). It is unsurprising, then, that our skin is our bodies’ largest organ (9, 10), comprising in part a diverse array of mechanoreceptive organelles, allowing people to feel skin deformations of different types, durations, and intensities (11). For example, human fingertips have over 200 mechanoreceptive units per square centimeter (10), perceiving static deformations of down to 0.1 mm (12) and vibrations of up to 400 Hz (10, 13).Despite the importance of touch, our visual and auditory senses dominate the experience of digital information. The most proliferate form of haptic actuation is vibrotactile, but this technique does not allow the type of displacement and persistence of touch required to provide a natural experience. Vibrations alone cannot meaningfully recreate the pressure felt from a bag on the shoulders or the impact of a ball caught in the hands. Because of this lack of haptic experience, at least two societal needs remain unfulfilled: artificial touch recreation in immersive virtual reality (VR) and braille displays that compete with analogous visual media. For braille, specifically, there are no full-page, affordable, portable, refreshable displays on the market (14).The dearth of available tactile display options is not from lack of trying; manufacturing arrays of actuators at the size and density suitable for reading computer information from a tactile screen requires reducing actuator volume, weight, power draw, and cost, all together. The diverse set of designs conceived to achieve this haptic challenge have employed an equally numerous suite of physical principles, and each actuation method has presented its own failure mode (14). For example, thermal actuators usually take seconds (without thermal management accessories) (15) to finish a work loop because of heat transport limitations (16). Pulsed electromagnetic systems suffer from low actuation forces and interference between individual actuators (crosstalk) when made close to the size of a braille dot (17). Piezoelectric devices have large production costs at scale (HyperBraille systems cost ∼$50,000), also needing long cantilevered geometries that impede their ability to be densely arranged (14, 18).Fluidic elastomer actuators (FEAs) displace rubber forms with liquids and gases, showing promise as dense actuator arrays because of their manufacturing simplicity and favorable mechanical characteristics (19). An elastomeric membrane ∼1 mm in diameter can displace more than 0.5 mm as was previously shown by ref. 20 in which a 1.5-mm diameter viscoelastic membrane displaced 0.56 mm in about 1 s. Beyond simplicity, these soft haptic devices also have the convenient ability to conform to complex body shapes (21). For example, HaptX has developed a commercial, tethered VR glove technology that integrates 130 individually addressable fluidic actuators into each glove (22). This paper’s lead author has experienced this technology and testifies to its natural feel (23). As designed, the glove’s microfluidic channels are tethered to a large box housing a pump and many valves, limiting the user’s range of motion. One major deficiency of FEAs is how the system scales with actuator number density: there is generally a linear relationship between the number of valves and actuators. As electromechanical valves are themselves actuators, the size, weight, power, and cost (SWaP-C) requirements of FEA arrays soon become untenable for portable tactile display systems. For example, the most popular valve choice for FEAs is the Parker X-Valve, with dimensions of 7.87 × 23.37 × 12.30 mm³ at a unit cost of ∼$40 (24); if a single, six-dot braille cell (∼6 × 10 × 10 mm³) was controlled by six valves, that is, one valve per actuator, the array of valves would take up 18.4 times the cell area and cost $240 (14). Though there are multiplexing solutions to the valving challenge (25), we are currently unaware of any high-resolution tactile interaction being enabled by these methods.Counterintuitively, microscale combustion could provide an alternative actuation motif for haptic arrays, given its own engineering tradeoffs. Combining high-energy density fuels (26, 27) with small-volume mechanical elements results in a potentially safe and enduring actuation mechanism. Previously, microscale combustion research primarily focused on replacing batteries with high–power density micro-electromechanical systems (MEMS) thermoelectric generators (28). These systems may have failed to become practical because of unwanted flame extinction, thermal degradation, and frictional wear (29). More recently, combustion has been used in FEAs for macroscale soft robots and pumps (30–34). A spark ignites a combustible gas mixture that rapidly heats the product gas and expands the soft FEA cavity to move a robot or separate fluid. This research direction, however, has not been previously expanded into the realm of small gas-powered FEAs (35).In this paper, we make two contributions: 1) the use of combustion in microliter-scale FEAs for powerful, high-stroke, millimeter-scale actuations and 2) the exploitation of rapid thermal quenching at these scales to individually actuate fluidically coupled arrays without valving. As we no longer need valves, we can space the actuators more closely because their flow and electrical control components occupy less area than the actuator footprint. Primarily composed of molded silicone and microfluidic liquid metal (LM) traces, our design is an inexpensive, thin rubber sheet that provides more favorable SWaP-C scaling than prior FEA systems. We elementally characterize our device’s mechanical performance as a general microactuation strategy. As tactile display systems represent one of the oldest, broadest, and most contemporary microactuator research initiatives, we focus our discussion and demonstration on this system’s potential to serve a similar purpose.