Thermopneumatic Pixels: Fast, Localized, Robust, Low-Voltage Touch Feedback

Max Linnander; Yon Visell

cs.HCMar 17, 2026

Thermopneumatic Pixels: Fast, Localized, Robust, Low-Voltage Touch Feedback

Authors: Max Linnander, Yon Visell

Organizations: Department of Mechanical Engineering, University of California, Santa Barbara, 93106 USA

Abstract

We present thermopneumatic pixels (TPPs) -- low-profile pixels and arrays that generate dynamic tactile feedback. These devices are thin, fast, reconfigurable, and output localized transient displacements at each pixel. Their parsimonious design -- a layered architecture without internal moving parts -- and low-voltage ( $\lesssim$ 10 V) operation may facilitate practical integration in a wide variety of interfaces. Each TPP converts brief electrical pulses into transient air pressure increases in an internal cavity, yielding out-of-plane forces and displacements for tactile feedback. We demonstrate TPPs that output displacements of 1 mm and forces exceeding 1 N, with millisecond response times, in packages that are less than 3 mm thick. Force and displacement increase with pixel surface area, facilitating tailorability. The pixels can also generate oscillating feedback at pulse rates up to 300 Hz range. We report designs for compact arrays of pixels at 4 mm spacing, and simple pulse driving architectures using miniature transistors driven by microcontrollers. We characterize the mechanical, dynamic, and thermal response of TPPs, and their robustness and consistency over tens of thousands of cycles. We report perceptual experiments on spatial localization and intensity as a function of driving power. Together, these results establish thermopneumatic pixels as a compact, adaptable tactile technology that blends performance and practicality.

Figures & tables

Fig. 1: A) Operating principle for thermopneumatic pixels (TPPs). TPPs deliver heat to air encapsulated within a small sealed cavity, driving gas expansion that yields localized forces and displacements. Heat is supplied to the gas (air) via transient Joule heating of a resistive Nickel-Chromium (NiCr) wire suspended in the cavity. Due to the small diameter of the wire (48 AWG), heat is rapidly transferred to the gas, driving a rapid pressure increase in the gas. The air pressure increase drives deflection of an elastic membrane sealing the top of the cavity, yielding localized forces and displacements. B) The design uses a simple, layered assembly of common materials. Exploded view of the multilayer TPP stack, comprising patterned structural, thermal, and compliant layers. C) Photograph of flexible printed circuit (FPC) layers with suspended NiCr wires, for a module with a single small TPP (cavity length $L=8$ mm). The dashed box shows an enlarged view of the NiCr wire. D) FPC layer for a quartet module with four TPPs ( $L=4$ mm). E) Photograph of the module. F) Top-down view of TPP during actuation ( $L=8$ mm, pixel diameter $D=6$ mm) G) Top-down view of the quartet module, with four TPPs, as it displays different patterns ( $L=4$ mm, $D=4$ mm). H) Top-down view of a 9-TPP module, as it displays a pattern ( $L=4$ mm, $D=3$ mm). I, J) Examples of how TPP arrays can be integrated in controllers. Green dashed squares: 3 $\times$ 3 arrays; circles: 2 $\times$ 2 arrays.

Fig. 2: A) Electronic driver board interfacing with up to ten modules, enabling simultaneous operation of up to forty TPPs in the quartet configuration. B) General schematic used to drive the TPPs. C) The single and quartet modules share a common electrical pinout to facilitate operation with the driver board. The photos show modules with a single TPP (top, $L=8$ mm) and quartet (bottom, $L=4$ mm). The connector length is application-dependent.

Fig. 3: Thermomechanical characterization of TPPs. A) NiCr wire temperature $T_{\mathrm{wire}}(t)$ , inferred cavity air temperature $T_{\mathrm{air}}(t)$ , isometric force $F(t)$ , and free displacement $z(t)$ in response to a 75-ms, 4.8 W electrical pulse. B) Force response $F(t)$ for increasing pulse durations $t_{p}$ at fixed electrical power. C) Peak force as a function of electrical power dissipated in the wire, $P_{\mathrm{el}}=V_{\mathrm{wire}}^{2}/R\mathrm{wire}$ , for $t_{p}=15$ ms. D) Peak force measured across actuators with varying cavity length $L$ and aperture diameter $D$ , at fixed power per unit length $\rho=0.5$ W/mm. E) Operating envelope of TPPs, defined by power per unit length $\rho$ and pulse duration $t_{p}$ . Shaded regions indicate viable operation (green) and empirical failure (red). Points denote measured failure events for varying $L$ , and the black curve shows a fit to an analytic thermal model.

Fig. 4: Characterization of TPPs under cyclic operation. A) Isometric force responses to pulse trains at pulse rates $f=10$ , $25$ , and $50$ Hz (top to bottom) at duty cycle $t_{p}/\Delta t=0.1$ . B) Peak-to-peak force component $F_{pp}$ as a function of pulse rate $f$ with duty cycle $t_{p}/\Delta t=0.1$ . Regression fit: $\log_{10}(F_{pp})=\alpha\log_{10}(f)+\beta$ , $\alpha=-1.12$ , $\beta=3.68$ , $r^{2}>0.99$ . Insets show the magnitude spectra of the measured force signals for pulse rates of 25 and 150 Hz. (C) Mean free displacement is consistent over 54,000 actuations. Each dot corresponds to the mean of 90 peaks.

Fig. 5: A) Surface temperature measurements of TPPs. (b) Max temperatures measured during a 5-s actuation window for varying $L$ and $\rho$ at a duty cycle of $t_{p}/\Delta t=0.2$ .

Fig. 6: Perceptual Evaluation. A) Experiment 1: Perceived intensity as a function of power, measured using the method of magnitude scaling. Regression fit: $I=\alpha P_{\mathrm{el}}+\beta$ , $\alpha=0.2677$ , $\beta=-0.151$ , $r^{2}=0.97$ . B) Experiment 2: Spatial localization of active TPP on the finger pad.

Appendix figures & tables2 assets

Supplementary material from the paper’s appendix.

Appendix

Component	Part Number	Quantity	Cost
Microcontroller	Teensy 4.1	1	$33.6
MOSFET	AO3400A	40	$1.97
Resistor	0402WGF3302TCE	40	$0.03
Connector	691137710002	1	$0.7
Connector	522070760	10	$2.79
Connector	PPPC241LFBN-RC	2	$4.4

Appendix

TABLE I: Electrical components used in the custom driving circuit for controlling up to 40 TPPs.

Component	Part Number	Unit Cost
FPC	JLCPCB Custom Design	$2.2
NiCr Wire	Wire Optim 354-23	$0.01
Polyimide	McMaster-Carr 2271K73	$0.24
Polysiloxane	McMaster-Carr 86915K24	$0.24
Adhesive	3M 2477	$0.01
PDMS	McMaster-Carr 86435K41	$0.12

Appendix

TABLE II: Components used for one TPP module

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