A Hybrid Soft Haptic Display for Rendering Lump Stiffness in Remote Palpation

TL;DR

This work addresses the fidelity gap in remote palpation by proposing a hybrid fingertip haptic display that combines a rigid platform for large-scale surface forces with a 4×4 soft pneumatic tactile array to render a hard lump beneath soft tissue. It evaluates three rendering strategies—Platform-Only, Hybrid A (Position + Force Feedback), and Hybrid B (Position + Preloaded Stiffness Feedback)—in a lump-detection task with 12 participants. Results show that both hybrid approaches dramatically improve lump detection accuracy (from 50% for Platform-Only to over 95% for hybrids) and uplift confidence and realism, though Hybrid B incurs potential latency due to event-based averaging. The findings suggest that spatially rich soft haptic cues are crucial for realistic lump perception and inform trade-offs between realism and real-time performance for remote palpation systems, with implications for telemedicine diagnostics and training.

Abstract

Remote palpation enables noninvasive tissue examination in telemedicine, yet current tactile displays often lack the fidelity to convey both large-scale forces and fine spatial details. This study introduces a hybrid fingertip display comprising a rigid platform and a $4\times4$ soft pneumatic tactile display (4.93 mm displacement and 1.175 N per single pneumatic chamber) to render a hard lump beneath soft tissue. This study compares three rendering strategies: a Platform-Only baseline that renders the total interaction force; a Hybrid A (Position + Force Feedback) strategy that adds a dynamic, real-time soft spatial cue; and a Hybrid B (Position + Preloaded Stiffness Feedback) strategy that provides a constant, pre-calculated soft spatial cue. In a 12-participant lump detection study, both hybrid methods dramatically improved accuracy over the Platform-Only baseline (from 50\% to over 95\%). While the Hybrid B was highlighted qualitatively for realism, its event-based averaging is expected to increase interaction latency in real-time operation. This suggests a trade-off between perceived lump realism and real-time responsiveness, such that rendering choices that enhance realism may conflict with those that minimize latency.

Figures (8)

• Figure 1: Hybrid soft tactile display. (a) A motorized rigid moving platform with a fingertip-size $4\times4$ soft tactile array. (b) Visualization of one bubble (top) and multi bubbles (bottom). (c) Time-lapse of one bubble inflating on the tactile array.
• Figure 2: Tissue phantom. (a) External view of the tissue phantom. The top surface was covered by a 1 mm skin layer of Ecoflex 00-10, while the interior cavity was filled with loose slime. (b) Diagram of the internal structure, showing a 20 mm diameter, 10 mm height polylactic acid (PLA) inclusion embedded within the 39 mm deep tissue phantom.
• Figure 3: Haptic display device characterization. (a) The rigid platform exhibits a nonlinear position-to-force response. (b) A pneumatic bubble shows a nonlinear pressure-to-force relationship. (c) The bubble's maximum vertical displacement is characterized across multiple frequencies.
• Figure 4: Haptic display hardware overview.
• Figure 5: Hybrid actuation methods. Top plots show force decomposition; bottom plots show pressure generation during a lump detection task (no-lump: 0-10s; with-lump: 13-23s). The Hybrid A method (purple) tracks residual force at 100 Hz, while the Hybrid B method (dark gray) holds a constant pressure per contact, based on the sustain-phase average.