Vibrotactile feedback enriches the use of wearable technologies for entertainment, navigation, and healthcare. The actuators of these portable systems, particularly fingertip devices, need to be compact, comfortable, and easy to integrate. Multiple vibrating elements could enhance perceptual realism, but how should they be arranged and oriented on the fingerpad? Here, we evaluate a simple approach that uses an audio input signal to drive an air coil that vibrates two magnets embedded in a soft fingertip sheath; the magnets are arranged in the radial-ulnar or proximal-distal direction with either the same or opposite polarity. We explore the effects of these new device configurations on both dynamic response and haptic perception. Experimental results indicate that the vibrations were perceived well across frequencies, with stronger sensations between 180 and 360 Hz, which aligns with the high vibration magnitudes our computational simulation predicts in this frequency range. Interestingly, perceptual responses showed that participants mainly classified vibrations based on the excitation frequency rather than the polarity of the magnets. Participants also rated vibrotactile feedback derived from recorded sounds and replayed for different interactions. Their evaluations offer promising evidence that this actuation approach could be used in extended-reality applications to improve transient user interactions with virtual objects.

Soft actuators offer lightweight, compliant, and safe alternatives to traditional mechanisms, but they often incur complicated actuation schemes, bulky support systems, and limited functionality when made solely from soft materials. Soft‑rigid designs that integrate rigid elements into primarily soft bodies are common, yet the potential of those rigid parts to shape actuation behavior without compromising the overall softness remains underexplored, and fabrication practices often lack reproducibility. This thesis presents two case studies of synergistic hybrid actuation systems that utilize the complementary roles of soft and rigid components to dictate temporal and spectral behavior in response to simple input commands. Between the soft and hard components, one is typically active, while the other is passive. The first case study implements a soft-active/rigid-passive approach for the medical robotics application of endoluminal locomotion. A thin hyperelastic balloon encased in an inextensible sleeve is coupled with a thicker, non-encased balloon on a single fluid supply to serve as front and rear anchors, respectively. Geometry and material selection reshape the pressure-stretch response so the rear anchor inflates and deflates before the front anchor, enabling asymmetric sequencing useful for peristaltic locomotion inside a lumen. Numerical simulation and experiments validate the characteristic curves of dip-molded balloons and alternating anchoring in rigid tubes. The approach can be extended to generate actuation patterns for sequential haptic feedback and other robotic applications. The second case study applies a soft-passive/rigid-active strategy in the domain of fingertip haptic actuation. A dip‑molded silicone sheath with embedded miniature magnets, excited by a single air‑core coil, produces localized, rich vibrotactile feedback. Simulations, mechanical measurements, and user experiments with a single-magnet design show consistent frequency‑dependent behavior and strong perceptual salience. In follow-on work, various dual‑magnet arrangements were also simulated, fabricated, and thoroughly evaluated. Classification tests indicate that frequency content is more important for perception than magnet orientation, while a realism‑rating experiment supports the feasibility of audio-driven simple commands for realistic haptic feedback. The device is demonstrated on the fingertip in virtual reality and could be adapted for other body locations for navigation, rehabilitation, or related applications. Together, these studies provide design rules, a simulation-fabrication-validation workflow, and reproducible fabrication practices for soft-rigid hybrid actuators that realize desired mechanical outputs from minimal actuation commands. The methods and findings generalize to other soft actuators and have potential applications in domains such as medical devices, wearable technologies, and soft sensing.

Predicting how the fingertip will mechanically respond to different stimuli can help explain human haptic perception and enable improvements to actuation approaches such as ultrasonic mid-air haptics. This study addresses this goal using high-fidelity 3D finite element analyses. We compute the deformation profiles and amplitudes caused by harmonic forces applied in the normal direction at four locations: the center of the finger pad, the side of the finger, the tip of the finger, and the oblique midpoint of these three sites. The excitation frequency is swept from 2.5 to 260 Hz. The simulated frequency response functions (FRFs) obtained for displacement demonstrate that the relative magnitudes of the deformations elicited by stimulating at each of these four locations greatly depends on whether only the excitation point or the entire finger is considered. The point force that induces the smallest local deformation can even cause the largest overall deformation at certain frequency intervals. Above 225 Hz, oblique excitation produces larger mean displacement amplitudes than the other three forces due to excitation of multiple modes involving diagonal deformation. These simulation results give novel insights into the combined influence of excitation location and frequency on the fingertip dynamic response, potentially facilitating the design of future vibration feedback devices.

Haptic displays act on the user's body to stimulate the sense of touch and enrich applications from gaming and computer-aided design to rehabilitation and remote surgery. However, when crafted from typical rigid robotic components, they tend to be heavy, bulky, and expensive, while sleeker designs often struggle to create clear haptic cues. This article introduces a lightweight wearable silicone finger sheath that can deliver salient and rich vibrotactile cues using electromagnetic actuation. We fabricate the sheath on a ferromagnetic mandrel with a process based on dip molding, a robust fabrication method that is rarely used in soft robotics but is suitable for commercial production. A miniature rare-earth magnet embedded within the silicone layers at the center of the finger pad is driven to vibrate by the application of alternating current to a nearby air-coil. Experiments are conducted to determine the amplitude of the magnetic force and the frequency response function for the displacement amplitude of the magnet perpendicular to the skin. In addition, high-fidelity finite element analyses of the finger wearing the device are performed to investigate the trends observed in the measurements. The experimental and simulated results show consistent dynamic behavior from 10 to 1000 Hz, with the displacement decreasing after about 300 Hz. These results match the detection threshold profile obtained in a psychophysical study performed by 17 users, where more current was needed only at the highest frequency. A cue identification experiment and a demonstration in virtual reality validate the feasibility of this approach to fingertip haptics.

Computational analysis of free and forced vibration responses provides crucial information on the dynamic characteristics of deformable bodies. Although such numerical techniques are prevalently used in many disciplines, they have been underutilized in the quest to understand the form and function of human fingers. We addressed this opportunity by building DigiTip, a detailed three-dimensional finite element model of a representative human fingertip that is based on prior anatomical and biomechanical studies. Using the developed model, we first performed modal analyses to determine the free vibration modes with associated frequencies up to about 250 Hz, the frequency at which humans are most sensitive to vibratory stimuli on the fingertip. The modal analysis results reveal that this typical human fingertip exhibits seven characteristic vibration patterns in the considered frequency range. Subsequently, we applied distributed harmonic forces at the fingerprint centroid in three principal directions to predict forced vibration responses through frequency-response analyses; these simulations demonstrate that certain vibration modes are excited significantly more efficiently than the others under the investigated conditions. The results illuminate the dynamic behavior of the human fingertip in haptic interactions involving oscillating stimuli, such as textures and vibratory alerts, and they show how the modal information can predict the forced vibration responses of the soft tissue.