Dissertation Title: Acoustics and Piezoelectricity for Soft Materials, Sensors, and Actuators

Abstract:

Piezoelectric materials (PMs) are smart materials with sensing and actuation capabilities, owing to their direct and converse piezoelectric effects, respectively. Lead zirconate titanate (PZT) and polyvinylidene fluoride (PVDF) are among the dominant materials in commercially available electroacoustic transducers, representing the piezoelectric ceramic and polymer classes, respectively. The former exhibits a high electromechanical coupling coefficient (k33 ≈ 0.52–0.74) and is capable of delivering high mechanical force as an actuator, but has a high Young’s modulus (60–90 GPa) and exhibits low strain tolerance. The latter offers a substantially lower coupling coefficient (k33 ≈ 0.14–0.19) but superior mechanical drapability over curvilinear geometries, owing to its intrinsic low modulus (2–3 GPa), making it better suited to wearable sensors. By harnessing the properties unique to each material class, this thesis introduced two novel applications of piezoelectric and acoustic technologies: (i) an energy-efficient, PZT-actuated approach for regenerating atmospheric water harvesting (AWH) hydrogels, addressing global water security; and (ii) a smart PVDF-based sensing textile that enabled quantitative transduction of motor biosignals from patients with limited mobility in a clinical setting.

The thesis began by mathematically deriving the thermodynamic limits of atmospheric water harvesting and the practical energy limits of the two dominant approaches: dewing (bounded by the coefficient of performance of Carnot refrigerator), and thermally driven sorption (bounded by the thermal limit). Then it examined alternative non-thermal technologies, including acoustics, that could offer lower-energy routes to efficient water harvesting from air. Understanding these limits is consequential for modelling and engineering AWH technologies that can operate at low energy cost in arid or semi-arid climates, and in regions where large-scale installations are impractical for economic or security reasons, including ongoing conflicts and regional wars. In such settings, shortages of freshwater severely inhibit land development and create harsh humanitarian conditions. This analysis was then extended to a critical examination of the potential impact and energy cost of introducing piezoelectric actuator technologies in the AWH domain.

Building on this, a novel high-efficiency AWH approach was designed, enabled by ~110 kHz ultrasonic actuation frequency using piezoelectric ceramics. A PZT-based piezoelectric actuator was simulated in COMSOL Multiphysics, physically prototyped, and experimentally validated. It simultaneously induced high mechanical strain and low-level Joule heating in laboratory-synthesized AWH hydrogels to generate water from air. This method was shown to be at least 45-times more energy-efficient than the state-of-the-art, which relied predominantly on heat-induced desorption at elevated temperatures. A preliminary technoeconomic analysis of the scaled-up system performance was conducted and indicated the viability of commercialization, with an estimated cost of producing water from air reaching $0.19/L— below the cost of bottled water in several countries, including the USA, UK, Canada and Australia.

The Joule heating contribution was then completely eliminated with another newly designed system operating in a broadband frequency range (20 Hz-12 kHz), showcasing how sound beats heat for near-ambient water production. A fundamentally different approach to sorbent regeneration was demonstrated, based on vibroacoustic actuation using audible acoustic waveforms in which externally applied mechanical energy directly drove moisture transport through hydrated porous media. The optimized system achieved moisture desorption at an energy consumption of 0.17 kWh/kg, breaking the thermal limit (0.63 kWh/kg)— the first ever reported demonstration of sub-latent-heat moisture desorption using audible acoustic waves. Through coupled experiments, finite-element modeling, rheological characterization, and transport analysis, moisture desorption was shown to be governed by frequency-dependent interactions among resonance, viscoelastic deformation, inertial particle dynamics, and non-Fickian moisture transport. Audible acoustic waveforms were further shown to be computationally optimizable using machine learning to enhance transport efficiency without increasing overall acoustic power, introducing waveform engineering as a new degree of freedom for controlling coupled mass transport processes.

Finally, the thesis turned to the flexible and conformable class of piezoelectric materials, i.e., PVDF homopolymers. While the preceding work exploited high-force actuation, this final strand exploited the compliant nature of PVDF polymers to produce wearable textile sensors that decoded fluctuating motor signals in patients with multiple sclerosis (MS), who experience motor impairment arising from demyelination and axonal damage within the central nervous system that slows or blocks the conduction of electrical nerve signals. Although wearable electronics for patients with MS remained dominated by silicon (Si)-based sensors, such devices were limited by the intrinsic rigidity and brittleness of Si, preventing conformability over the large curvilinear surface area of the human body, which subject the sensors to continuous deformation, movement, and mechanical contact. A textile form-factor circumvented this limitation, offering an alternative platform combining mechanical compliance, drapability, and seamless integration into garments that are already routinely worn in daily life and in clinical settings. As a representative and clinically relevant use case, this work focused on monitoring hand motor function in patients with MS, where fine and dynamic finger movements provided information that was difficult to capture with conventional methods. Because appropriate physical activity and rehabilitation can help preserve or improve motor function, sensitive and quantitative assessment of hand performance is clinically important. Current clinical assessment relies substantially on tools such as hand dynamometers, which measure gross grip strength but offer limited information about fine motor function or finger coordination— a research gap addressed in this thesis by a soft, easy-to-don, textile-native wearable piezoelectric sensing platform fabricated from β-phase PVDF textile yarns, demonstrating a sensitivity of 0.5 V/N and piezoelectric charge coefficient of 18 pC/N. Leveraging the direct piezoelectric effect and the conformability of this textile platform, a wearable sensing glove was designed to transduce mechanical signals, such as joint motion, finger bending, pressure, and vibration, and was evaluated in a clinical pilot study involving patients diagnosed with MS. Each glove integrated conformable sensors on three anatomically distinct digits (I, II, and V), enabling high-fidelity, simultaneous transduction of force and vibration biosignals during naturalistic hand use. In the pilot cohort, the glove yielded digital biomarkers of digit-level force and operating frequency that distinguished MS-affected hands from healthy controls, resolving motor features inaccessible to conventional grip dynamometry.

Svetlana Boriskina
Principal Research Scientist
Department of Mechanical Engineering

Joe Paradiso
Alexander W Dreyfoos (1954) Professor
MIT Media Lab

Mitch Thompson
CEO
Lionville Scientific, LLC