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On-demand drug delivery systems are promising for a wide range of therapeutic applications. When combined with wireless implants for controlled drug delivery, they can reduce overall dosage and side effects. Here, we demonstrate release of fluorescein from a novel on-demand release system for negatively charged compounds. The release system is based on a modified electroresponsive polypyrrole nanoparticulate film designed to minimize ion exchange with the stored compound - a major passive leakage mechanism. We further designed an ultrasonically powered mm-sized implant to electronically control the on-demand drug delivery system

Implantable medical devices (IMDs) and the concept of integrating electronics with the human body have been intriguing curiosities in medicine for many decades-slowly finding their way into mainstream medicine for several very specific conditions but not quite making it beyond "last resort" treatments. From the reality of today's cardiac pacemakers and deep-brain stimulation devices to the dream of sensor motes roaming around one's bloodstream (as described by researchers and writers like Isaac Asimov in his adaptation of Fantastic Voyage), the marriage of electronics and the human body has provided endless fascination for both science and science fiction alike. There is an abundance of potential applications, from electroceuticals to brain computer interfaces, and with advances in closed-loop systems and recent insights on neuromodulation, these devices are projected to make a huge impact on patient care for everything from neurological conditions all the way to diabetes and arthritis.

Multi-access networking with miniaturized wireless implantable devices can enable and advance closed-loop medical applications to deliver precise diagnosis and treatment. Using ultrasound (US) for wireless implant systems is an advantageous approach as US can beamform with high spatial resolution to efficiently power and address multiple implants in the network. To demonstrate these capabilities, we use wirelessly powered mm-sized implants with bidirectional communication links; uplink data communication measurements are performed using time, spatial, and frequency-division multiplexing schemes in tissue phantom. A 32-channel linear transmitter array and an external receiver are used as a base station to network with two implants that are placed 6.5 cm deep and spaced less than 1 cm apart. Successful wireless powering and uplink data communication around 100 kbps with a measured bit error rate below 10(−4) are demonstrated for all three networking schemes, validating the multi-access networking feasibility of US wireless implant systems.

Efficient ultrasonic power transfer to implantable devices requires precise transmitter beamforming to the receiver and can quickly degrade with small changes in implant location. Ultrasound localization can be used to find and track implants in the body to maintain an efficient link. We present a framework to calculate localization accuracy showing that sub-mm accuracy is obtainable using only three receive channels. A harmonic backscatter approach, which passively provides contrast in the frequency domain without active load modulation is compared to active uplink from the implant. The localization accuracy using both active uplink and harmonic backscatter from the implant power receiver is characterized using a linear array probe. The measured location standard deviation is nearly two orders of magnitude smaller than the half-power beamwidth of the array focal spot. Finally, beamforming using the measured location information increases the available power by over 20 × compared to an unfocused beam.

Miniaturized ultrasonic receivers are designed for efficient powering of implantable medical devices with reconfigurable power loads. Design parameters that affect the efficiency of these receivers under highly variable load conditions, including piezoelectric material, geometry, and operation frequency, are investigated. Measurements were performed to characterize electrical impedance and acoustic-to-electrical efficiency of ultrasonic receivers for off-resonance operation. Finally, we propose, analyze, and demonstrate adaptive matching and frequency tuning techniques using two different reconfigurable matching networks for typical implant loads from 10 $\mu \text{W}$ to 1 mW. Both simulations and measurements show a significant increase in total implant efficiency (up to 50 percentage points) over this load power range when operating off-resonance with the proposed matching networks.

We present the first fully wireless implantable imager for distributed long-term high-resolution imaging in the body. The imager consists of two mm-sized piezoelectric transducers (piezos) for ultrasound (US) wireless power and data communication; a capacitive micromachined ultrasound transducer (CMUT) array; and a 65 nm CMOS IC, which includes power harvesting, a high voltage charge pump, a low-power 16-channel front-end with up to $125\ \text{dB}\Omega$ gain and 13 MHz bandwidth, a memory system to store the digitized data at 8.6 Gbps, and a 120 kbps US transmitter. The on-chip realtime calibration, averaging, and interference mitigation help achieve high sensitivity. The system is characterized through a large 6 cm depth of acoustic tissue phantom and has an average power consumption of 1.5 mW at 1 fps.

Networks of miniaturized implantable medical devices (IMDs) capable of operating deep in the body are crucial for many novel diagnostic and therapeutic applications. An external portable base station to power, coordinate, and communicate with IMDs can enable these networks. Ultrasonic (US) wireless power transfer is utilized to shrink device dimensions, raise available power, and increase operation depth due to its small wavelength and low attenuation in the body. Building on previous demonstrations of US-powered IMDs for neuromodulation and active communication, we investigate and design an US linear array base station to interface with multiple implants. This allows for programmable focusing for multi-access control and higher power transfer efficiency (PTE), as well as bidirectional communication for closed-loop operation.

Miniaturized wirelessly powered implants capable of operating and communicating deep in the body are necessary for the next-generation of diagnostics and therapeutics. A major challenge in developing these minimally invasive implants is the tradeoff between device size, functionality, and operating depth. Here, we review two different wireless powering methods, inductive and ultrasonic power transfer, examine how to analyze their power transfer efficiency, and evaluate their potential for powering implantable medical devices. In particular, we show how ultrasonic wireless power transfer can address these challenges due to its safety, low attenuation, and millimeter wavelengths in the body. Finally, we demonstrate two ultrasonically powered implants capable of active power harvesting and bidirectional communication for closed-loop operation while functioning through multiple centimeters of tissue.

The next generation of implantable medical devices focuses on minimally invasive miniaturized solutions that operate reliably at large depths, provide duplex communication for closed-loop therapies, and enable multi-access for a network of implants to gather information or provide systemic interventions. Using ultrasound (US), power and data can be efficiently transferred through the body as its wavelength at MHz is comparable to a mm-sized receiver, resulting in improved focusing, coupling, and acoustic-to-electrical conversion efficiency. Furthermore, thanks to the low propagation loss (∼1dB/cm/MHz) and 7.2mW/mm2 safety limit, several mW of power is obtainable at the receiver, enabling high-power, complicated functionalities.

Carbon nanotube (CNT) field effect transistors (FETs) are anticipated to provide a viable alternative to silicon as CMOS technology begins to reach ultimate scaling lengths. With high carrier mobilities, current density, and tunable bandgaps, CNTs can allow for the growth of the transistor industry beyond traditional materials. However, these nanotubes are sensitive to their surroundings and require a protective passivation layer to isolate them from contamination by their external environment. Thin parylene films have been shown to be a practical passivation layer for CNT devices as a flexible, chemically inert, high dielectric strength, pinhole-free material. In this paper, we perform electrical characterization of single suspended CNT FETs before and after passivation with parylene-C thin-film deposition. Analysis shows moderate changes in the threshold voltage, subthreshold slope, and ON/OFF ratio with parylene passivation, indicating minimal effect on the CNT FET overall performance.

We explore graded index antireflection coatings that can reduce reflection of light at the ambient/cell or ambient/module boundary at all wavelengths and incidence angles. Simulations show that graded index coatings individually enhance absorption in solar cells across nearly the entire wavelength range. Silica sol-gel coatings with varying refractive indices deposited onto glass and silicon substrates are used to form graded index coatings, and the sol-gel layer processes have been adjusted so that the refractive indices are tunable from n=1.1 to n=1.5. Reflectance measurements also show that reflectivity decreases across the entire spectrum with added solgel multilayers.