Research Projects

Ultrasonic Neuromodulation

Physical Mechanisms of FUS Neuromodulation

Preclinical and clinical results are increasingly supporting the idea that low-intensity ultrasound can directly trigger cellular responses such as neuron firing, which can have downstream effects at a biological system level. This phenomenon is generating a lot of excitement for its clinical potential as a new form of precision therapy, with applications to many diseases including challenging neurological conditions. However, the physical mechanism of how the ultrasound interacts with cells in a tissue to produce these effects remains a mystery. I'm developing models along with stimulation and measurement hardware to address these open questions, in order to allow us to design effective therapies.

In Vitro Applications of LIFU

To tease apart the details of how ultrasound is interacting with cells, I've developed an in vitro focused ultrasound stimulation platform that we can use for precise stimulation of cell cultures while directly observing the cells in standard microscopy geometries. Along with collaborators in cell biology, I'm also exploring how the insights that emerge from these experiments could be adapted for use directly in cell culture, such as in stem cell differentiation or tissue engineering.

Read our review of ultrasound in tissue engineering

In Vivo Models for US Neuromodulation

In order to validate the hypotheses developed in vitro, I've developed a wearable focused ultrasound device that can be used for precision stimulation in preclinical experiments. This device can be used in vitro to concretely identify the role of specific biophysical pathways in ultrasonic neuromodulation. Beyond this, I'm using the device with collaborators for sonogenetic neuromodulation as well as to address open fundamental questions in neurobiology.

Ultrasound-Responsive Materials

Ultrasonically Controlled Release using Antibubbles

In fields such as medicine, biology, materials science, and chemistry, it is desirable to trigger chemical or biological processes noninvasively. To this end, ultrasound provides advantages over conventional approaches using light, electricity, or magnets. In addition to its high penetration depth in complex media, ultrasound can be patterned precisely and focused into small spatial regions. However, most ultrasonically-triggered processes to date require high ultrasound intensities, which is undesirable in many systems because they can trigger damaging effects such as cavitation or strong fluid flows. Recently, I have been exploring how we can overcome these limitations using cleverly-engineered microstructures. Antibubbles - air bubbles with a liquid core - are one such structure that respond strongly to ultrasound. Antibubbles can be loaded with diverse payloads and burst using low-intensity ultrasound, making them valuable new carriers for use in ultrasound-triggered chemical and biological processes.

Read the paper Read our review of ultrasound-responsive materials

Microactuation

Bubble-Based Acoustic Actuation

Ultrasound can provide many advantages for non-contact manipulation of objects in a sound field. However, there are typically constraints on the precision and size of objects that can be manipulated given a certain wavelength of sound. One way around these limitations is to make use of sub-wavelength scatterers, like bubbles. In a sound field, bubbles respond strongly to sound and will exert attractive or repulsive forces on other nearby objects or bubbles. These forces, however, are very weak and haven't been used for large-scale actuation. Here, we show how the forces can be amplified by patterning the bubbles into rigid structuers. The amplified forces are strong enough to manipulate and align centimeter-scale structures using 100-micron scale bubbles. We further show how the bubble patterns can be modified to create an acoustic motor, generating continuous rotation of one of the structures.

Read the paper Media Coverage Media Coverage (DE)

Light-Driven Distributed Pumping in Microchannels

As the size of fluid channels gets smaller, external pressure pumps need to do more work to fight viscous friction in the channels. At the same time, in complex fluid networks, it is difficult to locally control the flow, and the flow profiles are effectively set at design-time. Here we introduce a new approach to control flows in microchannels using distributed light-activated chemical pumps. When light shines on the pumps, they generate a unidirectional flow whose speed can be controlled by the light intensity. These micropumps offer not only the possibility to locally support external pumps within a microchannel, but could be used to locally change flow patterns and the flow profile within microfluidic networks.

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Acoustic Holograms & Field Synthesis

Dynamic Acoustic Holograms

Acoustic holograms are a powerful tool to shape ultrasonic pressure fields. Holograms can project significantly more complex patterns than state-of-the-art digital transducer arrays. Coupled with their simplicity and low cost, they provide several advantages in diverse settings from assembly to medical therapeutics.
One challenge for conventional acoustic holograms, however, is that once they are fabricated, the projected field pattern cannot be changed. For applications where real-time control over the pressure field is needed, it would be beneficial to be able to dynamically change the fields using acoustic holograms. Towards this end, I've worked on projects to create a spatial sound modulator, inspired by spatial light modulators used commonly in optical devices like digital projectors. One approach we took was to create bubbles at precise locations that can block sound on command, producing switchable binary amplitude holograms. Another approach I developed makes use of multiple hologram planes coupled to a multi-source array. By turning on different elements sequentially, multiple pre-defined fields can be projected. This technique can even be generalized to create more sophisticated compressive acoustic projectors.

Read the paper: Multiplane Acoustic Networks Read the paper: Bubble Sound Modulator

Acoustic Sensing

Acoustic/Elastic Sensing with Optical Breakdown Shocks

When managing ships or other ocean infrastructure, it is important to monitor the health of structures and to identify unknown objects on the surface. These types of inspection tasks currently require significant time and money to carry out, largely because the relevant sensors must make contact with the inspection target. In order to enable faster and more thorough inspections, I developed an acoustic sensing technique that uses a high-power laser to create sound at controllable points in the water, through a process called optical breakdown. By listening to how the short optical breakdown pulse scatters off of underwater objects, information about the material's composition and geometry can be calculated. This technique provides a new approach for underwater vehicles to remotely map structural and material properties along an underwater object.

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Medical Devices

Accessible Sleep Apnea Screening Device

Between 7 and 18 million Americans suffer from sleep disordered breathing (SDB), including those who suffer from obstructive sleep apnea (OSA). Despite this high prevalence of SDB, existing diagnostic techniques remain impractical for widespread screening. Furthermore, once a diagnosis is made, there are no readily available tools for routine monitoring of treatment efficacy on a longitudinal basis. As part of the ReDx initiative, I mentored a team in Mumbai to develop a wearable sleep mask with sensors for at-home screening and longitudinal monitoring of patients at risk for OSA. We developed a few prototypes and ran an early round of clinical testing to compare the performance of our device with existing at-home monitors.

Read the paper

Granular & Fluid Mechanics

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Vortex Interactions for Underwater Propulsion

Underwater robots can maneuver more precisely if they use pulsed jets instead of propellers to generate thrust. But in situations where multiple jets are needed - like when a robot needs to dock to a structure, or when a swarm of robots need to coordinate their activities - it is unclear how to best coordinate all the jets involved. In order to design and control underwater robots for efficient motion, it is critical to understand how the jets' wakes can interact, and how these interactions affect the thrusters' performance. To do this, I built an experiment to measure thruster performance when pulsed jets are ejected simultaneously. As the jets develop, vortex rings are formed in the wake; when the jets are close enough, the vortex interactions reduce thruster performance. To explain this behavior, I developed a physical model that agrees well with the data. My experiments show how some pulsing strategies can hurt performance, suggesting new strategies that leverage the vortex interactions to improve performance.

Read the paper Outreach Video Gallery of Fluid Motion Video

X-Ray Imaging of Granular Packings During Compression

To directly investigate how particle shape and particle interactions affect the mechanical properties of granular packings, I designed and built an x-ray tomography system that is able to image the 3D details of a granular packing while it is compressed in an Instron materials tester. This way, a packing's mechanical response (measured by the Instron) can be directly related to the internal geometry and rearrangements of grains.

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Granular Mechanics & Particle Shape

When you load a pile of sand (whether in the lab, at the beach, or on a construction site), the packing will slowly weaken until a slip-plane forms and the system cannot support the stress. However, if the shape of the grains is changed, the packing's behavior can change drastically, like we saw with the chains. An important question for designing granular systems is how the choice of grain shape will impact the mechanical response. To understand this relationship, we 3D-printed tens of thousands of mm-scale grains in 14 different shapes, and measured how bulk packings of each shape responded to mechanical compression. In addition to the broad collection of characteristic responses that we collected, we observed that local interactions between grains makes some packings more sensitive to confining pressure, suggesting that certain shapes can provide a more widely-tunable response for dynamic applications.

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Entanglement in Granular Chains

When a pile of chains is compressed, it will sometimes exhibit the unusual behavior of getting stronger the more it is compressed. This behavior is known as 'strain stiffening.' To figure out what causes strain stiffening in chains, we collected 3D x-ray measurements of the internal structure of chain packings at different stages during compression. These measurements revealed that the packings' increasing strength comes from interlocked clusters of chains that span the system. Moreover, if the chains are too short, they do not form large clusters, and we observed a corresponding loss of the strain-stiffening behavior.

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