Igor Bargatin

Untethered microscopic electronic circuits hold the potential for extraordinary advances in many fields such as neural transmitting and distributed sensing. However, establishing uplink communications from the microscale back to the macroscopic world remains challenging; existing micro-transmitters are difficult to integrate with semiconductor processing. Here we surmount this obstacle, introducing a strategy for modulating backscattered photons based on the electrochromic polymer PEDOT:PSS (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate) that is scalable to micron-order sizes and manufacturable using standard parallelizable methods. Our devices, which we call SPOTs (submillimeter polymer optical transmitters), actuate at low voltages (< +/-1 V), switch in as fast as 10 μs, can run for millions of cycles, and operate seamlessly in electrolytes. We achieve this design by emphasizing architectural simplicity and mass-manufacturability rather than traditional metrics such as data rates or energy costs. As a demonstration, we develop SPOT-equipped temperature-sensitive photovoltaic-powered foundry-fabricated microchips and use them to wirelessly measure and transmit local temperatures. These results represent an important step toward fully-integrable, micron-scale bidirectional communication.

Untethered microscopic electronic circuits hold the potential for extraordinary advances in many fields such as neural transmitting and distributed sensing. However, establishing uplink communications from the microscale back to the macroscopic world remains challenging; existing micro-transmitters are difficult to integrate with semiconductor processing. Here we surmount this obstacle, introducing a strategy for modulating backscattered photons based on the electrochromic polymer PEDOT:PSS (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate) that is scalable to micron-order sizes and manufacturable using standard parallelizable methods. Our devices, which we call SPOTs (submillimeter polymer optical transmitters), actuate at low voltages (< +/-1 V), switch in as fast as 10 μs, can run for millions of cycles, and operate seamlessly in electrolytes. We achieve this design by emphasizing architectural simplicity and mass-manufacturability rather than traditional metrics such as data rates or energy costs. As a demonstration, we develop SPOT-equipped temperature-sensitive photovoltaic-powered foundry-fabricated microchips and use them to wirelessly measure and transmit local temperatures. These results represent an important step toward fully-integrable, micron-scale bidirectional communication.

Abstract Achieving laser-driven relativistic light sails would represent a tremendous breakthrough for humankind. Numerous sail designs have been proposed, but none satisfy all the stringent optical, mechanical, and mass constraints. Here we demonstrate a class of nanolaminate sails with strong and flexible hexagonally-corrugated microstructures. Our prototypes, fabricated from alumina and molybdenum disulfide using scalable semiconductor processing techniques, feature areal densities of &lt; 1 g ⋅ m −2 , achieve experimentally-measured broadband reflectivities of &gt; 50%, and feature broadband absorptivities of &lt; 4% with a measurement uncertainty that overlaps with zero - indicative of our sail class’s potential for fast acceleration and ultra-low photon absorption. Moreover, we propose a sail’s maximum achievable relative velocity as a performance benchmark, and analyze optical, mechanical, and mass constraints for our design and others in the literature to highlight the strong potential of our class of sails. Our approach represents a promising step toward plausible relativistic interstellar propulsion.

We propose a tether-based structural architecture for orbital data centers operating in Dawn-Dusk Sun-Synchronous (DDSS) orbits under continuous sunlight. These space-based data centers, powered solely by solar energy, could provide multi-megawatt computing for artificial intelligence (AI) inference with minimal latency to Earth. The proposed design uses a tethered chain of computing nodes with photovoltaic panels to achieve uninterrupted ~2–20 MW of computing power, and employs radiative cooling and integrated shielding to manage heat and radiation. We detail the system architecture, including mass budgets, passive attitude control, and the dynamics induced by micrometeoroid collisions.

We present a proof-of-concept study showing that buckled aluminized polyimide films perforated with millimeter-scale cuts can redirect normally incident light obliquely and generate net in-plane force components parallel to the global solar sail surface. We use finite element simulations to obtain the buckled shapes of different periodic unit cell geometries and apply ray optics modeling to compute the resulting light-pressure forces. The simulations show that the buckled kirigami surfaces reflect light into different directions, producing a net in-plane force parallel to the direction of stretching. We verify these trends experimentally by illuminating a tensioned kirigami sample with a laser and observing reflected beam patterns consistent with the ray optics simulations. These results suggest that kirigami films may offer a low-power and lightweight way to achieve controllable in-plane forces for solar sail steering.

We propose a low-pretension reflective kirigami film as a material for the reflective surfaces of large deployable space reflector antennas with an operating frequency around 10 GHz. The kirigami cut pattern is based on the well-known rotating squares pattern but is augmented with diagonal cuts to enhance stretchability and allow control over the effective Poisson's ratio. Using finite element simulations, we analyzed how the geometric parameters of this pattern affected the reflectance of the film and the pretension required to resist thermal deformations. Tensile testing of selected designs, which are approximately half the weight of traditional metallic meshes, demonstrated a substantial reduction in the needed pretension to ~0.5 N/m and as low as ~0.1 N/m. Such low pretension represents an order-of-magnitude improvement over traditional metallic mesh reflectors and could enable the use of lighter antenna trusses. Free-space reflectance measurements also show that these perforated films can maintain power reflectance exceeding 90% at 10 GHz under the strains expected in the deployed configuration.

By harnessing the momentum of sunlight solar sails offer a lightweight alternative to chemical propulsion for long-duration space missions. Intuitively, larger sails are able to harness more power. However, as sails get larger they require more time and energy to rotate, making precise orientation and travel more time consuming and difficult. This project investigates tunable buckling in kirigami-inspired films as a way to achieve directional control without rotating the entire structure. Through light detection image-processing and a thermal sensor the relationship between in-plane reflected power and strain was quantified. sensor, the relationship between in-plane reflected power and applied strain was quantified. Results from rotating triangle geometries demonstrate controllable inclination angles and measurable shifts in reflected intensity with strain. These findings indicate that strain-induced buckling can serve as an effective actuation mechanism for solar sails. Future work aims to refine calibration accuracy and develop a dimensionless coefficient relating incident optical power to in-plane force across kirigami geometries.

Deployable space antennas demand materials that are lightweight, compact, and highly stretchable. This work explores kirigami-inspired mechanical metamaterials as potential reflector surfaces as alternatives to AstroMesh system. Using Finite Element Analysis (FEA) and physical prototyping, we evaluated five geometries by stretchability, isotropicity and manufacturability with the corresponding indices. Results reveal that additional diagonal and auxiliary cuts significantly improve stretchability but can reduce isotropicity. Geometry 5 achieved the highest stretchability, while Geometry 3 achieved the highest manufacturability, which demonstrates the balance required between mechanical properties and fabrication feasibility. Overall, we demonstrated that kirigami metamaterials hold strong promise for lightweight reflector applications, while also providing broader design insights for kirigami metamaterials.

We show theoretically that photophoretic aircraft would greatly benefit from a three-dimensional hollow geometry that pumps ambient air through sidewalls to create a high-speed jet. To identify optimal geometries, we developed a theoretical expression for the lift force based on both Stokes (low Reynolds number) and momentum (high Reynolds number) theory and validated it using finite-element fluid-dynamics simulations. We then systematically varied geometric parameters, including Knudsen pump porosity, to minimize the operating altitude or maximize the payload. Assuming that large vehicles can be made from nanocardboard material, as previously demonstrated at smaller scales, the minimum altitude such vehicles can levitate at is approximately 55 km, while the payload can reach approximately 1 kg at an altitude of 80 km for vehicles with a 10 m diameter. In all cases, the maximum areal density of the sidewalls cannot exceed a few grams per square meter, demonstrating the need for ultralight porous materials.

The goal of ultrathin lightweight photophoretic flyers, or light flyers for short, is to levitate continuously in Earth's upper atmosphere using only sunlight for propulsive power. We previously reported light flyers that levitated by utilizing differences in thermal accommodation coefficient between the top and bottom of a thin film, made possible by coating their lower surfaces with carbon nanotubes (CNTs). Such designs, though successful, had relatively high thermal emissivity (&gt;0.5), which prevented them from achieving high temperatures and resulted in their transferring relatively low amounts of momentum to the surrounding gas. To address this issue, we have developed light flyers with ultrathin undoped germanium layers that selectively absorb nearly 80% of visible light but are mostly transparent in the thermal infrared, with an average thermal emissivity of 0.1. Our experiments show that germanium-coated light flyers could levitate at up to 43% lower light irradiances than mylar-CNT disks with identical sizes. In addition, we simulated our experiments using a semiempirical model, which allowed us to predict that our 2-cm-diameter disk-shaped germanium-coated light flyers can levitate in the mesosphere (altitudes 67--75 km) under the natural sunlight (1.36 kW/${\mathrm{m}}^{2}$). Similar ultrathin selective-absorber coatings can also be applied to three-dimensional light flyers shaped like solar balloons, allowing them to carry significant payloads and thereby revolutionize long-term atmospheric exploration of Earth or Mars.

Abstract Faculty and staff can and do influence the climate of a department and achievement of students. Research shows the positive effects of choosing to implement evidence-based teaching practices like active learning and inclusive teaching, and having a growth mindset in relation to the abilities of students. However, research also shows that the local climate in a department could cause students of color to be driven from STEM, or that a chilly climate could have a disproportionate impact on female students. And while the focus of Diversity, Equity, and Inclusion (DEI) efforts tends to be on women and under-represented minorities (URMs, defined as non-white, non-Asian), populations with representation at or above the demographics of the general population face their own challenges. In this paper, we describe recent efforts in the Mechanical Engineering and Applied Mechanics (MEAM) Department at the University of Pennsylvania to address these issues. Most of our initial efforts in this area have focused on the undergraduate population as well as their intersection with faculty and staff. We have started exploring the departmental structures and practices and have some initial demographic data on students and faculty. We are interested in exploring how retention, graduation, and achievement in general overlap and intersect with gender, race, and socio-economic status. We have also recently implemented a DEI Scholars program that further engages undergraduate and graduate students in this process. This initial work establishes baseline numbers and describes the first cohort we will track from acceptance through graduation. It is our aim that sharing these early efforts may encourage others to take on similar endeavors, and will also provide a reference point for future work of ours in this area.

In this work, we propose a vehicle that is powered solely by light, has no moving parts, and is capable of carrying kg-scale payloads from 0 to 80 km above Earth’s surface. The vehicle originally employed 2D porous centimeter-scale plates that utilize photophoresis—the light-induced movement of gas particles. Photophoresis exploits temperature gradients created by sunlight to pump air through channels spanning the thickness of such plates, creating lift for payloads. We developed a model that predicts several kilograms of payload capabilities for 3D structures with radii of tens of meters. These structures also function as solar balloons, providing lift at altitudes below 50 km, while photophoretic forces provide most of the lift to the mesosphere (50-80 km). We also report experimental results showcasing solar buoyancy, as well as initial proof-of-concept reduced pressure testing. Proposed applications include extensive atmospheric measurements of winds, gas concentrations, and other state properties that are difficult to measure remotely.

Micro aerial vehicles are making a large impact in applications such as search-and-rescue, package delivery, and recreation. Unfortunately, these diminutive drones are currently constrained to carrying small payloads, in large part because they use propellers optimized for larger aircraft and inviscid flow regimes. Fully realizing the potential of emerging microflyers requires next-generation propellers that are specifically designed for low Reynolds number conditions and that include new features advantageous in highly viscous flows. One aspect that has received limited attention in the literature is the addition of roughness to propeller blades as a method of reducing drag and increasing thrust. To investigate this possibility, we used direct numerical simulation to conduct a numerical investigation of smooth and rough propellers. Our results indicate that roughness produces a 2% increase in thrust and a 5% decrease in power relative to a baseline smooth propeller operating at the same Reynolds number of Rec = 6500, held constant by rotational speed. We complement our numerical findings using thrust-stand-based experiments of 3D-printed propellers identical to those of the numerical simulations. Our study indicates that surface roughness is an additional parameter within the design space for micro-propellers, which may offer improved drone efficiencies and payloads.

The goal of ultrathin lightweight photophoretic flyers, or light-flyers for short, is to levitate continuously in Earth's upper atmosphere using only sunlight for propulsive power. We previously reported light-flyers that levitated by utilizing differences in thermal accommodation coefficient (TAC) between the top and bottom of a thin film, made possible by coating their lower surfaces with carbon nanotubes (CNTs). Such designs, though successful, were limited due to their high thermal emissivity (&gt;0.5), which prevented them from achieving high temperatures and resulted in their transferring relatively low amounts of momentum to the surrounding gas. To address this issue, we have developed light-flyers with undoped germanium layers that selectively absorb nearly 80% of visible light but are mostly transparent in the thermal infrared, with an average thermal emissivity of &lt;0.1. Our experiments show that germanium-coated light-flyers could levitate at up to 43% lower light irradiances than mylar-CNT disks with identical sizes. In addition, we simulated our experiments using a combined first-principles-empirical model, allowing us to predict that our 2-cm-diameter disk-shaped germanium-coated light-flyers can levitate in the mesosphere (altitudes 68-78 km) under the natural sunlight (1.36 kW/m2). Similar ultrathin selective-absorber coatings can also be applied to three-dimensional light-flyers shaped like solar balloons, allowing them to carry significant payloads and thereby revolutionize long-term atmospheric exploration of Earth or Mars.

Photophoretic levitation is a propulsion mechanism by which lightweight objects can be lifted and controlled through their interactions with light. Since photophoretic forces on macroscopic objects are usually maximized at low pressures, they may be tested in a vacuum chamber in close proximity to the chamber floor and walls. We report experimental evidence that the terrain under levitating microflyers, including the chamber floor or the launchpad from which the microflyer lifts off, can greatly increase the photophoretic lift forces relative to their free-space (midair) values. To characterize this so-called ``ground effect'' during vacuum-chamber tests, we introduce a miniature launchpad composed of three J-shaped (candy-cane-like) wires that minimize the microflyer's extraneous interactions with the underlying surfaces. We compare our J-shaped-wire launchpad with previously used wire-mesh launchpads for simple levitating Mylar-based disks with diameters of 2, 4, and 8 cm. Importantly, we discover that wire-mesh launchpads increase the photophoretic lift force by up to sixfold. A significant ground effect is also associated with the bottom of the vacuum chamber, particularly when the distance to the bottom surface is less than the diameter of the levitating disk. We provide guidelines to minimize the ground effect in vacuum-chamber experiments, which are necessary to test photophoretic microflyers intended for high-altitude exploration and surveillance on Earth or on Mars.

We show that photophoretic aircraft would greatly benefit from a three-dimensional (3D) hollow geometry that pumps ambient air through sidewalls to create a high-speed jet. To identify optimal geometries, we developed a theoretical expression for the lift force based on both Stokes (low-Re) and momentum (high-Re) theory and validated it using finite-element fluid-dynamics simulations. We then systematically varied geometric parameters, including Knudsen pump porosity, to minimize the operating altitude or maximize the payload. Assuming that the large vehicles can be made from previously demonstrated nanocardboard material, the minimum altitude is 55 km while the payload can reach 1 kilogram for 3D structures with 10-meter diameter at 80 km altitude. In all cases, the maximum areal density of the sidewalls cannot exceed a few grams per square meter, demonstrating the need for ultralight porous materials.