Chaofeng Lu

Injectable, Cellular-Scale Optoelectronics with Applications for Wireless Optogenetics

The Smaller, the Better New semiconductor device technology enables injection of light-emitting diodes, silicon devices, actuators, and sensors at precisely controlled locations within biological tissues, such as the brain. Kim et al. (p. 211) show how wireless control of animal models using these technologies and the techniques of optogenetics provide new insights into basic behavioral neuroscience. Miniaturized and implantable light-emitting diodes offer precise and flexible control of neurons, when used in combination with optogenetics. Successful integration of advanced semiconductor devices with biological systems will accelerate basic scientific discoveries and their translation into clinical technologies. In neuroscience generally, and in optogenetics in particular, the ability to insert light sources, detectors, sensors, and other components into precise locations of the deep brain yields versatile and important capabilities. Here, we introduce an injectable class of cellular-scale optoelectronics that offers such features, with examples of unmatched operational modes in optogenetics, including completely wireless and programmed complex behavioral control over freely moving animals. The ability of these ultrathin, mechanically compliant, biocompatible devices to afford minimally invasive operation in the soft tissues of the mammalian brain foreshadow applications in other organ systems, with potential for broad utility in biomedical science and engineering.

Digital cameras with designs inspired by the arthropod eye

In arthropods, evolution has created a remarkably sophisticated class of imaging systems, with a wide-angle field of view, low aberrations, high acuity to motion and an infinite depth of field. A challenge in building digital cameras with the hemispherical, compound apposition layouts of arthropod eyes is that essential design requirements cannot be met with existing planar sensor technologies or conventional optics. Here we present materials, mechanics and integration schemes that afford scalable pathways to working, arthropod-inspired cameras with nearly full hemispherical shapes (about 160 degrees). Their surfaces are densely populated by imaging elements (artificial ommatidia), which are comparable in number (180) to those of the eyes of fire ants (Solenopsis fugax) and bark beetles (Hylastes nigrinus). The devices combine elastomeric compound optical elements with deformable arrays of thin silicon photodetectors into integrated sheets that can be elastically transformed from the planar geometries in which they are fabricated to hemispherical shapes for integration into apposition cameras. Our imaging results and quantitative ray-tracing-based simulations illustrate key features of operation. These general strategies seem to be applicable to other compound eye devices, such as those inspired by moths and lacewings (refracting superposition eyes), lobster and shrimp (reflecting superposition eyes), and houseflies (neural superposition eyes).

Dynamically tunable hemispherical electronic eye camera system with adjustable zoom capability

Imaging systems that exploit arrays of photodetectors in curvilinear layouts are attractive due to their ability to match the strongly nonplanar image surfaces (i.e., Petzval surfaces) that form with simple lenses, thereby creating new design options. Recent work has yielded significant progress in the realization of such “eyeball” cameras, including examples of fully functional silicon devices capable of collecting realistic images. Although these systems provide advantages compared to those with conventional, planar designs, their fixed detector curvature renders them incompatible with changes in the Petzval surface that accompany variable zoom achieved with simple lenses. This paper describes a class of digital imaging device that overcomes this limitation, through the use of photodetector arrays on thin elastomeric membranes, capable of reversible deformation into hemispherical shapes with radii of curvature that can be adjusted dynamically, via hydraulics. Combining this type of detector with a similarly tunable, fluidic plano-convex lens yields a hemispherical camera with variable zoom and excellent imaging characteristics. Systematic experimental and theoretical studies of the mechanics and optics reveal all underlying principles of operation. This type of technology could be useful for night-vision surveillance, endoscopic imaging, and other areas that require compact cameras with simple zoom optics and wide-angle fields of view.

Unusual strategies for using indium gallium nitride grown on silicon (111) for solid-state lighting

Properties that can now be achieved with advanced, blue indium gallium nitride light emitting diodes (LEDs) lead to their potential as replacements for existing infrastructure in general illumination, with important implications for efficient use of energy. Further advances in this technology will benefit from reexamination of the modes for incorporating this materials technology into lighting modules that manage light conversion, extraction, and distribution, in ways that minimize adverse thermal effects associated with operation, with packages that exploit the unique aspects of these light sources. We present here ideas in anisotropic etching, microscale device assembly/integration, and module configuration that address these challenges in unconventional ways. Various device demonstrations provide examples of the capabilities, including thin, flexible lighting “tapes” based on patterned phosphors and large collections of small light emitters on plastic substrates. Quantitative modeling and experimental evaluation of heat flow in such structures illustrates one particular, important aspect of their operation: small, distributed LEDs can be passively cooled simply by direct thermal transport through thin-film metallization used for electrical interconnect, providing an enhanced and scalable means to integrate these devices in modules for white light generation.

Using nanoscale thermocapillary flows to create arrays of purely semiconducting single-walled carbon nanotubes

Among the remarkable variety of semiconducting nanomaterials that have been discovered over the past two decades, single-walled carbon nanotubes remain uniquely well suited for applications in high-performance electronics, sensors and other technologies. The most advanced opportunities demand the ability to form perfectly aligned, horizontal arrays of purely semiconducting, chemically pristine carbon nanotubes. Here, we present strategies that offer this capability. Nanoscale thermocapillary flows in thin-film organic coatings followed by reactive ion etching serve as highly efficient means for selectively removing metallic carbon nanotubes from electronically heterogeneous aligned arrays grown on quartz substrates. The low temperatures and unusual physics associated with this process enable robust, scalable operation, with clear potential for practical use. We carry out detailed experimental and theoretical studies to reveal all of the essential attributes of the underlying thermophysical phenomena. We demonstrate use of the purified arrays in transistors that achieve mobilities exceeding 1,000 cm(2) V(-1) s(-1) and on/off switching ratios of ∼10,000 with current outputs in the milliamp range. Simple logic gates built using such devices represent the first steps toward integration into more complex circuits.

Electronic sensor and actuator webs for large-area complex geometry cardiac mapping and therapy

Curved surfaces, complex geometries, and time-dynamic deformations of the heart create challenges in establishing intimate, nonconstraining interfaces between cardiac structures and medical devices or surgical tools, particularly over large areas. We constructed large area designs for diagnostic and therapeutic stretchable sensor and actuator webs that conformally wrap the epicardium, establishing robust contact without sutures, mechanical fixtures, tapes, or surgical adhesives. These multifunctional web devices exploit open, mesh layouts and mount on thin, bio-resorbable sheets of silk to facilitate handling in a way that yields, after dissolution, exceptionally low mechanical moduli and thicknesses. In vivo studies in rabbit and pig animal models demonstrate the effectiveness of these device webs for measuring and spatially mapping temperature, electrophysiological signals, strain, and physical contact in sheet and balloon-based systems that also have the potential to deliver energy to perform localized tissue ablation.

Exact Solutions for Free Vibrations of Functionally Graded Thick Plates on Elastic Foundations

Free vibration analysis of functionally graded thick plates on elastic foundation is carried out based on three-dimensional elasticity. An isotropic plate is assumed with Youngs modulus and mass density varying exponentially through the thickness, and Poissons ratio remaining constant. The state space method is used to derive an exact solution for a simply supported plate by expanding the state variables in trigonometric dual series. The effects of interaction between the plate surface and a Pasternak foundation are treated as the traction boundary conditions of the plate. The solution procedure is validated by comparing with established results for a homogeneous plate. Finally, the influences of foundation stiffness, foundation support, and gradient index on the natural frequencies are investigated.

Laser-Driven Micro Transfer Placement of Prefabricated Microstructures

Microassembly of prefabricated structures and devices is emerging as key process technology for realizing heterogeneous integration and high-performance flexible and stretchable electronics. Here, we report on a laser-driven micro transfer placement process that exploits, instead of ablation, the mismatch in thermomechanical response at the interface of a transferable microstructure and a transfer tool to a laser pulse to drive the release of the microstructure from the transfer tool and its travel to a receiving substrate. The resulting facile pick-and-place process is demonstrated with the assembling of 3-D microstructures and the placement of GaN light-emitting diodes onto silicon and glass substrates. High-speed photography is used to provide experimental evidence of thermomechanically driven release. Experiments are used to measure the laser flux incident on the interface. These, when used in numerical and analytical models, suggest that temperatures reached during the process are enough to produce strain energy release rates to drive delamination of the microstructure from the transfer tool.

Mechanics of Tunable Hemispherical Electronic Eye Camera Systems That Combine Rigid Device Elements With Soft Elastomers

A tunable hemispherical imaging system with zoom capability was recently developed by exploiting heterogeneous integration of rigid silicon photodetectors on soft, elastomeric supports, in designs that can facilitate tunable curvature for both the lens and detector. This paper reports analytical mechanics models for the soft materials aspects of the tunable lenses and detector surfaces used in such devices. The results provide analytical expressions for the strain distributions, apex heights and detector positions, and have been validated by the experiments and finite element analysis. More broadly, they represent important design tools for advanced cameras that combine hard and soft materials into nonplanar layouts with adjustable geometries.

Thermal properties of microscale inorganic light-emitting diodes in a pulsed operation

Light-emitting diodes (LEDs) in a pulsed operation offer combined characteristics in efficiency, thermal management, and communication, which make them attractive for many applications such as backlight unit, optical communication, and optogenetics. In this paper, an analytic model, validated by three dimensional finite element analysis and experiments, is developed to study the thermal properties of micro-scale inorganic LEDs (μ-ILED) in a pulsed operation. A simple scaling law for the μ-ILED temperature after saturation is established in terms of the material and geometrical parameters of μ-ILED systems, peak power, and duty cycle. It shows that the normalized maximum temperature increase only depends on two non-dimensional parameters: normalized μ-ILED area and duty cycle. This study provides design guidelines for minimizing adverse thermal effects of μ-ILEDs.