Zhan Kang

Waterproof AlInGaP optoelectronics on stretchable substrates with applications in biomedicine and robotics

Inorganic light-emitting diodes and photodetectors represent important, established technologies for solid-state lighting, digital imaging and many other applications. Eliminating mechanical and geometrical design constraints imposed by the supporting semiconductor wafers can enable alternative uses in areas such as biomedicine and robotics. Here we describe systems that consist of arrays of interconnected, ultrathin inorganic light-emitting diodes and photodetectors configured in mechanically optimized layouts on unusual substrates. Light-emitting sutures, implantable sheets and illuminated plasmonic crystals that are compatible with complete immersion in biofluids illustrate the suitability of these technologies for use in biomedicine. Waterproof optical-proximity-sensor tapes capable of conformal integration on curved surfaces of gloves and thin, refractive-index monitors wrapped on tubing for intravenous delivery systems demonstrate possibilities in robotics and clinical medicine. These and related systems may create important, unconventional opportunities for optoelectronic devices.

Multifunctional Epidermal Electronics Printed Directly Onto the Skin

Materials and designs are presented for electronics and sensors that can be conformally and robustly integrated onto the surface of the skin. A multifunctional device of this type can record various physiological signals relevant to health and wellness. This class of technology offers capabilities in biocompatible, non-invasive measurement that lie beyond those available with conventional, point-contact electrode interfaces to the skin.

Thin, Flexible Sensors and Actuators as ‘Instrumented’ Surgical Sutures for Targeted Wound Monitoring and Therapy

Sutures are among the simplest and most widely used devices in clinical medicine. All existing synthetic and natural forms use thread-like geometries, as purely passive, mechanical structures that are fl exible and resilient to tensile stress. Several recent reports describe strategies to incorporate advanced functionality into this platform through the employment of shape-memory polymers that offer mechanical actuation or through the release of bioresorbable compounds that carry growth factors and antibiotics to accelerate healing. [ 1–3 ]

Mechanics of Epidermal Electronics

Epidermal electronic system (EES) is a class of integrated electronic systems that are ultrathin, soft, and lightweight, such that it could be mounted to the epidermis based on van der Waals interactions alone, yet provides robust, intimate contact to the skin. Recent advances on this technology will enable many medical applications such as to monitor brain or heart activities, to monitor premature babies, to enhance the control of prosthetics, or to realize human-machine interface. In particular, the contact between EES and the skin is key to high-performance functioning of the above applications and is studied in this paper. The mechanics concepts that lead to successful designs of EES are also discussed. The results, validated by finite element analysis and experimental observations, provide simple, analytical guidelines for design and optimization of EES with various possible functionalities. [DOI: 10.1115/1.4005963]

Integrated Optimization of Material Layout and Control Voltage for Piezoelectric Laminated Plates

Topology optimization techniques have recently been successfully applied to the design of piezoelectric smart structures. However, in previous formulations, the material layout is optimized under the condition of given electric actuation voltages. This imposes a restriction to the design problem and may consequently limit application of these approaches, particularly in complex shape control problems. The present article investigates the integrated optimization of structural topology and control voltage of piezoelectric laminated plates. The finite element analysis formulation of laminated plates with surface bonded piezoelectric layers is introduced first. The optimal design problem is then formulated based on an artificial material model with penalization for both mechanical and piezoelectric properties. In the formulated problem, the topologies of both host and actuation layers are optimized simultaneously with spatial distribution of control voltage. Several special cases of the proposed design problem are considered, and numerical techniques for sensitivity analysis and optimal solutions are proposed. Numerical examples are presented to demonstrate the validity and applicability of the proposed methods.Topology optimization techniques have recently been successfully applied to the design of piezoelectric smart structures. However, in previous formulations, the material layout is optimized under the condition of given electric actuation voltages. This imposes a restriction to the design problem and may consequently limit application of these approaches, particularly in complex shape control problems. The present article investigates the integrated optimization of structural topology and control voltage of piezoelectric laminated plates. The finite element analysis formulation of laminated plates with surface bonded piezoelectric layers is introduced first. The optimal design problem is then formulated based on an artificial material model with penalization for both mechanical and piezoelectric properties. In the formulated problem, the topologies of both host and actuation layers are optimized simultaneously with spatial distribution of control voltage. Several special cases of the proposed design proble...

Mechanics of self-folding of single-layer graphene

The extreme out-of-plane flexibility makes single-layer graphene vulnerable to self-folding, driven by van der Waals interactions. Racket shaped bilayer graphene edges form after self-folding, which can significantly affect the electrical properties of graphenes. To study the self-folding behaviour, a theoretical model based on finite deformation beam theory is established. The critical folding lengths for both metastable and stable self-folding, as well as the edge profile of a folded single-layer graphene, are given. They all agree very well with MD simulations. MD simulations also show that folding directions do not have strong influence on the shape of folded graphene edges. (Some figures may appear in colour only in the online journal)

An analytical model of strain isolation for stretchable and flexible electronics

One important aspect of stretchable electronics design is to shield the active devices from strains through insertion of a soft layer between devices and substrate. An analytical model is established, which gives linear dependence of strain isolation on the reciprocal of strain-isolation layer thickness, and the reciprocal of device and substrate stiffness. Strain isolation is also linearly proportional to the shear modulus of strain-isolation layer and square of device length.

Topological design of compliant smart structures with embedded movable actuators

In the optimal configuration design of piezoelectric smart structures, it is favorable to use actuation elements with certain predefined geometries from the viewpoint of manufacturability of fragile piezoelectric ceramics in practical applications. However, preserving the exact shape of these embedded actuators and tracking their dynamic motions presents a more challenging research task than merely allowing them to take arbitrary shapes. This paper proposes an integrated topology optimization method for the systematic design of compliant smart structures with embedded movable PZT (lead zirconate titanate) actuators. Compared with most existing studies, which either optimize positions/sizes of the actuators in a given host structure or design the host structure with pre-determined actuator locations, the proposed method simultaneously optimizes the positions of the movable PZT actuators and the topology of the host structure, typically a compliant mechanism for amplifying the small strain stroke. A combined topological description model is employed in the optimization, where the level set model is used to track the movements of the PZT actuators and the independent point-wise density interpolation (iPDI) approach is utilized to search for the optimal topology of the host structure. Furthermore, we define an integral-type constraint function to prevent overlaps between the PZT actuators and between the actuators and the external boundaries of the design domain. Such a constraint provides a unified and explicit mathematical statement of the non-overlap condition for any number of arbitrarily shaped embedded actuators. Several numerical examples are used to demonstrate the effectiveness of the proposed optimization method.

Topology optimization of bending actuators with multilayer piezoelectric material

This paper investigates the topology optimization of bending actuators with multilayer piezoelectric material. In this problem, the distribution of the control voltage and the structural layout of the actuator are optimized simultaneously. Constraints with regard to energy consumption and material volume are imposed on the optimization problem. Numerical techniques for sensitivity analysis are presented and the proposed optimization problem is solved with a gradient-based mathematical programming approach. Illustrative examples are given to demonstrate the potential of the proposed approach.

Topology optimization of piezoelectric layers in plates with active vibration control

This article investigates topology optimization of the piezoelectric actuator and sensor layers in a plate for achieving the best vibration control performance. Therein, the actuator patches and sensor patches are symmetrically attached to the host layer, and the classical negative velocity feedback control strategy is adopted for reducing the vibration level of the structure. In the optimization model, the dynamic compliance under a specific excitation frequency or the aggregated dynamic compliance in a given frequency range is taken as the objective function. The relative densities of the elements in the actuator layer and the sensor layer are considered as topological design variables. The optimization problem is then formulated by using an artificial material model with penalization for both mechanical and piezoelectric properties. It is pointed out that the global-level damping property, consisting of the structural damping and the active damping effects, is a nonproportional one. For alleviating the computational burden involved in the frequency response analysis, the dynamic equations are solved with the complex mode superposition in the state space after a model reduction transformation. In this context, the sensitivity analysis scheme is also derived. The effectiveness and efficiency of the proposed method are demonstrated by numerical examples.