Human skin is a remarkable organ. It consists of an integrated, stretchable network of sensors that relay information about tactile and thermal stimuli to the brain, allowing us to maneuver within our environment safely and effectively. Interest in large-area networks of electronic devices inspired by human skin is motivated by the promise of creating autonomous intelligent robots and biomimetic prosthetics, among other applications. The development of electronic networks comprised of flexible, stretchable, and robust devices that are compatible with large-area implementation and integrated with multiple functionalities is a testament to the progress in developing an electronic skin (e-skin) akin to human skin. E-skins are already capable of providing augmented performance over their organic counterpart, both in superior spatial resolution and thermal sensitivity. They could be further improved through the incorporation of additional functionalities (e.g., chemical and biological sensing) and desired properties (e.g., biodegradability and self-powering). Continued rapid progress in this area is promising for the development of a fully integrated e-skin in the near future.

25th Anniversary Article: The Evolution of Electronic Skin (E-Skin): A Brief History, Design Considerations, and Recent Progress

Polydimethylsiloxane (PDMS) elastomers are extensively used for soft lithographic replication of microstructures in microfluidic and micro-engineering applications. Elastomeric microstructures are commonly required to fulfil an explicit mechanical role and accordingly their mechanical properties can critically affect device performance. The mechanical properties of elastomers are known to vary with both curing and operational temperatures. However, even for the elastomer most commonly employed in microfluidic applications, Sylgard 184, only a very limited range of data exists regarding the variation in mechanical properties of bulk PDMS with curing temperature. We report an investigation of the variation in the mechanical properties of bulk Sylgard 184 with curing temperature, over the range 25 ◦ C to 200 ◦ C. PDMS samples for tensile and compressive testing were fabricated according to ASTM standards. Data obtained indicates variation in mechanical properties due to curing temperature for Young’s modulus of 1.32‐2.97 MPa, ultimate tensile strength of 3.51‐7.65 MPa, compressive modulus of 117.8‐186.9 MPa and ultimate compressive strength of 28.4‐51.7 GPa in a range up to 40% strain and hardness of 44‐54 ShA.

/pdf/mechanical-characterization-of-bulk-sylgard-184-for-2mkxi71f19.pdf

Mechanical characterization of bulk Sylgard 184 for microfluidics and microengineering

Since the late 1980s there have been spectacular developments in micromechanical or microelectro-mechanical (MEMS) systems which have enabled the exploration of transduction modes that involve mechanical energy and are based primarily on mechanical phenomena. As a result an innovative family of chemical and biological sensors has emerged. In this article, we discuss sensors with transducers in a form of cantilevers. While MEMS represents a diverse family of designs, devices with simple cantilever configurations are especially attractive as transducers for chemical and biological sensors. The review deals with four important aspects of cantilever transducers: (i) operation principles and models; (ii) microfabrication; (iii) figures of merit; and (iv) applications of cantilever sensors. We also provide a brief analysis of historical predecessors of the modern cantilever sensors.

Cantilever transducers as a platform for chemical and biological sensors

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Introduction To Electrodynamics

Word of mouth and interpersonal communication: A review and directions for future research

A technique that provides a first approximation to the mean, , and gradient, , components of residual stress in a thin-film material is discussed. In this method, measurements are made on a single micromachined cantilever, as opposed to an array of structures as used in the related critical-length buckling approach, to find tensile, compressive and gradient stresses. The measured deflection profile of a cantilever is reduced to rotation and curvature components, which are shown to derive independently from and , respectively. Essential to this method is the observation that a micromachined structure with a `nominally-clamped' boundary undergoes subtle rotation at its junction with the portion of the thin film that remains bonded to the substrate but is contiguous with the structure. This boundary rotation effect occurs through in-plane expansion or contraction of the bonded film following relief of residual stress. Thus, the deformation of the micromachined cantilevers considered here and of more general bulk- or surface-micromachined devices, can be strongly influenced by the state of stress in the still-bonded film.

https://iopscience.iop.org/article/10.1088/0960-1317/6/3/002/pdf

Determining mean and gradient residual stresses in thin films using micromachined cantilevers

The static deformation of micromachined beams under prescribed in-plane compressive stress is studied through analytical and experimental means over the prebuckling, transition, and postbuckling load ranges. The finite amplitude of the beam in its postbuckled state is predicted by modeling the non-linear dependence of the out-of-plane deformation on the compressive stress. In addition, the model explicitly considers the net effect of slight imperfections, which can include fabrication defects, geometric irregularities, or non-ideal loading, on the beam's behavior in the near-buckling regime. As an application, clamped-clamped silicon dioxide beams are fabricated through conventional bulk micromachining, and their deflected profiles are measured through three-dimensional optical profilometry. The measurements are compared to the postbuckled amplitudes and shapes that are predicted by the model, and by existing simpler models that do not include the effects of either non-linearity or imperfection. As borne out by the data, when imperfections are considered, the beams exhibit continuous growth of the out-of-plane amplitude during transition from the prebuckled state to a postbuckled one, in contrast to sudden bifurcation at a critical load. By accounting for this behavior, the estimate of residual stress in the thin film from which the beams are fabricated can be improved, and the amplitude of common postbuckled micromachined structures can be predicted during the design phase.

https://iopscience.iop.org/article/10.1088/0960-1317/4/3/004/pdf

Post buckling of micromachined beams

The coefficient of thermal expansion CTE is an important mechanical property for thin film materials. There are several problems that arise from the thermal expansion effect; for example, the mismatch of thermal expansion between the thin films and the underlying substrate may lead to residual stresses in the thin films. On the other hand, the thermal expansion effect can be exploited to drive microactuators. The CTEs of Al and Ti thin films were determined in the present study using the bilayer microcatilever technique. The contribution of this paper is to demonstrate the variation of the thin film CTE with the film thickness. The CTE of the Al thin film changes from 18.23= 10 y6 r8C to 29.97= 10 y6 r8C, when the film thickness increases from 0.3 to 1.7 mm. The CTE of the Ti thin film changes from 21.21= 10 y6 r8C to 9.04 = 10 y6 r8C, when the film thickness increases from 0.1 to 0.3 mm. Further, the concept that thin film CTE may be influenced by the defects in thin film materials is proposed. Thus, a desired thin film CTE can be obtained by tuning the film thickness as well as the deposition conditions. q 2000 Elsevier Science S.A. All rights reserved.

/pdf/on-the-thermal-expansion-coefficients-of-thin-films-4qk917qlul.pdf

On the thermal expansion coefficients of thin films

This paper reports a new solder bonding method for the wafer level packaging of MEMS devices. Electroplated magnetic film was heated using induction heating causing the solder to reflow. The experiment results show that it took less than 1 min to complete the bonding process. In addition, the MEMS devices experienced a temperature of only 110 °C during bonding, thus thin film materials would not be damaged. Moreover, the bond strength between silicon and silicon wafer was higher than 18 MPa. The step height of the feed-through wire (acting as the electrical feed-through of the bonded region) is sealed by the electroplated film. Thus, the flatness and roughness of the electroplated surface are recovered by the solder reflow, and the package for preventing water leakage can be achieved. The integration of the surface micromachined devices with the proposed packaging techniques was demonstrated.

https://iopscience.iop.org/article/10.1088/0960-1317/15/2/020/pdf

Localized induction heating solder bonding for wafer level MEMS packaging

http://mdl.pme.nthu.edu.tw/NTHU_PME_lab_ENG/papers/99.pdf

Weileun Fang

Papers

Determining mean and gradient residual stresses in thin films using micromachined cantilevers

Post buckling of micromachined beams

On the thermal expansion coefficients of thin films

Localized induction heating solder bonding for wafer level MEMS packaging

Static and dynamic mechanical properties of polydimethylsiloxane/carbon nanotube nanocomposites