Plastic Dissipation Mechanisms in Periodic Microframe‐Structured Polymers

Novel lightweight micro‐ and nanostructured materials are being used as constituents in hierarchically structured composites for providing high stiffness, high strength, and energy absorbing capability at low weight. Three dimensional SU‐8 periodic microframe materials with submicrometer elements exhibit unusual large plastic deformations. Here, the plastic dissipation and mechanical response of polymeric microframe structures is investigated using micromechanical modeling of large deformations. Finite element analysis shows that multiple deformation domains initiate, stabilize, and then spread plasticity through the structure; simulated deformation mechanisms and deformation progression are found to be in excellent agreement with experimental observation. Furthermore, the geometry can be used to tailor aspects of 3D behavior such as effective lateral contraction ratios (elastic and plastic) during tensile loading as well as negative normal stress during simple shear deformation. The effects of structural geometry on mechanical response are also studied to tailor and optimize mechanical performance at a given density. These quantitative investigations enable simulation‐based design of optimal lightweight material microstructures for dissipating energy.     

Characterization of flexural and thermo-mechanical behavior of plastic ball grid package assembly using moiré interferometry

Flexural and thermo-mechanical behavior of a wire-bond plastic ball grid array (WB-PBGA) package assembly is characterized using moiré interferometry. Fringe patterns are recorded and analyzed at several bending loads and temperatures. Detailed global and local deformations of the assembly are investigated. The deformations caused by the thermally induced bending are compared with those caused by the mechanical bending. The results reveal that global bending modes are similar but the deformations at the critical locations are significantly different; the sign (direction) of the shear strain caused by the mechanical bending is the opposite of that caused by the thermal loading. The implication of the opposite bending on board level reliability is discussed.     

Mechanical Characterization of Amyloid Fibrils Using Coarse‐Grained Normal Mode Analysis

Recent experimental studies have shown that amyloid fibril formed by aggregation of β peptide exhibits excellent mechanical properties comparable to other protein materials such as actin filaments and microtubules. These excellent mechanical properties of amyloid fibrils are related to their functional role in disease expression. This indicates the necessity of understanding how an amyloid fibril achieves the remarkable mechanical properties through self‐aggregation with structural hierarchy. However, the structure‐property–function relationship still remains elusive. In this work, the mechanical properties of human islet amyloid polypeptide (hIAPP) are studied with respect to its structural hierarchies and structural shapes by coarse‐grained normal mode analysis. The simulation shows that hIAPP fibril can achieve the excellent bending rigidity via specific aggregation patterns such as antiparallel stacking of β peptides. Moreover, the length‐dependent mechanical properties of amyloids are found. This length‐dependent property has been elucidated from a Timoshenko beam model that takes into account the shear effect on the bending of amyloids. In summary, the study sheds light on the importance of not only the molecular architecture, which encodes the mechanical properties of the fibril, but also the shear effect on the mechanical (bending) behavior of the fibril.     

Designing Bio‐Inspired Adhesives for Shear Loading: From Simple Structures to Complex Patterns

The gecko has inspired numerous synthetic adhesive structures, yet under shear loading conditions, general design criteria remains underdeveloped. To provide guidance for bio‐inspired adhesives under shear, a simple scaling theory is used to investigate the relevant geometric and material parameters. The total compliance of an elastic attachment feature is described over many orders of magnitude in aspect ratio through a single continuous function using the superposition of multiple deformation modes such as bending, shear deformation, and tensile elongation. This allows for force capacity predictions of common geometric control parameters such as thickness, aspect ratio, and contact area. This superposition principal is extended to develop criteria for patterned interfaces under shear loading. Importantly, the adhesive patterns under shear are controlled through the compliance in the direction of loading. These predictions are confirmed experimentally using macroscopic building blocks over an extensive range of aspect ratio and contact area. Over 25 simple and complex patterns with various contact geometries are examined, and the effect of geometry and material properties on the shear adhesion behavior is discussed. Furthermore, all of these various attachment features are described with a single scaling parameter, offering control over orders of magnitude in adhesive force capacity for a variety of applications.     

Biomimetic Chitin–Silk Hybrids: An Optically Transparent Structural Platform for Wearable Devices and Advanced Electronics

The cuticles of insects and marine crustaceans are fascinating models for man‐made advanced functional composites. The excellent mechanical properties of these biological structures rest on the exquisite self‐assembly of natural ingredients, such as biominerals, polysaccharides, and proteins. Among them, the two commonly found building blocks in the model biocomposites are chitin nanofibers and silk‐like proteins with β‐sheet structure. Despite being wholly organic, the chitinous protein complex plays a key role for the biocomposites by contributing to the overall mechanical robustness and structural integrity. Moreover, the chitinous protein complex alone without biominerals is optically transparent (e.g., dragonfly wings), thereby making it a brilliant model material system for engineering applications where optical transparency is essentially required. Here, inspired by the chitinous protein complex of arthropods cuticles, an optically transparent biomimetic composite that hybridizes chitin nanofibers and silk fibroin (β‐sheet) is introduced, and its potential as a biocompatible structural platform for emerging wearable devices (e.g., smart contact lenses) and advanced displays (e.g., transparent plastic cover window) is demonstrated.     

Behaviour of anisotropic conductive joints under mechanical loading

Anisotropic conductive films (ACFs) received a great deal of attention in recent years for interconnection applications in electronic packaging. This paper reports the behaviour of ACF joints under various mechanical loading, i.e., die shear and cyclic fatigue in shear. The mechanical behaviour of ACF joints that have been exposed to environmental effects, i.e., high moisture and elevated temperature (autoclave test conditions) has been examined using die shear test. The maximum shear force prior to fracture was determined as 465.0 N, at which the contact resistance is found extremely high and can be considered as open circuits. Epoxy based ACF exhibits insignificant plastic deformation, especially for samples that have undergone autoclave test. Reduction trend was observed in the shear moduli over autoclave test time for ACF joints. Fracture surface of ACF that failed in shear test shows spalling and less plastic deformation after exposed to autoclave test. For cyclic fatigue test, the endurance limit is determined at about 143.5 N and the corresponding calculated endurance ratio is around 32.0%.     

Computation of full-vector modes for bending waveguide using cylindrical perfectly matched layers

A new full-vector approach to calculate leaky modes on three-dimensional bending waveguides is developed and demonstrated with the help of the cylindrical perfectly matched layer (CPML) numerical boundary conditions. By utilizing the complex coordinate stretching technique in the cylindrical system, a new set of full-vector wave equations for the bending waveguide structures are derived for the perfectly matched layer regions. Numerical solutions by the finite-difference schemes for the new wave equations are shown to yield highly accurate complex propagation constants (e.g., the bending-induced phase shifts and leakage losses) and modal field patterns, due primarily to the effective CPML.     

Design and optimization of color lookup tables on a simplex topology

An important computational problem in color imaging is the design of color transforms that map color between devices or from a device-dependent space (e.g., RGB/CMYK) to a device-independent space (e.g., CIELAB) and vice versa. Real-time processing constraints entail that such nonlinear color transforms be implemented using multidimensional lookup tables (LUTs). Furthermore, relatively sparse LUTs (with efficient interpolation) are employed in practice because of storage and memory constraints. This paper presents a principled design methodology rooted in constrained convex optimization to design color LUTs on a simplex topology. The use of n simplexes, i.e., simplexes in n dimensions, as opposed to traditional lattices, recently has been of great interest in color LUT design for simplex topologies that allow both more analytically tractable formulations and greater efficiency in the LUT. In this framework of n-simplex interpolation, our central contribution is to develop an elegant iterative algorithm that jointly optimizes the placement of nodes of the color LUT and the output values at those nodes to minimize interpolation error in an expected sense. This is in contrast to existing work, which exclusively designs either node locations or the output values. We also develop new analytical results for the problem of node location optimization, which reduces to constrained optimization of a large but sparse interpolation matrix in our framework. We evaluate our n -simplex color LUTs against the state-of-the-art lattice (e.g., International Color Consortium profiles) and simplex-based techniques for approximating two representative multidimensional color transforms that characterize a CMYK xerographic printer and an RGB scanner, respectively. The results show that color LUTs designed on simplexes offer very significant benefits over traditional lattice-based alternatives in improving color transform accuracy even with a much smaller number of nodes.     

A Stair-Building Strategy for Tailoring Mechanical Behavior of Re-Customizable Metamaterials

Re-customizable mechanical behavior is critical for versatile materials with tunable functions and applications, but inverse design for varying targets is often hindered by complex coupling between structural topologies and mechanics. In this work, a novel “stair-building” strategy for customizing as well as re-customizing target mechanical behavior for mechanical metamaterials is proposed. Similar to building a stair with bricks, customizing or re-customizing a target stress–strain (force–displacement) curve for the material can be realized by stacking the brick-like loading curves of bistable units visually. The mechanical feasibility of the “stair-building” strategy is firstly physically realized in a type of array-structured multistable mechanical metamaterial and then carefully verified by theoretical mechanics analysis. Accordingly, three specific simple design schemes are further proposed for implementation. The “stair-building” strategy is proved to be rapid, effective, and accurate for mechanical behavior customization by both experiments and finite element simulations. Moreover, re-customization for diverse mechanical behaviors in a wide range can be realized by the same piece of metamaterial. This design strategy provides a novel approach for tailoring metamaterials with re-customizable target mechanical behaviors and applies to a variety of bistable units.     

Novel Safeguarding Tactile e‐Skins for Monitoring Human Motion Based on SST/PDMS–AgNW–PET Hybrid Structures

A novel versatile electrical skin (e‐skin) with safeguarding and multisensing properties based on hybrid structures is developed by assembling Ag nanowires (AgNWs), polyester (PET) film with hybrid shear stiffening polymer/polydimethylsiloxane (SST/PDMS) matrix. The hybrid SST/PDMS polymer shows stable configuration. Storage modulus of the SST/PDMS increases from 5.5 kPa to 0.39 MPa when the shear frequency changes from 0.1 to 100 Hz, exhibiting typical rate‐dependent behavior. e‐Skin functions as a human‐monitoring device by detecting various motions such as gentle touching, stroking, elbow bending, as well as speaking. More importantly, due to the shear stiffening characteristic, e‐skin with high damping capacity exhibits safeguarding performance, which can dissipate impact force from 720 to 400 N and increase buffer time (from 0.9 to 2 ms). Meanwhile, distinguishable resistance values can reveal the level of harsh impact applied on the e‐skin. In addition, the visible thermosensation effect of e‐skin similar to chameleon epidermis is convenient for assessing environmental temperature. e‐Skin arrays can precisely map the dynamic impact location and pressure distribution. Finally, the high electrical sensitivity and shear stiffening performance are attributed to the disturbance of AgNW effective conductive paths and dynamic B? O bonds, respectively.     

Mechanically Versatile Soft Machines through Laminar Jamming

There are two major structural paradigms in robotics: soft machines, which are conformable, durable, and safe; and traditional rigid robots, which are fast, precise, and capable of applying high forces. Here, the paradigms are bridged by enabling soft machines to behave like traditional rigid robots on command. This task is accomplished via laminar jamming, a structural phenomenon in which a laminate of compliant strips becomes strongly coupled through friction when a pressure gradient is applied, causing dramatic changes in mechanical properties. Rigorous analytical and finite element models of laminar jamming are developed, and jamming structures are experimentally characterized to show that the models are highly accurate. Then jamming structures are integrated into soft machines to enable them to selectively exhibit the stiffness, damping, and kinematics of traditional rigid robots. The models allow jamming structures to efficiently meet arbitrary performance specifications, and the physical demonstrations illustrate how to construct systems that can behave like either soft machines or traditional rigid robots at will, such as continuum manipulators that can rapidly have joints appear and disappear. This study aims to foster a new generation of mechanically versatile machines and structures that cannot simply be classified as “soft” or “rigid.”     

Viscoelastic Cell Microenvironment: Hydrogel-Based Strategy for Recapitulating Dynamic ECM Mechanics

The native extracellular matrix (ECM) generally exhibits dynamic mechanical properties and displays time-dependent responses to deformation or mechanical loading, in terms of viscoelastic behaviors (e.g., stress relaxation and creep). Viscoelasticity of the ECM plays a critical role in development, homeostasis, and tissue regeneration, and its implication in disease progression has also been recognized recently. Hydrogels with tunable viscoelastic properties hold a great promise to recapitulate such time-dependent mechanics found in native ECM, which have been recently used to regulate cell behavior and guide cell fate. Here the importance of tissue viscoelasticity is first highlighted, the molecular mechanisms of hydrogel viscoelasticity are summarized, and characterization techniques used at the macroscale and microscale are reviewed. Then, recent advances in developing novel hydrogels with tunable viscoelasticity through varying crosslinking strategies, engineering of viscoelastic cell microenvironment and its substantial effects on cell behavior and fate are described, and the underlying mechanobiology mechanisms are subsequently discussed. Finally, the ongoing challenges and future perspectives on the design and modulation of viscoelastic hydrogels and the mechanobiology mechanisms on cellular responses to viscoelastic cell microenvironment are proposed.     

Tri-Layer Structures Subjected to Combined Temperature and Mechanical Loadings

Thermal stress has been a concern in electronic packaging for decades. More recently, mechanical bending of printed circuit board (PCB) assembly has attracted increased interest due to the drop impact failure of interconnects in mobile products. Analytical solutions are available in the literatures for a PCB assembly modeled as a tri-layer structure, consisting of IC components, PCBs, and an interconnect layer, subjected to either thermal stress or mechanical bending, but there are no known reports for combined loadings. This paper presents a comprehensive treatment for a PCB assembly subjected to combined temperature and mechanical loadings, taking into account the axial, shear, and flexural deformation of the interconnects. Solutions are provided for two types of interconnect layer: one in which the interconnect layer is made of a continuous element such as adhesive, and another in which the interconnect layer is made of discrete elements such as solder joints. The solutions were successfully validated with finite-element analysis, and design analyses were performed for both types of interconnect layers.      

High-temperature and humidity change the microstructure and degrade the material properties of tin‑silver interconnect material

The present work elucidates the microstructural changes and their impact on electrical resistivity and mechanical behavior of Sn-3.5 wt% Ag electronic interconnect material after exposure at high-temperature and relative humidity (85 °C/85%) environment. An in-depth structural observation is performed through electron microscopy e.g., SEM, EBSD and TEM techniques. The microstructural analysis shows that the as-received sample contained sub-micron size ε-Ag₃Sn intermetallic compound (IMC) and dendritic structure having a special orientation 〈100〉60° relationship with the matrix grains. However, it is found that after exposing the material at the harsh service environment for 60 days, the morphology, and size of the matrix grains and the ε-Ag₃Sn IMC phase are significantly altered. Such microstructural changes impact negatively on their material properties e.g., electrical resistivity, elastic and shear moduli, hardness and creep performance. An assessment between the as-cast and the aged material demonstrated that the degradations in hardness and elastic modulus are approximately 21.8 and 31.7%, respectively. Subsequently, the heat-treated material displays a higher temperature and strain amplitude-dependence damping characteristic as compared to the as-cast solder material.     

Electrically Activated Paper Actuators

This paper describes the design and fabrication of electrically controlled paper actuators that operate based on the dimensional changes that occur in paper when the moisture absorbed on the surface of the cellulose fibers changes. These actuators are called “Hygroexpansive Electrothermal Paper Actuators” (HEPAs). The actuators are made from paper, conducting polymer, and adhesive tape. They are lightweight, inexpensive, and can be fabricated using simple printing techniques. The central element of the HEPAs is a porous conducting path (used to provide electrothermal heating) that changes the moisture content of the paper and causes actuation. This conducting path is made by embedding a conducting polymer (PEDOT:PSS) within the paper, and thus making a paper/polymer composite that retains the porosity and hydrophilicity of paper. Different types of HEPAs (straight, precurved, and creased) achieved different types of motions (e.g., bending motion, accordion type motion). A theoretical model for their behavior is proposed. These actuators have been used for the manipulation of liquids and for the fabrication of an optical shutter.     

Isothermal bending fatigue response of solder joints in high power semiconductor test structures

Fatigue and cyclic delamination behavior of PbSnAg solders which are typically used as die attach material in power semiconductors was investigated. Isothermal bending fatigue tests were performed by using multilayered model test structures consisting of Si chips soldered on ceramic substrates and failure probability curves were obtained up to 1e8 loading cycles. The fatigue experiments were conducted by using an ultrasonic fatigue testing machine equipped with a three point bending set-up at a constant testing temperature of 80 °C. Detailed failure analysis of the fatigued samples revealed a dependency of the failure mode on the chemical composition of the high-Pb soft solders. The main failure modes included interfacial delamination of the Si-chip from the die attach, degradation due to crack propagation in the solder layer and in some cases partial fracture of the chip. Finally the feasibility of high frequency mechanical fatigue testing for screening and evaluation of solder joints in multilayered electronic systems is discussed.     

Programmable Granular Metamaterials for Reusable Energy Absorption

Designing metamaterials with programmable features has emerged as a promising pathway for reusable energy absorption. While the current designs of reusable energy absorbers mainly exploit mechanical instability of flexible beams, here is created a new kind of metamaterial for reusable and programmable energy absorption by integrating rigid granular materials and compliant stretchable components. In each unit cell of the metamaterial, the stretchable components connect the granular particles to maintain the integrity and control the deformation pattern of the material. When the metamaterial is subjected to an external load, the input energy is partially trapped as elastic energy in the stretchable components, and partially dissipated by friction between the granular particles, forming hysteresis between the loading and unloading force–displacement curves. Through tuning the structural design of the metamaterial, the pretension and stiffness of the stretchable components, and the size of and friction between the particles, a vast design space is achieved to program the mechanical behavior of the metamaterial, such as the load–displacement curve, the multistability, and the amount of energy dissipation. Experimental impact tests on a thin glass panel confirm energy‐absorbing capability of the proposed metamaterial. This design strategy opens a new avenue for creating reusable energy‐absorbing metamaterials.     

A Small Polymeric Ridge Waveguide With a High Index Contrast

In this paper, an optimal design of a small polymeric ridge waveguide with a high index contrast is presented. In order to reduce the leakage to the substrate and pure bending losses, the buffer layer of the present waveguide is etched partially. The single-mode condition, the bending characteristics, and the birefringence of the present small polymeric ridge waveguide are also studied. By adjusting the core width and the core height, it is possible to obtain a polarization-insensitive small polymer ridge waveguide. For the bending loss, the numerical results show that the dominant part is the transition loss between the straight and bending sections (other than the pure bending loss) and the transition loss could be reduced greatly by introducing a lateral offset. However, the transition loss is still too large to obtain a very small bending radius (e.g., 10 $mu $m). When only the pure bending loss is necessary to consider in some special case (e.g., microrings without any transitions), one can have a bending radius less than 10 $mu$m due to the possibility of low pure bending loss.      

空间相机大型轻质框架的选型与设计

针对某空间相机的光学设计结果,基于不同材料的机械加工特性,设计了几种结构形式的框架并进行了选型。采用有限元法对各框架的谐振频率进行了分析,对框架的经济性也进行了分析,最终确定了框架的结构形式。选定的框架为全桁架杆结构,重量为57 kg,一阶谐振频率为90.4 Hz。框架组件的力学试验验证表明,其一阶谐振频率为88.7 Hz。分析和试验结果均表明,该框架能够满足设计要求。     

Recent Advances in Design of Functional Biocompatible Hydrogels for Bone Tissue Engineering

Bone related diseases have caused serious threats to human health owing to their complexity and specificity. Fortunately, owing to the unique 3D network structure with high aqueous content and functional properties, emerging hydrogels are regarded as one of the most promising candidates for bone tissue engineering, such as repairing cartilage injury, skull defect, and arthritis. Herein, various design strategies and synthesis methods (e.g., 3D-printing technology and nanoparticle composite strategy) are introduced to prepare implanted hydrogel scaffolds with tunable mechanical strength, favorable biocompatibility, and excellent bioactivity for applying in bone regeneration. Injectable hydrogels based on biocompatible materials (e.g., collagen, hyaluronic acid, chitosan, polyethylene glycol, etc.) possess many advantages in minimally invasive surgery, including adjustable physicochemical properties, filling irregular shapes of defect sites, and on-demand release drugs or growth factors in response to different stimuli (e.g., pH, temperature, redox, enzyme, light, magnetic, etc.). In addition, drug delivery systems based on micro/nanogels are discussed, and its numerous promising designs used in the application of bone diseases (e.g., rheumatoid arthritis, osteoarthritis, cartilage defect) are also briefed in this review. Particularly, several key factors of hydrogel scaffolds (e.g., mechanical property, pore size, and release behavior of active factors) that can induce bone tissue regeneration are also summarized in this review. It is anticipated that advanced approaches and innovative ideas of bioactive hydrogels will be exploited in the clinical field and increase the life quality of patients with the bone injury.