Tae-Ik Lee

Highly Sensitive, Flexible, and Wearable Pressure Sensor Based on a Giant Piezocapacitive Effect of Three-Dimensional Microporous Elastomeric Dielectric Layer.

We report a flexible and wearable pressure sensor based on the giant piezocapacitive effect of a three-dimensional (3-D) microporous dielectric elastomer, which is capable of highly sensitive and stable pressure sensing over a large tactile pressure range. Due to the presence of micropores within the elastomeric dielectric layer, our piezocapacitive pressure sensor is highly deformable by even very small amounts of pressure, leading to a dramatic increase in its sensitivity. Moreover, the gradual closure of micropores under compression increases the effective dielectric constant, thereby further enhancing the sensitivity of the sensor. The 3-D microporous dielectric layer with serially stacked springs of elastomer bridges can cover a much wider pressure range than those of previously reported micro-/nanostructured sensing materials. We also investigate the applicability of our sensor to wearable pressure-sensing devices as an electronic pressure-sensing skin in robotic fingers as well as a bandage-type pressure-sensing device for pulse monitoring at the human wrist. Finally, we demonstrate a pressure sensor array pad for the recognition of spatially distributed pressure information on a plane. Our sensor, with its excellent pressure-sensing performance, marks the realization of a true tactile pressure sensor presenting highly sensitive responses to the entire tactile pressure range, from ultralow-force detection to high weights generated by human activity.

Hydrogel-laden paper scaffold system for origami-based tissue engineering

Significance This work describes an intriguing strategy for the formation of hydrogel-laden multiform structures utilizing paper sheets and suggests a route for trachea tissue engineering. It combines concepts extracted from paper origami, functional thin polymer coating, and thin hydrogel layering on top of the paper scaffolds. A computer-aided design-based lock-and-key arrangement was used for folding the sheets into multiform structures with spatial arrangements. With encapsulating cells in hydrogel-laden paper, the scaffold system was able to deliver biological cues in vivo. In this work, we have successfully applied an origami-based tissue engineering approach to the trachea regeneration model. In this study, we present a method for assembling biofunctionalized paper into a multiform structured scaffold system for reliable tissue regeneration using an origami-based approach. The surface of a paper was conformally modified with a poly(styrene-co-maleic anhydride) layer via initiated chemical vapor deposition followed by the immobilization of poly-l-lysine (PLL) and deposition of Ca2+. This procedure ensures the formation of alginate hydrogel on the paper due to Ca2+ diffusion. Furthermore, strong adhesion of the alginate hydrogel on the paper onto the paper substrate was achieved due to an electrostatic interaction between the alginate and PLL. The developed scaffold system was versatile and allowed area-selective cell seeding. Also, the hydrogel-laden paper could be folded freely into 3D tissue-like structures using a simple origami-based method. The cylindrically constructed paper scaffold system with chondrocytes was applied into a three-ring defect trachea in rabbits. The transplanted engineered tissues replaced the native trachea without stenosis after 4 wks. As for the custom-built scaffold system, the hydrogel-laden paper system will provide a robust and facile method for the formation of tissues mimicking native tissue constructs.

Extremely Robust and Patternable Electrodes for Copy-Paper-Based Electronics.

We propose a fabrication process for extremely robust and easily patternable silver nanowire (AgNW) electrodes on paper. Using an auxiliary donor layer and a simple laminating process, AgNWs can be easily transferred to copy paper as well as various other substrates using a dry process. Intercalating a polymeric binder between the AgNWs and the substrate through a simple printing technique enhances adhesion, not only guaranteeing high foldability of the electrodes, but also facilitating selective patterning of the AgNWs. Using the proposed process, extremely crease-tolerant electronics based on copy paper can be fabricated, such as a printed circuit board for a 7-segment display, portable heater, and capacitive touch sensor, demonstrating the applicability of the AgNWs-based electrodes to paper electronics.

Anomalous Stretchable Conductivity Using an Engineered Tricot Weave

Robust electric conduction under stretching motions is a key element in upcoming wearable electronic devices but is fundamentally very difficult to achieve because percolation pathways in conductive media are subject to collapse upon stretching. Here, we report that this fundamental challenge can be overcome by using a parameter uniquely available in textiles, namely a weaving structure. A textile structure alternately interwoven with inelastic and elastic yarns, achieved via a tricot weave, possesses excellent elasticity (strain up to 200%) in diagonal directions. When this textile is coated with conductive nanomaterials, proper textile engineering allows the textile to obtain an unprecedented 7-fold conductivity increase, with conductivity reaching 33,000 S cm(-1), even at 130% strain, due to enhanced interyarn contacts. The observed stretching conductivity can be described well using a modified 3D percolation theory that reflects the weaving effect and is also utilized for stretchable electronic interconnects and supercapacitors with high performance.

Bending Properties of Anisotropic Conductive Films Assembled Chip-in-Flex Packages for Wearable Electronics Applications

In this paper, a chip-in-flex (CIF) assembly that has an excellent bending performance, including a minimum bending radius without a chip fracture and the capacity to withstand dynamic bending, is developed. Chip-on-flex (COF) and CIF assemblies are fabricated using anisotropic conductive films (ACFs) as interconnection materials. The COF package is composed of 40-μm-thin silicon chips, ACF, and flexible substrates. The CIF package is fabricated by attaching a cover adhesive film and a polyimide film on the COF package to encapsulate the silicon chip. Through static bending tests, the optimal thickness of the cover adhesive film is established. The optimized CIF assembly allows a minimum bending radius of 4 mm without a chip fracture, while the chip in the COF assembly fractures at a bending radius of 10 mm. A finite-element analysis of the static bending test is performed to understand the internal stress state of the assemblies. A bending reliability test of the CIF package is also conducted at a bending radius of 7.5 mm for 160k cycles, by measuring the daisy-chain resistance during the test. The effect of the elastic modulus of the ACF resin on the fatigue endurance is investigated through the bending fatigue test. The higher modulus of the ACF resin resulted in excellent fatigue reliability with stable ACF joints showing neither delamination nor resin crazing after 160k cycles of bending.

Effects of the Mechanical Properties of Polymer Resin and the Conductive Ball Types of Anisotropic Conductive Films on the Bending Properties of Chip-in-Flex Package

Ultrathin chip-in-flex (CIF) packages using anisotropic conductive film (ACF) as an interconnecting material were demonstrated as one of the flexible electronic packages for wearable electronics applications. In this paper, the effects of ACF resin material and conductive ball type on the CIF package bending properties were investigated. Various ACFs with different moduli were fabricated by adding silica particle to polymer resin. For the conductive ball type, solder ball, metal-coated polymer ball, and Ni ball were used. To quantify the bending properties of the CIF packages, dynamic bending test was performed. It was found that the lowest modulus resin (0.54 GPa) resulted early delamination between the conductive ball and the electrode interface. For the resin modulus of 1.04 GPa, the conductive balls and electrodes interface delamination was significantly suppressed due to lower deformation of ACFs. However, when the modulus was increased to 1.3 GPa, chip crack failure occurred presumably due to the internal stress increase at the chip area. Therefore, it is suggested that the optimal modulus of the ACF resin was around 1 GPa. In addition, among the various conductive ball types, metal-coated polymer balls showed no electrical and mechanical failure even after 160k dynamic bending cycles. The metal-coated polymer balls had better compliance compared with the solder ball and Ni ball, which eventually prevented chip crack and ACF contact loss to occur.

Effect of Nanofiber Orientation on Nanofiber Solder Anisotropic Conductive Films Joint Properties and Bending Reliability of Flex-on-Flex Assembly

Anisotropic conductive films (ACFs) have been widely used as an interconnection adhesive material due to its light weight and simple and low-temperature assembly processes. However, because of the higher demands for further miniaturization, short-circuit problem of ACFs interconnection has been a major issue, when it comes to very fine pitch assembly. Also, due to the fast development of wearable devices, demands for flexible packaging as well as flexible interconnection methods such as flex-on-flex (FOF) are growing more, where the bending characteristics are important. Nanofiber incorporated ACFs were previously introduced by our research group. The nanofiber incorporated ACFs showed excellent conductive particle movement suppression capability that not only prevents short circuit but also improves the conductive ball capture rate, which eventually improves the joint reliability of fine pitch FOF assembly. In order to maximize the conductive particle movement suppression capability of the nanofiber, nanofiber was oriented using a drum-type receiver. In addition, the bending reliability of FOF assembly using nanofiber/solder ACFs with different nanofiber orientations has been also investigated. As a result, parallel nanofiber/solder ACFs showed excellent joint properties as well as bending reliability due to the stable metallurgical solder joint formation and high conductive particle movement suppression capability, and this new type of ACF technology will provide a promising solution for future flexible electronic packaging.

Ultra-thin chip-in-flex (CIF) technology using anisotropic conductive films (ACFs) for wearable electronics applications

For improving the flexibility and compact package size, chip-on-flex (COF)/chip-in-flex(CIF) assembly using Anisotropic Conductive Films (ACFs) as interconnection materials are investigated for flexible packaging technology of wearable electronics applications. The fatigue reliability of COF/CIF packages under dynamic bending environment was investigated. COF packages were successfully fabricated using ACFs with 40 um thin silicon chips and flexible substrates. By attaching the cover adhesive film with a polyimide film to COF packages, CIF packages were successfully fabricated as well. Through both convex and concave dynamic bending test, the thickness of cover adhesive film was optimized and then the minimum bending radius was obtained at no chip-crack condition. After fabrication of CIF packages with optimal cover adhesive film thickness of 30 um, and the effect of 3 types of ACFs on the dynamic bending performance of CIF packages up to 160 k cycles was also investigated. CIF packages using ACFs of higher modulus showed excellent fatigue reliability and stable ACF joints without delamination or resin crazing during dynamic bending test conditions.

The Effect of Anisotropic Conductive Films Adhesion on the Bending Reliability of Chip-in-Flex Packages for Wearable Electronics Applications

In this paper, the effects of adhesion properties of anisotropic conductive films (ACFs) interconnection on the chip-in-flex (CIF) bending reliability were investigated. Oxygen plasma treatment was conducted to increase the adhesion strength between ACFs and Si chip or ACFs and flexible printed circuit (FPC) substrates. In order to characterize the enhanced adhesion properties of the CIF packages, surface energy, surface roughness, elemental composition, and peel strength were measured. A digital image correlation method was used with cross-sectional scanning electron microscopy images to visualize the stress development at the ACFs interconnection. It was found that the interface of ACFs resin and FPC substrate showed the weakest adhesion, where the delamination was initiated. As a result of the improved adhesion at the ACFs resin and FPC substrate, the location of stress concentration was changed to the interface of Si chip and ACFs resin, leading to better dynamic bending reliability. When the oxygen plasma was treated both on the Si chip and FPC substrate, the stress concentration was observed not at the ACFs interfaces, but inside of the ACFs resin, resulting in further improved dynamic bending reliability. With the optimized plasma treatment condition and the ACFs materials, the dynamic bending reliability of the CIF packages was successfully demonstrated up to 160 000 bending cycles at a 7.5-mm bending radius without any electrical failures.

Effects of ACFs Adhesion on the Bending Reliability of Chip-in-Flex Packages for Wearable Electronics Applications

In this paper, the effects of adhesion properties of ACFs on CIF bending reliability were investigated by enhancing the adhesion between ACFs and Si chip or flexible substrate by an oxygen plasma treatment. In order to analyze the enhanced adhesion properties of CIF packages, several techniques such as contact-angle, surface-energy, surface-roughness, and SEM Digital image correlation (DIC) were used. The contact angle measurements indicate that the surface energies of Si chip and flexible substrate can be increased by an oxygen plasma treatment. And the Digital Image Correlation technique also measures the degree of deformation and strain mathematically by comparing the reference and deformed configuration digital images. As a result, by increasing adhesion strength using a plasma treatment, the stress concentration reduced at the ACF interface and the delamination location also changed. Moreover, when the oxygen plasma was treated on the Si chip and flexible substrate, stress concentration changed from the ACF interface to the ACF resin itself. With the optimized plasma treatment conditions and the ACF materials, the bending reliability of the CIF package was successfully demonstrated up to 160,000 bending cycles.