Thomas Vervust

Stretchable Electronics Technology for Large Area Applications: Fabrication and Mechanical Characterization

The development and mechanical characterization of a novel technology for stretchable electronics is presented, which can be used for the realization of wearable textile electronics and biomedical implants. The stretchable devices consist of rigid or flexible component islands interconnected with stretchable meander-shaped copper conductors embedded in a stretchable polymer, polydemethylsiloxane. The technology uses standard printed circuit board manufacturing steps and liquid injection molding techniques to achieve a robust and reliable product. The conductors in the device are designed to accommodate strains up to 10-15%. Spin-on photo-definable polyimide as mechanical support for the stretchable interconnects and the functional flexible islands are introduced. By use of polyimide, the reliability of the stretchable interconnects, the straight interconnects on the flexible islands and the transitions between the stretchable and nonstretchable parts are improved. Long-term endurance behavior of the stretchable interconnects is studied by cyclic elongation at strain ranges of up to 20% while monitoring the electrical connectivity. Its shown that the lifetime of the polyimide supported interconnects is at least two times better compared to the nonsupported.

Integration of stretchable and washable electronic modules for smart textile applications

Today electronics in “wearable systems” or “smart textiles” are mainly realised on traditional interconnection substrates, like rigid printed circuit boards or mechanically flexible substrates. The electronic modules are detachable to allow cleaning and washing of the textile. In order to achieve a higher degree of integration and user comfort, a technology for flexible and stretchable electronic circuits was developed. The electronic system is completely embedded in an elastomer material like polydimethylsiloxane (PDMS, silicone), resulting in soft and stretchable electronic modules. The technology uses standard packaged components (ICs) and meander-shaped copper tracks, so that stretchable systems with complex functionality can be achieved. This paper describes how these stretchable modules can be integrated in textiles. Also the influence of the PDMS on the stretchability of the textile has been examined. A basic electronic module with blinking light emitting diodes (LEDs) was designed and produced to illustrate the textile integration and to perform initial washing tests.

Wearable, Small, and Robust: The Circular Quarter-Mode Textile Antenna

A miniaturized wearable antenna, entirely implemented in textile materials, is proposed that relies on a quarter-mode substrate integrated waveguide topology. The design combines compact dimensions with high body-antenna isolation, making it excellently suited for off-body communication in wearable electronics/smart textile applications. The fabricated antenna achieves stable on-body performance. A measured on-body impedance matching bandwidth of 5.1% is obtained, versus 4.8% in free space. The antenna gain equals 3.8 dBi in the on-body and 4.2 dBi for the free-space scenario. High radiation efficiency, measured to be 81% in free space, is combined with a low calculated specific absorption rate of 0.45 mW/g, averaged over 1 g of tissue, with 500 mW input power.

Reliability of a stretchable interconnect utilizing terminated, in-plane meandered copper conductor

The pursuit for reliable, deformable electronic systems took two major paths, utilizing either conductive elastomers or metal conductors. In the case of the latter, a mechanical robustness trade-off is made in return for metallic conductor native low resistivity allowing for realization of power demanding and large area applications as well (e.g. conformable lighting and signage). The mechanical trade-off stems from the metal conductor intrinsic inability for significant elongation without failure. One of many present attempts at enabling a metal conductor to perform in an elastomeric medium without failure is the SMI (Stretchable Molded Interconnect), a PCB compatible technology developed at the CMST. Its concept relies on an in-plane, meandered, metal track embedded into a soft, elastomeric material. This work focuses at cyclic, uniaxial elongation endurance and reliability assessment (Weibull analysis) of such interconnect in its most simple form - utilizing unsupported, meandered copper tracks embedded in PDMS (Polydimethylsiloxane). The tracks are evaluated as short interconnects (a few meander wavelengths long) terminating between flexible (non-stretchable) regions to incorporate the effect of flex-stretch transition mechanics on reliability. This is an important assessment for optimizing the interconnect geometry for practical applications where flex-stretch transitions will be inevitable, and reliability under repeated deformation is of interest (e.g. stretchable circuits for integration in textile). An attempt is made to reinforce the meander geometry by tapering the transitions, but a negative impact on reliability is observed. It is clearly demonstrated that the wearout of the interconnect is strongly related to the amount of copper present in the interconnect.

SCB and SMI: two stretchable circuit technologies, based on standard printed circuit board processes

Purpose – In the past 15 years stretchable electronic circuits have emerged as a new technology in the domain of assembly, interconnections and sensor circuits and assembly technologies. In the meantime a wide variety of processes with the use of many different materials have been explored in this new field. The purpose of the current contribution is for the authors to present an approach for stretchable circuits which is inspired by conventional rigid and flexible printed circuit board (PCB) technology. Two variants of this technology are presented: stretchable circuit board (SCB) and stretchable mould interconnect (SMI).Design/methodology/approach – Similarly as in PCB 17 or 35 μm thick sheets of electrodeposited or rolled‐annealed Cu are structured to form the conductive tracks, and off‐the‐shelf, standard packaged, rigid components are assembled on the Cu contact pads using lead‐free solder materials and reflow processes. Stretchability is obtained by shaping the Cu tracks not as straight lines, like ...

Laser based fast prototyping methodology of producing stretchable and conformable electronic systems

For user comfort and reliability reasons, electronic circuits for human body related applications should ideally be soft, elastic and stretchable, for smart textile application, but also for applications which need a high level of biocompatibility. We are developing several roll-to roll technologies using MID (Molded Interconnect Device) and low cost standard PCB technology (Printed Circuit Board) to produce soft, stretchable, human-compatible packaging. All those technologies are based on a sacrificial layer on where meander shaped interconnections are patterned. Those stretchable interconnections are connecting together non-stretchable functional islands on where SMD components are soldered. All the system is then embedded in stretchable polymer matrix, silicone rubber or polyurethane. All these technologies are using standards methodologies for PCB productions (lamination, photolithography, copper etching, reflow oven lead free soldering). A fast prototyping technology has been developed to ease the development of stretchable electronic circuits. Less than 1 day is necessary from CAD design to finalization: Rigid or flexible standard components or electronic sub-systems are interconnected with YAG laser shaped meander interconnections and molded in silicone rubber afterwards. From any kind of flexible circuit, stretchable circuit can be produced using this methodology. The stretchable meander interconnection can be stretched more than 100% and can sustain at least 3000 cycles at 20% of deformation. This paper presents a general overview of stretchable electronic process, a detailed view of the fast prototyping technology using YAG laser cutting, and the demonstrators developed in the frame of the European project STELLA (Stretchable Electronic for large area) [7] and in the Belgian project SWEET (Stretchable and washable electronic in textile) [8] and BIOFLEX (Biocompatible stretchable electronic system) [9].

Stretchable Circuits with Horseshoe Shaped Conductors Embedded in Elastic Polymers

Conformable electronics, i.e., electronics that can be applied on curved surfaces, is demanded nowadays in place of conventional rigid printed circuit board (PCB) based electronics for a number of applications. In the field of stretchable electronics there has been a swift progress in recent years. In this paper we are presenting our contribution to this ever growing topic, including thin-film based polyimide (PI), supported Au stretchable meanders as well as PCB based Cu meanders. These meanders are supported by PI or poly(ethylene naphthalate)/poly(ethylene terephthalate) (PEN/PET) films. Thin-film based stretchable interconnects is targeting mainly the biocompatible environments with demands for strong miniaturization while the PCB based technology is used more for large area applications. Both approaches are reviewed in this paper in terms of fabrication processes, materials and cyclic fatigue reliability. For each technology fabricated demonstrators are presented as well.

Impact of geometry on stretchable meandered interconnect uniaxial tensile extension fatigue reliability

This work investigates the impact of geometry on the reliability of a high conductivity, meandered, stretchable interconnect. Meandered copper conductor interconnects of varying geometries that have been encapsulated into a PDMS matrix, are evaluated for reliability under tensile stretching conditions to 10% elongation. We present results that support our earlier findings by experiment and FEM simulation. Following, we vary interconnect parameters related to the encapsulation geometry, such as encapsulation hardness, thickness and stretchable zone perimeter, to assess impact on fatigue life of the embedded meandered copper lines. Results confirm and refine the prior simulation findings. Combinations of interconnect geometry parameters critical for stretching reliability are identified. Among others, we find that the meander radius (R) and encapsulation thickness are strongly coupled, causing very large meanders with thick encapsulation to fail very early. We show that, depending on the design of the meander transition, the characteristic life of an interconnect can differ 50 times under moderate, 10% cyclic elongation. Finally, we indicate the significance of our findings for the design of reliable, stretchable electronic systems.

Vision: smart home control with head-mounted sensors for vision and brain activity

Today, an increasing number of household appliances is being connected to the Internet to form a smart home. Intelligent control algorithms in the cloud adapt the configuration of this Internet-of-Things to our daily routines and personal preferences. Frequently, there are unforeseen situations where the control algorithms will not capture the actual desired configuration. In these cases, the user must intervene in the control algorithms and manually adjust the connected objects setting. Browsing to the appropriate web service or launching the vendor-specific companion app for even a simple interaction like lowering the temperature setting is a tedious process. In this paper, we report on our early insights in building a mobile system that provides a common, intuitive interface to all actuators in the smart home. Using a head-mounted camera and a commercial Emotiv EEG neuro-headset, we let the user configure the IoT by merely looking at an object and performing a related facial expression. This way, users only need to look at an object and think about the desired action. We leverage on the home cloudlet for the compute-intensive signal processing for object detection.

Improved Stretchable Electronics Technology for Large Area Applications

A novel technology for stretchable electronics is presented which can be used for the realization of wearable textile electronics and biomedical implants. It consists of rigid or flexible component islands interconnected with stretchable meander-shaped copper conductors embedded in a stretchable polymer, e.g. PDMS. The technology uses standard PCB manufacturing steps and liquid injection molding techniques to achieve a robust and reliable product. Due to the stretchable feature of the device, conductors and component islands should be able to withstand a certain degree of stress to guarantee the functionality of the system. Although the copper conductors are meander-shaped in order to minimize the local plastic strain, the lifetime of the system is still limited by the occurrence of crack propagation through the copper, compromising the connectivity between the functional islands. In order to improve the lifetime of the conductors, the most important feature of the presented technology is the use of spin-on polyimide as a mechanical support for the stretchable interconnections and the functional flexible islands. In this way, every stretchable copper connection is supported by a 20μm layer of polyimide being shaped in the same manner as the above laying conductor. The grouped SMD components and straight copper tracks on the functional islands are also supported by a complete 20 μm polyimide layer. By use of the polyimide, the reliability of the stretchable interconnections, the straight interconnections on the flexible islands and the transitions between the stretchable and non-stretchable parts is improved. This approach results in a significant increase of the lifetime of the stretchable interconnections as it is doubled. In this contribution, the different process steps and materials of the technology will be highlighted. Initial reliability results will be discussed and the realization of some functional demonstrators containing a whole range of different components will further illustrate the feasibility of this technology. The advantages and disadvantages in terms of processability, cost and mechanical strength of the photo-definable polyimide will be covered.