Brian Elolampi

Novel Strain Relief Design for Multilayer Thin Film Stretchable Interconnects

Most electronic systems are rigid and inflexible. Many applications, however, require or benefit from conformable designs. To create efficient conformable systems, multilayer stretchable interconnects are necessary. A novel strain relief structure for multilayer stretchable interconnects is proposed. The numerical analysis shows that the proposed structure will function indefinitely when stretched as much as 20% of its initial length. Electromechanical measurements demonstrate that the onset of microcrack formation in the interconnects occurs, on average, after 89% elongation. These measurements also show that the structures are able to withstand elongations of up to 285%. Additionally, precise failure mechanisms, including interconnect straightening and microcrack formation are documented.

Epidermal electronics: Skin sweat patch

An ultrathin, stretchable, and conformal sensor system for skin-mounted sweat measurement is characterized and demonstrated in this paper. As an epidermal device, the sweat sensor is mechanically designed for comfortable wear on the skin by employing interdigitated electrodes connected via stretchable serpentine-shaped conductors. Experimental results show that the sensor is sensitive to measuring frequency, sweat level and stretching deformation. It was found that 20kHz signals provide the most sensitive performance: electrical impedance changes 50% while sweat level increases from 20 to 80. In addition, sensor elongation from 15 up to 50% affected the measurement sensitivity of both electrical impedance and capacitance.

Design for reliability of multi-layer thin film stretchable interconnects

To date, nearly all electronic systems have been rigid and inflexible. However, there are many areas such as in biomedical devices in which these rigid electronics are less than ideal and which require new conformable electronic systems. In order to create effective, compact, and complex systems, stretchable interconnects must be designed to overlap one another in multiple layers. The circular strain relief structure described in this paper effectively redistributes the strain to the crest of the horseshoes of the interconnects themselves. Numerical analysis and simulations of the strain relief structures described in this paper indicate that the structures will function indefinitely when stretched up to a 20% elongation. In-situ electromechanical measurements show that the structures are able to withstand elongations of 285% or more before failing. Precise failure mechanisms including straightening of the interconnects and micro-crack formation are documented with images taken during the electromechanical tests.

Epidermal electronics for seamless monitoring of biopotential signals

Medical deployment of electronics is often hampered by boxy and rigid packaging. Biological tissues are soft and curved, while electronic components are hard and angular. The mechanical mismatch can be improved by re-packaging electronics in radical new form factors. We present a technology platform using ultra-thin components linked with conformal interconnects and embedded in low modulus polymers to provide an excellent match to biological tissues. This technology platform builds on the pioneering work by Prof. John Rogers @ UIUC. [1, 2] Rather than developing novel semiconducting, conducting and insulating materials, the platform exploits the concept that only the top 5-15 μm of a silicon IC contributes to functional behavior. Similar considerations apply to other high performance components such as LEDs and photodiodes. The thin active layer can be removed and transferred to polymer by various processes, which we discuss below. The resulting thin and flexible silicon islands can be interconnected using metallization patterned to permit substantial macro-scale deformation while experiencing minimal micro-scale deformation, just as a coiled spring can stretch several times its own length while keeping the local metal strain within the elastic limit. On-body and in-body applications are both well suited to the technology platform. Epidermal electronics are skin-mounted systems that resemble electronic tattoos, and can be worn for extended periods without discomfort while providing continuous monitoring. In this paper, we discuss in detail, the following technologies and concepts that enable epidermal electronics: (1) Advanced die preparation methodologies that allow for thinning, placement, and attachment of sub-50μm commercial IC devices (COTS); (2) die embedding methods in flexible polymer substrates; (3) use of conformal metal interconnects to connect components; and (4) elastomer stacking optimized for application strain and compatibility with biological tissue. We present data of thinned COTS ICs embedded in flex test circuits to demonstrate the technology.