Hello! My name is Kunpeng and I'm from the Hybrid Body Lab at Cornell University. Today,
I am delighted to introduce our project: Delocalizing Strain in Interconnected Joints of
On-Skin Interfaces. On-skin interfaces, often in the form of smart tattoos or bandages, are a form
of wearable computing that directly contacts the skin to detect physiological signals and assess
a person's activities. However, one of the key obstacles to the scalable and robust fabrication
of on-skin interfaces is the durability of the transition area, or the interconnection joints,
between the rigid electronic components and the soft on-skin substrate.
Our research builds upon related works in three areas. Several attempts have developed on-skin
interfaces in soft, stretchable forms that can adhere seamlessly to the skin. Previous
research has explored film-based approaches with thin silicone or conductive films layered on top
of each other. An emerging alternative approach that takes advantage of yarn-level structures,
more often produced in constructed textiles, can easily conform to body contours. In particular,
weaving, which we adopt in this work, presents a rich platform for versatile circuit topographies,
and affords scalable manufacturing, providing unique benefits over film-based approaches.
Weaving's unique structure of intersecting yarns offers easy incorporation of delicate electronic
components and thin materials as well as diverse trace routing. In our project, interconnect
refers to the actual joint making a mechanical and electrical bond to interlock components.
Integration of rigid electronics into soft interfaces has been examined in the form of
garment-integrated and on-skin interface systems. Garment integration incorporates PCBs or discrete
components into fabrics via stitching, sewing, soldering, and non-conductive adhesive under
pressure. Studies have identified that the transition areas between rigid electronics
and textiles are typically subjected to higher stress. Efforts of accommodating high mechanical
stress to improve the stability of interconnects include the use of a stabilizer or interposer,
as well as manipulating the geometry and material of the traces. Interconnections
for on-skin interfaces are typically implemented with makeshift temporary wires or attached via
Z-tape or conductive fabric tape. While previous works report that areas most prone to breakage
are the interconnect joints between the rigid PCBs and soft on-skin traces, to date, no research
has centered on characterizing the design of interconnects for improved durability, which this
work aims to contribute. In this paper, we present a systematic study evaluating the durability and
reliability of 5 interconnection designs for on-skin systems under tensile and bending tests
as well as design guidelines on PCB interconnects. We explored different weaving techniques,
pad shapes, sizes of PCBs, techniques for handling and mounting the PCB components onto the
on-skin substrate, and circuit design features to maximize the durability of the interconnections.
The design goals of our implementation include mass-producibility,
seamless integration of technology, and durability and long-term usability.
To fabricate the on-skin interface, we extent the weaving techniques explored in the related works
and developed a woven on-skin substrate with embedded conductive materials. The substrate
is made of silk yarns in plain weave. We used copper wires as the conductive material and
adopted a serpentine pattern to allow more flexibility and stretchability.
The main goal of designing the circuit layouts for PCBs was to distribute the interconnection
joint stress in the surrounding areas within the on-skin system. By adjusting the shapes and sizes
of the PCBs, we designed and fabricated flexible circuit boards with three different layouts,
Minimal, Elongated, and Extended, with pads on the top surface. For the Minimal design, we also built
and tested flex PCBs with pads on the bottom surface as well as a rigid PCB. The groups of
4 pads on each end were connected to the I2C bus of an IMU as well as an LED power indicator. Then,
the PCB was integrated onto the woven on-skin substrate. For boards with pads on top, the copper
wires go through the non-conductive holes from the bottom and were then soldered onto the pads using
a soldering iron. For boards with pads on the bottom, we used an additional layer of flex PCB
with only conductive pads on top as an interposer. To test the interconnects under stretching, the
Minimal designs for the rigid and flexible PCBs were tested using an Instron machine. In addition,
to simulate the repeated bending occurring at high-flexion regions, such as the wrist,
we built a bending machine that bends the sample from opposite sides. For both tests, the IMU
readings were constantly monitored, and the eight interconnects were checked with a multimeter.
As the serpentine pattern of copper wires distributed the strain,
both the rigid and flexible PCBs remained functional after the tensile test with a maximum
24% stretching. The interconnects on the flex PCBs were more durable to bending than rigid boards,
but internal trace breakages were observed. Both the Elongated and Extended flex PCBs performed
slightly better than the Minimal design in the bending test. The interposing 2-layer flex PCB
was more bend resistant than the rigid board, and had fewer internal breakages
than other flex PCB designs due to the increased thickness. For both the rigid and flexible PCBs,
wires broke at the surface of the solder joints or adjacent to the interconnects, which is consistent
with previous studies. The manual fabrication of weaving, soldering, and the reflow process
might introduce minor variables that can affect the durability performance. Thorough exploration
can be achieved with the mass production of more configurations of interconnect design,
along with repeated, fully automatic testing procedures to allow more design parameters to be
examined. Lastly, based on our experiment results, we provide a set of recommendations for PCB design
for on-skin interfaces, including incorporating serpentine pattern for circuit wires weaved or
knitted in fabric, and the choice of PCB structure and material determined by circuit complexity and
the frequency of bending. Thank you for listening! Please check out our paper for more details.