Soft Modular Electronic Blocks (SMEBs): A Strategy for Tailored Wearable Health‐Monitoring Systems

Abstract Precise monitoring of human body signals can be achieved by soft, conformal contact and precise arrangement of wearable devices to the desired body positions. So far, no design and fabrication methodology in soft wearable devices is able to address the variations in the form factor of the human body such as the various sizes and shapes of individual body parts, which can significantly cause misalignments and the corresponding inaccurate monitoring. Here, a concept of soft modular electronic blocks (SMEBs) enabling the assembly of soft wearable systems onto human skin with functions and layouts tailored to the form factors of individuals' bodies is presented. Three types of SMEBs are developed as fundamental building blocks for functional modularization. The physical design of SMEBs is optimized for a mechanically stable island‐bridge configuration. The prepared SMEBs can be integrated onto a target body part through rapid, room‐temperature (RT) assembly (<5 s) using an oxygen plasma‐induced siloxane bonding method. A soft metacarpophalangeal (MP) joints flexion monitoring system that is tailored to allow for accurate monitoring for multiple individuals with unique joint and hand sizes is demonstrated.


Experimental Section
Measurement of hand dimensions: All subjects-50 adult males in their 20s and 30s-were asked to place their hands on a flat and hard surface with fingers extended, and put together their four fingers except thumb. All measurement was executed by one observer to avoid inter-observer bias. Three parameters of hand dimension were measured using vernier calipers as follows [1,2] : (i) Breadth of hand: the distance between the radial side of second MP joint and the ulnar side of fifth MP joint. The maximum distance was measured through a marked line across the four head of metacarpal. (ii) Interval between MP joints: the distance between second and third MP joints. When the MP joints were bent to the maximum angles, the middle points of the MP joints were marked by an observer that are on the front view of dorsum of the hands. Then, the distance was measured with their MP joints extended. (iii) Length of middle finger: the distance between the center of third MP joint and the fingertip of middle finger.
Fabrication of circuit blocks: For preparation of the circuit blocks, PEN flim (thickness =50 m, Q65H, Teijin DuPont Films) was cut into the proper size and UV/O 3 -treated (power = 28 mW cm -2 ) for 10 minutes for the (3-Aminopropyl)triethoxysilane treatment (ATPES, Sigma-Aldrich). After UV/O 3 -treatment, APTES solution was drop casted on the PEN film. After 5 mins in ambient air, the PEN film was rinsed with deionized (DI) water. To remove the remaining APTES solution or DI water, we blew the PEN with N 2 spray gun. After the same process was conducted on the opposite side of the PEN, we obtained APTES-functionalized PEN.
After UV/O 3 treatment for 5 mins on the top side of PEN film, silver electrodes and pads were formed on it using a piezoelectric inkjet printer (DMP-2831, Dimatix Corp.) and annealed at 125 for 30 mins. Then, silver epoxy is printed at exact positions on pads via pneumatic dispenser (SHOTmini 200Sx, Musashi Engineering, Inc.) or manually and pure epoxy was also printed between the pads not only to prevent electrical short between silver epoxy but also to bond IC chips to the PEN film robustly. [3][4][5] After annealing at 185 for 30 mins, the bottom side of PEN film and strain-relief PDMS mixed at 10:1 ratio (Sylgard 184, Dow Corning)-All PDMS utilized in this work were annealed at 100 for 2 hours-was treated by oxygen plasma (CUTE-1MP, Femto Science) for 60 seconds at a 60W and was bonded to strain relief PDMS to obtain the circuit blocks successfully.
Fabrication of bending sensor blocks: The capacitive-type bending sensor has a similar structure with our previous bending sensor. [6] Bottom plane was fabricated using inkjet printing on the APTES-functionalized PEN film. The ANP ink was printed with dimension of 2×16mm. Dielectric PDMS layer (10:1) was spin-coated on glass at 1000rpm for 60sec, followed by curing on a hotplate. Top plane was flat-structured AgNWs-embedded PDMS, and the fabrication process was conducted with almost the same process of fabricating interconnects blocks. Instead of pre-stretched mold PDMS, a flat glass was used as a mold.
The AgNWs of top electrodes were spray-coated with width of 2mm. After preparation of three layers, bottom plane and dielectric PDMS layer were activated through O 2 plasma treatment and put together to form siloxane bond; surrounding region of contact pad of the bottom electrode was not activated due to PDMS masking during O 2 plasma treatment. After exposed to O 2 plasma, top plane was bonded to dielectric PDMS on bottom plane.
Fabrication of interconnect blocks: For preparation of interconnect blocks, mold PDMS (at a ratio of 10:1) was stretched up to 50% uniaxially and UV/O 3 treatment was conducted 45mins.
We spray-coated AgNWs solution (YURUI chemical, AgNW length 5-10 m) on the mold dried the AgNWs film at 50 for 30 minutes. After releasing the mold PDMS to its initial state (zero strain), the liquid PDMS mixture (10:1 ratio of weight) was casted onto the AgNWs film, followed by curing at 100 for 2 hours. Because the liquid PDMS can penetrate into the void of AgNWs film, the AgNWs film can be embedded into the PDMS after curing process. After peeled off and cut into moderate size, the AgNWs-embedded PDMS with corrugated microstructure-interconnect blocks-were obtained.  Table S1 in Supporting Information for detailed chip information) were bonded onto inkjet-printed silver pads via silver epoxy on the APTES-functionalized PEN (25×25mm) of a circuit block. The circuit block was bonded to a 300-m-thick substrate PDMS and the substrate was slowly elongated up to 30% on the automatic stretching equipment. The voltage at SDA and SCL node were measured by using an oscilloscope (TEKTRONIX DPO4104). For cyclic measurement, the repetitive stretching and releasing test (up to strain of 30%) was conducted at a stretching speed of 300mm min -1 .
For measurement of characteristics of bending sensor block, the bending sensor block was attached onto automatic bending equipment. The capacitance of the sensor was measured at 1MHz with a 1V a.c. signal by using Agilent 4980a LCR meter during bending deformations.
For cyclic measurement, the repetitive bending test (up to bending radius of 6.5mm) was conducted at a bending speed of 10mm min -1 . For measurement of electrical properties of interconnect blocks, the substrate PDMS, which had two circuit blocks connected by an interconnect block (2×20mm), was elongated up to 50% strain. While the substrate was elongated, electrical resistance of the interconnect block was continuously measure by Keithley 2420 and Labview program.
MP joint flexion monitoring system: A signal-process circuit block, two LED gauge blocks, and two bending sensor blocks, and interconnect blocks were prepared, and their circuit schematic is shown in Figure S6 Figure S10. Circuit diagram of a MP joints flexion monitoring system. A micro-controller unit (MCU) was programmed to communicate with a capacitance-to-digital converter (CDC) through I 2 C. The bending sensor data are updated every 20 ms and comprared with the calibrated data. After comparison, the MCU turns LED chips on and off according to the results. Figure S11. Calibration process of a MP joints flexions monitoring device. Calibration begins when the external power is applied. Right after that, the LED display block blinks for (a) one, (b) three, and (c) two LEDs for 6 seconds each, indicating that calibration is in progress. We asked the subject to bend the MP joints at 0, 90, and 45° at each step (a-c). Figure S12. The sensor signals that monitor the hand flexion to 30, 60, 90° of the wearable device with (a,c,e,g, and i) a fixed and (b,d,f,h, and j) a tailored design for five different subjects. The personal data and form factors of the subjects are shown in the following formats: (Sex, age, distance between 2nd and 3rd MP joints) Table S1. Chip information used in this work