Improvement and Verification of Reliability in Conductive Liquid Metal based Soft Sensors

This thesis presents reliability improvement in a conductive liquid metal, Eutectic gallium-indium (eGaIn), based soft sensors and verification of sensor lifetime. To predict the service life of the soft sensor, we performed an accelerated life test, a cyclic loading test until the failure of the sensor. Then, the relationship between the number of cycles and the applied voltage was represented by the inverse power mode.

Consequently, the lifetime of the soft strain sensor was divided into 3 phases related failure modes.

Introduction

As described, the performance of the DIW-based eGaIn soft sensor has been actively investigated. Also, the soft sensor that has a predictable lifespan is a foundation of the durable wear system. Even without such an initial failure, repeated movements can cause the soft sensor to suddenly fail.

Then we performed an accelerated life test with respect to the voltage range to propose a life model of the soft sensor.

Figure 1.1: Wearable application of soft sensor: (a) body suit [21], (b) finger motion [40], (c) swallowing motion [40], (d) wrist motion [45], (e) elbow joint bending motion [5], (f) finger motion [41].

Manufacturing Technologies for

Improved Reliability of eGain Soft Sensors ∗

Introduction of Direct Ink Writing (DIW) Fabrication Process

Manufacturing Technologies for Reinforcement of Connecting Parts

Even the direct wiring of liquid metal, eGaIn leakage problems in connecting parts have occurred frequently due to non-stick elastomers. Therefore, additional reinforcement was essential to prevent the leakage of eGaIn in the direct wiring connection. In order to find appropriate adhesions and corresponding procedures for strengthening connecting parts, adhesion test was performed to evaluate the adhesion for 4 types commercially available; Epoxy (VT-146; Vital Technical [43]), silicone bond (Silpox; Smooth op [38]), instant glue (Super Glue Gel; LOCTITE [19]), and silicone sealant (VT-132; Vital Technical [33]).

For the adhesive test, the custom plates which had 10 gold coated electrodes were attached to the coated Dragon Skin 30 using any adhesive. After heat curing the coated elastomer, the adhesion between the elastomer and the adhesive was confirmed by pulling the end of the elastomer.

Figure 2.2: Breakage in cyclic loading: 80 % and 110 % strain at the speed of 25 mm/s

Short

Performance Verification with Different Elastomers

We experienced the fracture in the glove type soft sensor assembled Dragon Skin 30 as shown in Figure 2.7 (a). However, the sensor is easily ruptured in repeated adduction and abduction motion, because the required stress is greater than elastomer elasticity. In addition, to measure human movement of walking and running, the soft strain sensor needs a large strain for robust application of more than 300 % as shown in Table 2.2 (Figure 2.7 (b) [21]).

Therefore, the load sensor must achieve a wide load range for wearable systems without breakage. The stretchability of the soft sensor can be determined as a deformable conductive material and elastomer. Here, eGaIn is limited as the conductive material for the soft sensor due to irreplaceable excellent electrical rheological properties.

Whereas, in the case of elastomer, different candidates can be applied to have a wide range of deformation. Therefore, we expect that the eGaIn soft sensor made of highly stretchable elastomer can provide reliable lifetime to measure human motion. Because different elastomers have different mechanical properties such as stretch, hardness, viscosity, tensile strength and Young's modulus.

Elastomers have different mechanical properties such as hardness, 115% modulus, tensile strength, elongation at break and viscosity as shown in Table 2.3. Acrylic molds attached to both sides of the sensor were designed to compress the electrode when the sensor was gripped by experimental equipment (Figure 2.8 (c)).

Figure 2.7: Fracture of the soft sensor during measuring human motion: (a) fracture of the soft sensor to measure finger motion (b) fracture of the soft sensor to measure walking and running

Amplifier board

Strain jig

Motor

Sensor

This may be caused by differences in the shape of the sensor and the use of the acrylic mold. All samples showed a similar trend of the curve regardless of the elastomers, the elongation range only differs (Figure 2.11 (a)). We assume that the sum of the resistance of eGain in the microchannel was lower than the value of the model due to the pattern design.

The sensor pattern which is in the red circle in Figure 2.11(c) is vertical against the pull direction of the tensile test at failure. When the sensor was stretched, the resistance of the pattern in the red circle decreased due to the increased cross-sectional area and decreased length of the pattern. For this reason, the part of the vertical pattern is made below the value of the sensor signal.

The second test was a cyclic load test to compare signal variation at different speeds. The sensor signal was obtained by the same procedure with a pull-to-failure test, and the elongation was measured with a linear potentiometer (KTC-300; Shenzhen Minuo Electronics [7]) attached to the movable stage of the machine. In contrast, the degree of hysteresis was similar at the same speed regardless of the samples.

Because the conductive material, eGaIn, was highly conductive and micro-channel reconfigurable, the only difference was the alignment of the sensor. Therefore, we can select a suitable sensing elastomer to apply the target coating systems considering the required strain range.

Figure 2.10: Maximum strain range and elongation at break of 4 samples

Verification of Soft Sensor Lifetime

Accelerated Tests of Soft Strain Sensors

Strain (S)

Accelerated condition

Actual condition

StrainStress

Maximum usable strain

Level 2 Level 3

• Accelerated Life Models

Therefore, we have to establish our own criteria for the load range for accelerated lifetime testing. Because the substrate of the soft sensor consisted of an elastomer, we assume that the stress-strain curve of the elastomer is similar to that of the soft sensor (Figure 3.1 (b)). While the tension after the interval increases drastically due to cross-linking of the polymer chain.

All samples have a dogbone shape with the same size of the modified sensor in the tensile failure test of Chapter 2 (Figure 2.11 (d)). We used the tensile tester (AGX-100NX; SHIMADZU [30]) (Figure 3.2), which is suitable for lamination, rubber and graphene. In Figure 3.3, the stress-strain curves of the sensor and the elastomer sheet are plotted on the same graph.

Therefore, the effect of the small size microchannel may not be significant on the sensor voltage distribution. The accelerated life test, which is a cyclic load test to sensor failure, was performed 9 times for every 3 load levels. Data acquisition for sensor signal and sensor elongation was the same as for the cyclic loading test in Chapter 2.

The similarity of the shape factor can be verified by a visual method using the Weibull probability plot (Figure 3.5 (b)). As a result, the plot shows the corresponding slope in 3 strain level with each median, which is the representative value of the lifetime data with 50% error rate until the cycle.

Figure 3.2: Tensile test setup

200 ห

200 ห(a)

Analysis of Failure Modes

When the knife moved in the elastomer, the elastomer was easily pressed due to low hardness like 10A (Figure 3.5 (a), (b)). However, Dragon Skin 30 did not have any boron on the edge of the sensor (Figure 3.7 (b)), even the same manufacturing process. If the knife cutting is changed to laser cutting, the drill on the elastomer can be avoided to extend the life of the sensor.

To verify the proposed lifetime model of the KE-1190 sensor, we performed another accelerated test at 150% strain, which is a lower strain range than the previous 3 strain levels. Although the scale factor was slightly reduced due to the gap between the change of low peak and high peak, the linearity of the sensor signal did not change. We hypothesize that the reason for the increase in resistance is the oxidation of eGaIn as repeated large deformations.

Since the overall length of the sensor had not changed after the test, we believed that the microchannel was also not deformed. Since the color of EGaIn changes to dark gray in the ambient air due to oxidation [34], the color change in the microchannel of eGaIn can be regarded as the oxidation of eGaIn. As a result, the failure mode of the sensor was changed 2 from crack propagation caused by boron at large stress to the increase in resistance due to eGaIn oxidation in a small stress range.

Since the elastomers had pores between the polymer chains, the pore surface area increases over a large strain range. In order to verify the effect of deformation on resistance increase, the rate of change of resistance was recorded every 10 times the increase cycles for different strain range and 200.

50หKnife

50 ห

Comparing the resistance value of the change ratio in 105 cycles, it was higher as the voltage increased. The failure mode can be avoided by using an elastomer that has a relatively high hardness or replacing the knife cutting process with a laser cutting method. When a small strain was applied, the increase in resistance occurred before crack propagation.

Finally, no change in resistance was detected even in 106 cycles, which can be called an infinite lifetime.

Figure 3.8: Microscope images of knife cutting: (a) Knife cutting of KE-1190 (b) cutting surface of KE-1190 (c) Knife cutting of Dragon Skin 30 (d) cutting surface of Dragon Skin 30

Conclusion and Open Issues

KE-1190 had edge scratches that occurred during the knife cutting process due to low hardness. However, the Dragon Skin 30 had a smooth edge due to its relatively high stiffness which was sufficient not to deform under the movement of the knife. Thus, the life data that failed from the crack propagation from the nozzle to the rim can be expressed in the inverse power model.

In the region of smaller deformation than the tested level, we found another mode of failure, in which the value of the resistance in the large cycle increased. We assume that the increase in the resistance value was caused by the oxidation of eGaIn, which changed the color to dark. As a result, we divided the lifetime of the soft sensor into 3 stages depending on the failure modes: (1) crack propagation, (2) increase in resistance value, (3) infinite lifetime.

If the cutting process is replaced by other methods such as laser cutting or compression cutting to avoid drilling, the life of the soft sensor would be extended without crack propagation. However, a soft sensor may have another stress factor expected from the load; temperature, moisture, shear strength, pressure, etc. Depending on the application in external conditions, other stressors must be used to express the life model.

Therefore, life model expressed by multi-variable should be proposed to spend diverse conditions for wearable systems.

Bibliography

Consistent and Reproducible Direct Ink Writing of Eutectic Gallium-Indium for High-Quality Soft Sensors”. Suzuki et al. “A fast-responsive, wide-stretchable sensor of aligned MWCNTs/elastomer composites for human motion detection”. Tanget al. "Coaxial Printing of Silicone Elastomer Composite Fibers for Stretchable and Wearable Piezoelectric Sensors".

Acknowledgment