Dynamic load

Electronic properties dependence on dynamic load, which is a more realistic comparison to a biological host tissue, was carried at different frequencies and

displacements. Presented in Figure 3.9 are fluctuations in the resistance of a circuit subjected to 180 μm displacement (corresponds to a strain of 1.8%) at 0.02 Hz frequency. The fluctuations are almost fully reversible, although a lag is observed when the stress is removed, which is due to the viscoelastic behavior of the PVA substrate. After removal of the stress at midpoint of each cycle, the PVA substrate recovers from the elongated state back to the neutral state; this recovery, however, is not immediate. As evident from the data, a large portion of recovery (60-70%) occurs instantaneously and the remaining occurs over a period of time.

Figure 3.9 Variations in electric resistance of a silver microparticle conductive path

subjected to 1.8% strain at frequency of 0.02Hz.

Overall behavior at higher frequency (1 Hz) and different (12 μm and 180 μm) displacements (corresponding to strains of 0.12% and 1.8% respectively) was very similar to that at lower frequency. As shown in Figure 3.10, a correlation exists between strain and resistance; a clear correlation between frequency and resistance, however, is not evident from

our experiments. For high frequency testing, the sample was subjected to cyclic low and high strains; a lag was observed in low-strain data at high frequency testing; i.e. the resistance peak in low-strain data occurs slightly after that of the high-strain data. The second peak on each cycle corresponds to the period of time when the stress is evidently removed; however, despite initial drop of resistance reading, it does slightly increase before decreasing again to the neutral (initial value). The nature of the lag and secondary peaks are still unknown to us. They may be due to equipment limitations and/or nonlinear behavior of the substrate or circuit.

Figure 3.10 Electrical response of a straight conductive path as a function of low and high

strain at frequency of 1Hz.

Also, over numerous cycles an overall increase in resistance was observed. Increase in overall resistance, although very small and detectable only after tens of cycles (Figure 3.11), may be an indication of change in electronic properties in long-term; a longer

relaxation period may be needed to fully recover from this state, such condition (tens of consecutive cycles) is rare in biological systems except for few organs like heart.

Figure 3.11 The overall resistance increases after numerous cycles of 1.8% strain.

What makes soft and flexible electronics vulnerable to malfunction is variations in electronic properties when subjected to strain. Elastic modulus of soft electronics is orders of magnitude less than that of ordinary silicon-based electronics; thus, even small forces can cause significant deformation and elongation in soft electronics that will affect their electronic properties. Our studies presented in this paper suggest existence of a reversible correlation between electronic properties and strain; yet, non between electronic properties and frequency of the applied stress. This observation is mainly due to the mismatch of elastic moduli between polymeric substrates and electronic components. Under larger elongation of PVA substrate, silver microparticles are distanced from each other, which causes a drop in

conductivity; upon removal of force, the elasticity of polymeric membrane brings the

electronic components back together and results in the recovery of electrical conductivity. At higher frequency (1 Hz) a similar behavior was observed. Viscoelasticity of polymeric membrane resulted in a lag in recovering from the stress after it was removed. Considering significant variations in electrical resistance as a function of strain, development of

conductive material with elastic moduli closer to that of soft polymers appears to be necessary for fabrication of, especially large area, soft electronics and bioelectronics.

3.4. Conclusion

Circuits consisting of silver microparticle-based electronics were fabricated on PVA- based substrates and subjected to static and dynamic mechanical strain while their electronic properties were monitored and recorded. Conductivity of circuits was monitored and utilized as a means to deduce electronic properties when the circuits were subjected to different extents of mechanical strain at different frequencies. A correlation between electronic properties and strain was observed; yet, no correlation was evident between electronic properties and the frequency of the applied mechanical strain. It was also demonstrated that the composition of the polymeric substrate could be used as a means to control the transiency rate of bioelectronic circuits. This work is expected to contribute to design and fabrication of transient soft bioelectronics with metastable electronic properties and controlled transiency rate.

Acknowledgements

This material is based upon work supported in part by the Department of Mechanical Engineering at Iowa State University; and a funding from Health Research Initiative and Presidential Initiative for Interdisciplinary Research at Iowa State University.

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CHAPTER 4. ACTIVE TRANSIENCY: A NOVEL APPROACH TO EXPEDITE