This dissertation is focused on design, implementation and characterization of transient soft electronics. Controlling the transiency behavior of the devices as well as investigating mechanics of dissimilar materials in soft transient electronics were two of the main goals. We also conducted application-based studies and demonstrated successful implications of transient materials as constructing elements of a transient Li-ion battery, platforms for controlled release and even a PCB substrate. The concluding remarks of each study is summarized below. And lastly, some ideas and suggestions for future studies are presented.
9.1. Conclusions
In chapter 2 we demonstrate the control of transiency through the dissolution behavior of the substrate, for electronic devices composed of colloidal metal particles on PVA-based substrates. The results suggested that if colloidal metal particles are used for conductive paths, the transiency of polymer-based substrates, which are easier to control, can be used to program transiency of the whole system; and, that unlike conventional transient electronics, transiency rate of substrate can be a controlling and dominating factor. An appealing aspect of this approach is that it allows realization of controlled transiency by eliminating limiting factors posed by dissolution rate of metals. This study can provide a scientific foundation toward fabrication of conductive components of transient electronics, utilizing colloidal metal particles to achieve fast and coordinated deconstruction and
transiency of polymeric substrate and electronic components in transient electronic devices. In chapter 3 we observed correlations between electrical functionality and applied mechanical strain, for the prototype circuits mentioned earlier. It was also demonstrated that
the composition of the polymeric substrate could be used as a means to quantify the loss of functionality of the electronic devices upon exposure to aqueous solvent.
In chapter 4 we presented a materials system that exhibit secondary reactions when undergoing transiency, aiming to expedite redispersion of colloidal metal particles in exposure to solvent. It was observed that the polymer composite containing 50% additives (sodium bicarbonate and citric acid) to polymer matrix (GPVA and PEO) concentration, demonstrated the fastest transiency (100% in 300 s).
In chapter 5, we studied correlations between mechanical strain, electrical properties, and failure of soft electronics in detail when electronic components are fabricated on pre- strained substrates, with attention to mechanical cycling and extent of pre-straining. It was concluded that three-dimensional conductive paths allow typically brittle, colloidal silver paint to be stretched with the substrate it is adhered to, without declining the electrical properties of the path.
In chapter 6 we presented a transient Li-ion battery based on polymeric constituents. The results showed that electrode layer sprayed onto PVA film can disintegrate when the PVA film swelled in water. The transiency property of the battery was similar to that of the single electrode. The disintegration time of the transient Li-ion battery was designed to be within one day, a timescale much smaller than that of the previous designed battery. The output potential of the transient Li-ion battery was designed to be between 1.0 V-2.5 V. Based on the literature previously discussed, single battery output voltage lower than 1.1 V. With a fast disintegration time and high output voltage, the transient Li-ion battery reported in this chapter is a promising candidate for transient energy storage elements that can be applied in the non-biological fields such as environmental sensors.
In chapter 7 we demonstrated GA cross-linked polymer-based interpenetrating network films (IPNFs) with programmable degradation which enable controlled release of therapeutic proteins or vaccines. The long-term, slow release of the protein was achieved by the adjustment to the concentration of the cross-linking agent in its solvent and also the duration of dipping the polymer inside the crosslinking agent. This method was simple enough to be applied to bulk production.
In chapter 8, we presented a proof of concept transient PCB, based on PVA and gelatin with polyurethane coating, and proved that all electronic components (resistors and LED), copper, and solder could be recovered after complete transiency of the substrate in water. This encourages the efforts to produce transient PCBs in large scale manufacturing, impeding the contribution to health problems caused by improper WEEE disposal.
9.2. Future Work
Considering the significant variation in electrical properties as a function of strain caused by the mechanical mismatch between the substrate and the electronic component (presented in chapter 3 and 5), development of conductive material with elastic moduli closer to that of the polymer is necessary. For this purpose, conjugated polymer inks like poly (3,4ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), can be promising
substitutes for metal inks, however deposition of PEDOT:PSS ink on PVA substrate was not compatible with the spray coating method that we used for fabrication of our prototype circuits. Therefore, we would recommend electrohydrodynamic jet printing method which allows high precision and accurate printing of the conjugated polymer ink, to exactly where it is needed on the substrate. An interesting investigation to be considered here, is to compare electrical functionality as a function of strain, between the substrate-metal ink system and the substrate-conjugated polymer ink system; and to correlate the results with interfacial bonding
strength in these two systems. For the next step, we would recommend to use surface modification methods like plasma cleaning and evaluate the resulting interfacial bonding strength in the aforementioned systems.
Another recommendation for future work would be to conduct further systematic study on mechanical properties of transient PCBs and to investigate flame retardant
properties; in an effort to make a large-scale replacement for the current glass epoxy resins used for PCBs. Comparing the recovery efficiency of electronic components from the
transient PCB and the conventional PCB, as well as evaluating the environmental impact and economic impact of switching to transient PCB would be part of the future investigations in this field.