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Flexible electronics has attracted tremendous attention due to the fast-growing market for portable devices and sensors. However, these devices usually suffer from internal and external forces, which can lead to the failure of interconnects in the circuit. To address this issue, some passive and active strategies have been developed for improving the reliability of interconnects in circuits. Although all these strategies are effective in tackling problems related to the mechanical stress, they have inherent limit in dealing with other failure causes or repairing the open fault. Therefore, in this dissertation, a particle-based self-healing (PBSH) technique has been developed. The detailed study starts with an overall review of the state of the art in techniques developed for improving the reliability of interconnects in flexible circuits. Then the mechanisms of the PBSH technique are analysed and the chemical treatment of metal particles are explored. The physical modelling of the healing is established, and relations between the healing time with the suspension concentration, electric field, length of open gap, and external resistance are verified by experimental results. In addition, to avoid the inherent conductivity of the suspension and the aggregation of micro-particles, they are modified by the oleic acid. This chemical treatment of metal particles improves the uniformity of the suspension and sets a threshold electric field for the occurrence of self-healing. As a key component in flexible circuits, an organic thin-film transistor (OTFT) fabricated by all inkjet-printing process has been developed and characterised in this study. This transistor uses 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPSPentacene) as the semiconductor, poly (vinyl cinnamate) (PVC) as the dielectric, silver as the electrodes, and CYTOP as the encapsulation. Based on this device, two basic circuits: the current mirror and the common-source (CS) amplifier have been successfully built on flexible substrates. The proper functioning of the printed devices and circuits becomes the base for the study of the self-healing in flexible circuits. Subsequently, the PBSH in OTFT device and circuit interconnects is verified. This technique can successfully repair open faults at both source and drain sides in OTFT device interconnects. Simultaneously, it can heal open faults occurring at different locations in flexible current mirror and common-source amplifier circuits biased at the static state. Moreover, the self-healing is also proved to be effective and stable in common-source amplifier when processing alternating current (AC) signals. In order to demonstrate the feasibility of the PBSH technique in real-world applications, more performance characterisations have been conducted. The bending test shows the reliability and stability of the healing in the bent condition. And the dualfaults test verifies the ability of the PBSH technique for healing more than one open fault in interconnects. In addition, methods of packaging the circuit and confining the healing material in the expected region are also developed, which prove the feasibility and manufacturability of the PBSH technique in integrated circuits. This study sets a new benchmark for improving the reliability of inkjet-printed flexible circuits.

We consider a scenario in which several external service vendors are contracted to repair purchased items which fail under warranty. We develop and analyze various allocation models concerning how the repair work should be distributed among the vendors in a cost-effective manner. Furthermore, we depart from previous work by arguing the importance of approaches to the modelling of goodwill costs which penalize long waits experienced by individual customers. We firstly study a simple static allocation model with a fixed warranty population. Both theoretical considerations and numerical results show that a simple greedy approach to the distribution of items under static models works outstandingly well. However, such a static formulation ignores the stochastic nature of the warranty population. Hence, we develop a second allocation model in which new equipment purchases are made according to a compound Poisson process. As in the static allocation model, the current information regarding the repair queue at each vendor is not available to inform decisions. The resulting dynamic optimization problem is non-standard. We develop an effective allocation procedure to this non-standard problem using a dynamic programming policy improvement approach. We report representative results from a simulation investigation to evaluate the status of the allocation heuristic developed in comparison to two simpler heuristics suggested by static models. Thirdly, we propose a dynamic allocation model which utilizes data on the queue length at each vendor for decisions on the routing of real-time failures to the vendors. Due to the problem size and state space in practice, traditional stochastic dynamic programming is not a realistic and computationally viable option. Hence, Whittle’s restless bandits approach is deployed to develop the index-based heuristic for this dynamic allocation problem. A crucial theoretical result in this part of the study is that the system considered is indeed indexable.