**4. Reconfigurability with liquid metals**

There are two ways liquid metals can be used for microwave circuit reconfigurability: contacting and contactless [8]. These refer to whether or not the liquid metal enters in contact with conductors in the microwave circuit, the contacting approach, or is brought in close proximity to these conductors without touching them, the contactless method. This latter method is similar in principle to using dielectric fluids, whereby the liquid metal can behave as a capacitive load on top of or below the transmission line's

conductors, thereby altering the propagation characteristics. This requires thin dielectric layers between the liquid metal's hosting cavities and the rest of the circuit's conductors to avoid contact resulting in undesirable reactions, e.g., corrosion. This approach can be highly effective, particularly when the liquid metal is placed near the maximum electrical field value resulting in a good tuning range [9, 10]. In the contacting approach, liquid metals are used to short connections between conductors, i.e., like contact switches [11–13] in a microwave circuit or extend a single conductor with a custom shape determined by the encapsulating cavity [14, 15]. This allows altering signal paths in a circuit, thereby enabling the reconfiguration of their electrical length and propagation delays, which reconfigures the overall component.

While liquid metals can offer significant advantages for reconfigurable microwave components and deformable printed electronics, they are relatively complex to handle and suffer from fast-oxidation when exposed to oxygen. Therefore, they require specific packaging and manipulation guidelines to overcome these limitations [16, 17].
