Flexible and tunable Split Ring Resonators

The recent development of flexible electronic technologies has created a strong need for flexible/stretchable electromagnetic (EM) components such as resonators, antennas, filters and oscillators. Flexible electronic technologies require devices and structures to reside on non-planar, conformal and elastic surfaces platform. Thus, an ideal solution is to realise the EM components on flexible/stretchable materials or substrates. The ability to bend and stretch EM components enables a new dimension of manipulating electromagnetic waves and promises new potential functionalities of the devices.

The scope of this PhD research is to investigate microfluidic flexible/tunable liquid metal Split Ring Resonators (SRRs) in elastomeric polymer. A rigid substrate is replaced with an elastomeric substrate Polydimethylsiloxane (PDMS), and the conventional metal conductor is substituted with the liquid metal alloy Galinstan. By employing PDMS and liquid metal, it is expected to obtain tunable/stretchable and reversely deformable characteristics in the designed resonators without electrode cracking.

First, a mechanically tolerant fluidic SRR meta-atom that can be stretched, mechanically strained, and reversibly deformed (bent, stretched and twisted) is presented. Applying mechanical deformation to the SRR results in minimal deviation of the transmission response. The frequency response remained within 3% of its original value under strains of up to 18%, 1 cm radius curvatures and 45° degree twisting. This offers a stable and predictable response for flexible electronic applications where mechanical deformation or conformity is inherent.

Second, a fluidic SRR is developed to achieve frequency tunability. A built-in microfluidic channel in between the gaps of the fluidic SRR is employed to harness pneumatic tuning by air injection/suction. A frequency tuning of 3% have been achieved. Further, a contactless and fully integrated PDMS microfluidic sensor based on a meta-atom SRR is proposed for dielectric characterization applications. The integrated microfluidic sensing channel is internally designed through a dual gap SRR as this area provides significantly sensitive to dielectric changes. Thus, exploiting the electric field sensitivity of the devices can be used to predict the dielectric properties of liquid samples. This microfluidic sensor offers a potential lab-on-chip solution for liquid dielectric characterization without external electrical connections.

As PDMS may be limited by its significantly high dielectric loss at microwave frequencies, application flexible composite solution is explored. The challenges associated with creating PDMS-Alumina and PDMS-PTFE composites to form flexible-low loss materials are studied in terms of their structural, mechanical and electrical properties. The incorporation of either low dielectric loss filler in the PDMS matrix (up to 50 wt% filler loading) is shown to reduce the dielectric loss by 25% while maintaining the flexibility of the host matrix. The fillers can also control the permittivity of the composite, either increasing or decreasing the relative permittivity from that of PDMS (~2.7) in the range of 2.4 to 3.7. Such low-loss composites offer a promising solution for flexible microwave substrate applications.