|Abstract:||Biologically-inspired microvascular networks have found many uses in synthetic materials, including self-healing, thermal management, and electromagnetic reconfiguration. In particular, reconfigurable radio-frequency (RF) systems enabled by microvascular networks containing conductive liquids overcome many of the challenges faced by traditional reconfiguration mechanisms such as microelectromechanical systems (MEMS), varactors, and diodes which can interfere with device operation. However, microfluidics and vascular systems in structural and thermally stable materials for RF reconfiguration have not been widely explored. Applications of microfluidics for reconfigurable RF systems are also generally limited to active devices, such as antennas, and tend to experience a low number of repeatable reconfiguration cycles before permanent shorts occur due to residual conductive fluid build-up within channels.
Two examples of polarization-reconfigurable antennas which utilize eutectic gallium indium (EGaIn) displacement in microfluidic networks are first developed and characterized. Microfluidic networks contained in a structural epoxy are fabricated by direct-write assembly of a fugitive ink. One antenna is a cavity-backed substrate integrated waveguide (SIW) slot which is able to switch polarization states. Displacement of EGaIn surrounded by a non-conductive fluid in a serpentine channel results in a portion of the slot being shorted out, creating a signal polarization. Further displacement of the EGaIn shorts a different portion of the slot, resulting in a 90° signal polarization shift. The reconfiguration mechanism is integrated with an electrical automated fluid positioning system which provides positional feedback and commands to a peristaltic pump. The second antenna is a cross-patch microstrip which utilizes two separate fluidic networks to connect discrete copper patches. The long axis of the microstrip, and thus the polarization of the emitted signal, is formed by completely filling one network with EGaIn while leaving the other empty. Polarization reconfiguration is demonstrated by switching which network is filled with EGaIn.
Due to the corrosive nature and rheological properties of EGaIn, its use in many microvascular reconfiguration applications is limited, leading to the development and characterization of other fluids for adaptive RF devices. The dielectric properties of 11 aqueous solutions of conductive polymers and suspensions of conductive particles are characterized at frequencies ranging from 50 MHz to 12 GHz. High dielectric permittivities and loss tangents are observed for most fluids tested, with concentrated suspensions of carbon nanotubes (CNTs) and silver nanoparticles exhibiting exceptionally high loss characteristics. The RF reflectance from 2.6 – 3.95 GHz of of three of these fluids and EGaIn are compared in a 3D printed array of 7 microchannels. Reflectance increase of the array when all 7 channels filled with fluid were found to be 81%, 46%, 33%, and 22% for EGaIn, a 15 wt% CNT suspension, a 40 wt% silver nanoparticle suspension, and a 3.5 wt% poly(3,4-ethylenedioxythiphene): polystyrene sulfonate (PEDOT:PSS) solution, respectively.
More complex microvascular networks containing conductive fluid are used to impart adaptive RF properties to materials for applications such as EM shielding and polarization-selective surfaces. Eight microvascular network designs consisting of two arrays of straight or semi-circular channels with different inter-channel spacing (ICS) are fabricated through a VaSC process. The four fluids characterized in the microchannel array are also used in these vascular networks for adaptive RF properties from 2.6 – 3.95 GHz. In general the reflectance of networks containing fluids increased with decreasing ICS. Networks filled with EGaIn were found to have the highest reflectance, with a straight channel network (ICS = 1.5 mm) containing EGaIn found to be the most reflective with an 88% reflectance increase over the empty network. Semi-circular networks were slightly more reflective than straight channel networks in most cases. Polarization selective reflection was demonstrated by only filling channel arrays either perpendicular or parallel to the direction polarization of radiation in straight channel networks. Fluid contained in channels arrays parallel to the direction of polarization reflect on average 12.7x more radiation than fluid contained in perpendicular channel arrays. This difference in reflectivity between states was dependent on the network ICS and fluid. The parallel channel array containing EGaIn reflected 73 times more radiation than EGaIn in perpendicular channel array for the 1.5 mm ICS network, the largest difference observed in this study. Injecting fluids into either one or both arrays of semi-circular networks revealed adaptive broadband reflectance. A 71% increase in reflectance was seen when both channel arrays are filled with fluid compared to only one array on average. Significant frequency dependent reflectance is observed for both straight and semi-circular networks, especially when filled with EGaIn. Straight networks behaved similar to a band-pass frequency-selective surface (FSS) when both channel arrays were filled, but less frequency dependence was observed when only the parallel array was filled. Semi-circular networks showed high frequency dependence when only one channel array contains EGaIn, which decreased significantly when both channel arrays are filled.
Finally, a non-wetting coating was developed and applied to the interior walls of microchannels to enable repeatable electrical switching using EGaIn. Facile and inexpensive coatings consisting of fumed silica with a polydimethylsiloxane (PDMS) surface treatment in a poly(2-vinylpyridine) (P2VP) binder are developed and characterized. Coated substrates exhibit a high RMS roughness (110 nm), high static contact angle (SCA) (170°), and low sliding angle (SA) (14°) for EGaIn. Coatings are applied to the interior of high aspect ratio (L/D = 66.7) microchannels and electrical switching capabilities evaluated. Channels coated with 12.5:87.5 P2VP:STFS by weight achieve an average of 424 switching cycles, over 120x more than controls. MicroCT scans reveal rapid accumulation of EGaIn residue in controls, which is not present in coated samples. Use of a non-wetting coating prevented accumulation of EGaIn in microchannels, enabling highly repeatable electrical switching using EGaIn displacement for adaptive electronic applications.