Review of Recent Metamaterial Microfluidic Sensors

Abstract

Metamaterial elements/arrays exhibit a sensitive response to fluids yet with a small footprint, therefore, they have been an attractive choice to realize various sensing devices when integrated with microfluidic technology. Micro-channels made from inexpensive biocompatible materials avoid any contamination from environment and require only microliter-nanoliter sample for sensing. Simple design, easy fabrication process, light weight prototype, and instant measurements are advantages as compared to conventional (optical, electrochemical and biological) sensing systems. Inkjet-printed flexible sensors find their utilization in rapidly growing wearable electronics and health-monitoring flexible devices. Adequate sensitivity and repeatability of these low profile microfluidic sensors make them a potential candidate for point-of-care testing which novice patients can use reliably. Aside from degraded sensitivity and lack of selectivity in all practical microwave chemical sensors, they require an instrument, such as vector network analyzer for measurements and not readily available as a self-sustained portable sensor. This review article presents state-of-the-art metamaterial inspired microfluidic bio/chemical sensors (passive devices ranging from gigahertz to terahertz range) with an emphasis on metamaterial sensing circuit and microfluidic detection. We also highlight challenges and strategies to cope these issues which set future directions.

Keywords: biosensor; chemical sensor; dielectric perturbation; metamaterial; microfluidic.

Figure 1

Metamaterial (unit cell) Topologies: (a) split ring resonator (SRR); and (b) complementary split ring resonator (CSRR).

Figure 2

(a) Microstrip-coupled SRR etched on the top of a substrate; (b) Equivalent circuit of the proposed resonator with Lm represents the inductance of microstrip line, {RLC}s for the parasitic elements of the ring resonator, and M for the mutual inductance between SRR and microstrip line; (c) Simulated transmission magnitude of the proposed resonator, and in inset electric field magnitude at 2.1 GHz (resonant frequency) with the highest electric field at the split-gap of ring resonator; (d) Measured transmission magnitude showing various profiles of the proposed chemical sensor, 100% ethanol being the lossy chemical showing the lowest Q factor as compared to air and water; (e) Fabricated prototype assembled with two SMA (SubMiniature version A) connectors; (f) A zoom-in view showing the microfluidic channel accurately positioned on the SRR gap. The red arrow indicates the direction of fluidic flow; (g) Fully assembled sensor with polymer tubes delivering liquid into the chamber and coaxial cables connected to the vector network analyzer (Redrawn from [33]).

Figure 2

(a) Microstrip-coupled SRR etched on the top of a substrate; (b) Equivalent circuit of the proposed resonator with Lm represents the inductance of microstrip line, {RLC}s for the parasitic elements of the ring resonator, and M for the mutual inductance between SRR and microstrip line; (c) Simulated transmission magnitude of the proposed resonator, and in inset electric field magnitude at 2.1 GHz (resonant frequency) with the highest electric field at the split-gap of ring resonator; (d) Measured transmission magnitude showing various profiles of the proposed chemical sensor, 100% ethanol being the lossy chemical showing the lowest Q factor as compared to air and water; (e) Fabricated prototype assembled with two SMA (SubMiniature version A) connectors; (f) A zoom-in view showing the microfluidic channel accurately positioned on the SRR gap. The red arrow indicates the direction of fluidic flow; (g) Fully assembled sensor with polymer tubes delivering liquid into the chamber and coaxial cables connected to the vector network analyzer (Redrawn from [33]).

Figure 3

(a) Bird’s-eye view of the CSRR-loaded patch with an adhesive film and a microfluidic channel (top layer); (b) zoom-in view of channel alignment with CSRRs; (c) fabricated prototype of CSRR-loaded microfluidic patch as ethanol chemical sensor; and (d) side view with nanoport assembly (Redrawn from [4]).

Figure 4

An OSRR coupled with microstrip line and integrated with a microfluidic channel proposed as a chemical sensor. The Spacing between OSRR elements with reference to outermost points (Dy) is optimized for all the proposed designs. (a) Simultaneous detection of multiple fluids, a pictorial view; and (b) insertion loss of the microfluidic sensor when permittivity of Fluid B was varied while keeping the permittivity of Fluid A constant (Redrawn from [37]).

Figure 5

Metamaterial Based Microfluidic Sensor Design consists of a microstrip-coupled square shape ring resonator with one opening gap. A Microfluidic channel is positioned in the middle of the split ring resonator for a strong interaction of electric field with target analyte: (a) top view; (b) design dimensions; and (c) back view (redrawn from [38]).

Figure 6

(a) Geometry of the splitter/combiner configuration: L = 86 mm, W = 62 mm, l₁ = 27 mm, w₁ = 2.22 mm, ls = 25 mm, Ws = 9 mm, c = 1.4 mm, g = 2.4 mm, d = 0.2 mm, l₂ = 9.21 mm, w₂ = 1.34 mm. (b) Electric field distribution at SRR resonance (the region with highest intensity at SRR gap is shown). (c) Fabricated prototype of metamaterial inspired microfluidic chemical sensor (redrawn from [39]).

Figure 7

(a) Illustration of a unit cell meta-atom SRR based fully integrated microfluidic sensor with g = 0.8 mm, and w_f = 0.4 mm; and (b) an integrated meta-surface SRR based sensor, consisting of 16 unit cells (redrawn from [40]).

Figure 8

(a) SRR based sensor array (operating at THz frequency) integrated with a microfluidic system consisting of trapezoidal shaped structure to entrap the liquid particles of analyte. (b) Geometrical parameters of a unit cell (a SRR), and aligned-position of corresponding trapping structure, g = t = w = 5 µm, L = 30 µm; and (c) array design by integrating several unit cells from (b) with p = 50 µm (redrawn from [42]).

Figure 9

Fabrication process of metamaterial based microfluidic chemical sensor developed on paper substrate: (a) wax printer used for wax-printing on chromatography paper; (b) polyimide Sheet cut by CO₂ laser used as interface layer on wax printed paper; (c) painting silver ink; (d) peeling polyimide sheet off the paper; (e) silver-painted paper after peeling off polyimide sheet; (f) flow of water through the microfluidic channels; (g) unit cell of the proposed sensor, the thickness of the chromatography paper and conductive silver disks are 90 μm and 100 μm, respectively; (h) fabricated sample (dimensions of 11.5 × 11.5 cm²), inhomogeneity of the silver disks due to low resolution in painting are shown on the inset figure; (i) flow of dye on microfluidic channel; and (j) channels are filled with dye, and the source of the dye is also shown (redrawn from [3]).

Figure 10

Flexible metasurface absorber is proposed as an ethanol sensor. SRCR is inkjet-printed on Kodak premium photo paper, serving as middle layer. Microfluidic channel is engraved on top layer (1 mm thick PDMS), and bottom PDMS layer is added to increase the absorbance. (a) Three-dimensional view of unit cell design; (b) electric field magnitude distribution of unit cell without loading a microfluidic channel; (c) simulation setup for boundary conditions and excitations; and (d) fabricated prototype filled with DI-water. Red ink is mixed with DI-water to make the fluidic path observable (redrawn from [48]).

Figure 11

Microfluidic MM absorber as an ethanol sensor: (a) simulated model is shown; (b) unit cell consists of the microfluidic layer, bonding layer, and MM absorber layer with dimensions: c = 1.5 mm, d = 2 mm, g = 1.13 mm, and h = 1.5 mm; (c) top view of the fabricated prototype (overall structure in 6 × 6 array configuration with inlet/outlet tubes); and (d) fluid flow in the channel of the unit cell (blue color arrows) (redrawn from [9]).

Figure 12

Antigen-antibody binding illustration: (a) before binding (with immobilized antibody around SRR electrode); and (b) after binding under analyte flow through a microfluidic channel (redrawn from [77]).

Figure 13

Hyperbolic metamaterial plasmonic biosensor integrated with microfluidics: (a) a 3D view of the fabricated biosensor with a fluid flow channel and a SEM image of the fabricated 2D subwavelength gold diffraction grating on top of the sensor surface; (b) photograph of the grating coupled hyperbolic metamaterial sensor device fully integrated with a microfluidic channel and sample tubing; (c) real parts of effective permittivity of biosensor, which shows a hyperbolic dispersion at λ ≥ 520 nm (dashed vertical line). In the inset the side view of fabricated biosensor is shown; and (d) reflectance spectra of the grating coupled HMM at different angles of incidence. It shows four prominent reflectance dips, corresponding to the bulk plasmon polariton modes, and two weak reflectance minima in the shorter wavelengths, corresponding to the SPP modes, all six modes are guided modes (redrawn from [85]).

Figure 13

Hyperbolic metamaterial plasmonic biosensor integrated with microfluidics: (a) a 3D view of the fabricated biosensor with a fluid flow channel and a SEM image of the fabricated 2D subwavelength gold diffraction grating on top of the sensor surface; (b) photograph of the grating coupled hyperbolic metamaterial sensor device fully integrated with a microfluidic channel and sample tubing; (c) real parts of effective permittivity of biosensor, which shows a hyperbolic dispersion at λ ≥ 520 nm (dashed vertical line). In the inset the side view of fabricated biosensor is shown; and (d) reflectance spectra of the grating coupled HMM at different angles of incidence. It shows four prominent reflectance dips, corresponding to the bulk plasmon polariton modes, and two weak reflectance minima in the shorter wavelengths, corresponding to the SPP modes, all six modes are guided modes (redrawn from [85]).

Figure 14

(a) THz metamaterials microfluidics biosensor; and (b) equivalent circuit with {RLC}s for the SRRs (redrawn from [86]).