Integrated contact lens sensor system based on multifunctional ultrathin MoS2 transistors

Abstract

Smart contact lenses attract extensive interests due to their capability of directly monitoring physiological and ambient information. However, previous demonstrations usually lacked efficient sensor modalities, facile fabrication process, mechanical stability, or biocompatibility. Here, we demonstrate a flexible approach for fabrication of multifunctional smart contact lenses with an ultrathin MoS₂ transistors-based serpentine mesh sensor system. The integrated sensor systems contain a photodetector for receiving optical information, a glucose sensor for monitoring glucose level directly from tear fluid, and a temperature sensor for diagnosing potential corneal disease. Unlike traditional sensors and circuit chips sandwiched in the lens substrate, this serpentine mesh sensor system can be directly mounted onto the lenses and maintain direct contact with tears, delivering high detection sensitivity, while being mechanically robust and not interfering with either blinking or vision. Furthermore, the in vitro cytotoxicity tests reveal good biocompatibility, thus holding promise as next-generation soft electronics for healthcare and medical applications.

Keywords: 2D semiconductor materials; flexible and wearable devices; glucose sensing; integrated sensor systems; molybdenum disulphide; photodetection; smart contact lens; soft bioelectronics; temperature monitoring.

Graphical abstract

Figure 1

Structural design of a smart contact lens with ultrathin MoS₂ transistors-based serpentine mesh sensor system (A) Schematic illustration of the different layers of smart contact lens structure attached to an eyeball. The dashed region highlights the gold-mediated mechanical exfoliation of monolayer MoS₂. (B) Optical image of the serpentine electrode and sensor structure. (C) Photograph of the sensor layer transferred onto a dome-shaped PDMS substrate. (D) Photograph of the system placed on an artificial eye. (E) Illustration of the smart contact lens and the corresponding dimensions with human eyes.

Figure 2

Transfer process and characterization of an exfoliated large-area MoS₂ flake transistor (A) Schematic illustration of the gold-mediated exfoliation process. (B) Optical image of an exfoliated MoS₂ flake with a lateral size of ~200 × 100 μm on a SiO₂/Si substrate. (C) Room-temperature Raman spectrum of MoS₂ showing the A_1g mode at 408 cm⁻¹ and the E¹_2g mode at 385 cm⁻¹. (D) High-resolution TEM image of the atomic layer of MoS₂. Inset is the zoom-in view exhibiting a lattice d100 of 0.27 nm. (E) Histogram comparing flake area size between traditional mechanical exfoliation (in red) and the gold-assisted exfoliation method (in blue). (F) 3D schematics of the back-gated FET device on the SiO₂/Si substrate. (G) Optical image of the MoS₂ FET device from the top. Different channel lengths were fabricated for optimization of the design. (H) Output curve (source-drain current I_DS versus source-drain voltage V_DS) with the back-gate voltage V_G sweep from 0 to 60 V. (I) Transfer curve (I_DS as a function of V_G) with V_DS = 5 V, shows n-type behavior with an on/off ratio of 10⁷.

Figure 3

The “pattern-release-transfer” process and mechanical analysis of a smart contact lens (A) Illustration of the fabrication process of a smart contact lens. We began by coating the bottom PI layer on a glass substrate. The MoS₂ and gold mesh electrodes were consecutively placed and patterned to form the sensor and the interconnect layer. Subsequently, the top PI layer was coated and patterned, followed by release from the glass substrate. Finally, the device was picked up by the PDMS contact lens, oven baked, and enzyme modified to complete the process. (B) Illustration for the sensor layer with the radial strain (x, y axis) (left) and the FEA strain map under a radial strain of 25% (right). (C) Illustration of the sensor layer with the longitudinal strain on the perpendicular plane (z axis) (left) and the FEA strain map with 4 mm of z axis displacement (right).

Figure 4

The characteristics of the multifunctional sensor system (A) Schematic illustration of the multifunctional sensor structure. (B and C) 3D schematic illustration (B) and optical image (C) of the two-terminal MoS₂ photodetector. (D) Photoswitching behavior under pulsed illumination by 365 nm wavelength UV light with different V_DS values. (E) Drain-source characteristics in the dark and under illumination with different wavelengths of light. (F) Drain-source characteristics of the photodetector under different illuminating light intensities. (G) The photoresponsivity of the MoS₂ phototransistor, exhibiting a high photoresponsivity of 4.8 A/W for a UV power of 20 nW. (H) Schematic illustration of ac MoS₂ glucose sensor. (I) Illustration of the sensing mechanism of the device with oxidation of glucose. (J) Optical image of the MoS₂ glucose sensor. (K) Time versus current curve based on changes in glucose levels. (L) The sensitivity (|$\Delta$R|/R₀) of the glucose sensor with a buffer solution, where R₀ is the initial resistance at zero glucose concentration. (M) Sensitivity of the sensors with the PBS buffer solution (blue) and the artificial tear solution (red). Five devices with each solution were tested and the standard deviation is represented by error bars. (N and O) Schematic illustration (N) and optical image (O) of the temperature sensor. (P) The temperature-dependent resistance curve for the Au thermal resistor. (Q) The resistance of the temperature sensor versus strain under different temperatures.

Figure 5

The optical transmittance/haze test and in vitro cytotoxicity test of the smart contact lens (A) Schematic illustration of optical transmittance testing. (B) Optical transmittance (black) and haze (red) spectra from 400 to 800 nm. (C) Optical properties at 55 nm as a function of tensile strain up to 30%. (D) The optical images (upper) and fluorescent images (lower) of the HUVECs at different times. In a live/dead assay, green and red fluorescence indicate living and dead cells, respectively. (E) The average viability (percent of living cells) at days 1, 4, and 7. The percentage of living cells remained the same during the 7 days. (F) Cell count (number of cells per area) at different times. The increasing number of living cells demonstrates cell culture reliability.