**Background:** The paper reviews the emerging field of smart contact lenses (SCLs) as wearable devices for non-invasive continuous health monitoring. Traditional diagnostic tools rely on blood draws and bench-top assays, but SCLs offer a unique interface via the cornea and tear fluid, which contains biomarkers (e.g., glucose, lactate, ions) correlated with systemic conditions. The global SCL market was valued at USD 115.0 million in 2018 and is projected to reach USD 1603.4 million by 2026 (CAGR 38.9%). The review focuses on two major applications: intraocular pressure (IOP) monitoring for glaucoma and glucose monitoring for diabetes.
**Methods:** The paper is a narrative review summarizing materials, electrical components, fabrication methods, and applications of SCLs. Key materials include silicone, poly(2-hydroxyethyl methacrylate) (PHEMA) hydrogels, graphene, and polydimethylsiloxane (PDMS). Electrical components involve wireless systems (e.g., RFID, inductive coupling), power sources (microbatteries, energy harvesting, biofuel cells), and sensors (capacitive, piezoresistive, strain gauge, electrochemical). Fabrication techniques include photolithography, microfluidics, injection molding, cast molding, CO2 laser ablation, and femtosecond laser ablation. The review cites specific studies: e.g., a flexible multifunctional lens using γ-Fe2O3@NiO nanosheets achieved glucose detection limit of 0.43 μmol, eye movement accuracy 95.27%, and IOP sensitivity 0.17 MHz mmHg⁻¹. Another study used MoS2 transistor-based serpentine mesh sensors for glucose, temperature, and photodetection. A power-free SCL with multiple electrochromic electrodes detected glucose from 0.05 mM to 0.9 mM with correlation coefficient 0.99543.
**Key Results:** For IOP monitoring, SCLs use sensors (capacitance, piezoresistive, strain gauge, micro-inductor) to detect corneal curvature changes. A theragnostic SCL with gold hollow nanowire IOP sensor and drug delivery system (timolol) was tested on glaucoma-induced rabbits, showing effective monitoring and control. Another smart soft contact lens enabled 24-hour IOP monitoring during sleep, integrated into commercial brands without altering lens power, biocompatibility, or oxygen permeability. For glucose monitoring, tear glucose concentration (0.01–0.05 mM) correlates with blood glucose. A wireless SCL with NFC antenna and electrochemical sensor quantified glucose in artificial tears (0.2–1 mM, limit of detection 66 μM). A bio-chargeable battery using enzymatic reactions showed discharging capacity 45 μA cm⁻² and maximum power 201 μW cm⁻² over 15 cycles. A soft SCL with transparent nanostructures and LED display wirelessly monitored tear glucose in vivo, with transmission distance <9 mm. Challenges include high cost, limited battery life, comfort issues, data privacy, and regulatory hurdles. Notably, Mojo Vision paused its SCL project due to funding difficulties, and Verily halted glucose-sensing lens research in 2018 because tear glucose measurements lacked consistency with blood glucose.
**Clinical Implications:** SCLs have potential to revolutionize non-invasive diagnostics for glaucoma and diabetes by enabling continuous, real-time monitoring without finger-prick tests or bulky equipment. For glaucoma, SCLs could provide 24-hour IOP tracking and on-demand drug delivery, improving management. For diabetes, tear glucose monitoring offers a painless alternative to blood sampling, with potential for early detection of diabetic retinopathy. However, clinical translation faces significant barriers: accuracy and reliability of tear biomarkers, biocompatibility, power supply, and regulatory approval. The review emphasizes that while SCLs are promising, they remain in early development, requiring further clinical trials and technological refinement before widespread adoption.