A Review of Thin Films Used in Smart Contact Lenses

Smart contact lenses incorporate a diversity of thin‐film components and are also fabricated utilizing different thin films. They have been developed for the sensing and treatment of diseases such as intraocular pressure and glaucoma treatment using the tear fluid. This report provides a review of how these thin films are processed to develop functional characteristics relevant for their application in smart contact lenses. Types of thin films used, as well as various applications, are discussed from the historical developments leading to the invention of smart contact lenses to the state‐of‐the‐art technologies related to thin films used in them. Finally, challenges with contact lenses and improvements to tackle these challenges using thin films are presented.


Introduction
A smart contact lens is a type of wearable, health monitoring device. [1]2a] Based on what has been achieved with other wearable devices such as skin patches that interact with body sweat, the following are some of the proven and potential capabilities of smart contact lenses due to their interaction with human tears and detection of its biomarkers: [4] 1) detecting lactate levels in tears, indicative of increasing stress; [5] a) potentially useful in detecting driver fatigue based on eye focus while driving; b) possibly usable in monitoring doctors' concentration when performing multiple surgeries one after the other; 2) indicating dangerous level of blood sugar in diabetic patients, as well as automatically releasing drugs in response to this; [6] and 3) detecting ailments such as allergies, dry eye syndrome, and degenerative diseases like keratoconus. [7]1a] The distinction of smart contact lens (SCLs) from normal contact lenses, aside the "smart" appendage, lies in the incorporation of devices like sensors and thin-film electrolytes into them for functions which go beyond eye sight correction to include wireless control of external devices and health monitoring by detection of eye fluid compositional variations. [8]8a,9] However, some issues are associated with the use of smart contact lenses including low-temperature burn or dehydration due to the exposure to electromagnetic (EM) waves, as well as the dry-eye syndrome. [10]ome common terms usually encountered in the discussion of studies and applications of SCLs include 1) iris and sclera of the eye: the colored and whitish parts of the eye, [11] respectively (see Figure 1A), which work together to produce images from entering light.It is desired for contact lenses to be placed on these parts of the eye in a way that light transmittance is not compromised significantly; 2) biomarkers: they could be physical like IOP or chemical like components of the tear fluids such as glucose and lactase [12] that can be indicative of health conditions in the body.They can be detected and quantified by SCLs for diagnosis, or to trigger components in the SCL to give a response; and 3) electrodes, electrolytes, and antennas: these are conductive components of the SCL that enable charge transfer for signal processing and communication within the SCL [13] (see Figure 1B).While they are usually external and nonbiological components, the tear fluid due to its constituent ions (of sodium and potassium) is also used as electrolyte for SCLs.

History and Key Developments in SCL
The "smart" reinvention of contact lenses started with the prototype developed at Google[x] a team within Google of Mountain View, California, which comprised a conventional lens with a tiny wireless chip, a glucose sensor and a tiny battery in 2014. [14]An early research work on developing thin films [15] used polyethyleneteraphthalate (PET) sheets as substrate upon selfassembled monocrystalline silicon field-effect transistors, which were synthesized aiming to make micro-sized functional components applicable in systems like smart contact lenses. [16]It appears that a development of this technology went on to the Google invented SCLs as some of the authors of this report were listed as inventors of the Google SCL.4a] MojoVision in 2015 about a year after Google's announcement was reported to have created as a startup company to create smart contact lens. [17]They successfully created a prototype that has seen investment from big industry players toward creating am augmented reality product.Some of the thin-film components in mojo Vision's Smart Contact Lenses called Mojo Lens were 1) 14 000 pixel-per-inch Micro-LED (light-emitting diode) display: measuring less than 0.5 mm in diameter with a pixel-pitch of 1.8 μ and was described as the world's smallest and densest display ever created for dynamic content; 2) application-specific integrated circuit (ASIC): incorporated a 5 GHz radio and ARM Core M0 processor that transmitted sensor data off the lens and streamed augmented reality (AR) content to the MicroLED display; 3) custom-configured trackers for eye movements including accelerometer, gyroscope, and magnetometer working continuously; 4) unique and intuitive interface based on eye tracking: likely comprising thin films for scleral coils and for electrodes in electro-oculography; and 5) proprietary power management system: this included medical-grade micro-batteries and power management-integrated circuits.
4a] They thereafter redirected their focus from the use of electrically conductive components to passive sensors comprising organic materials, eliminating the need for batteries in the sensors.Beyond and building on these inventions, scientist in different parts of the world have developed SCLs tested for military application (laboratories of Defence Advanced Research Projects Agency) and utilization in the metaverse. [18]Some of the details that went into these inventions as related to their thin-film components are discussed in the next section.

Thin-Film Batteries for Powering SCL
In thin-film batteries, [19] off axis illumination and annealing temperature affect quality of thin films.poly(chloro-p-xylylene) (parylene C) surface coating provided practical protection of the batteries from water that would typically be encountered in the contact lens interacting with tears.
Thin-film batteries were also used along with thin-film electrode layers comprising gold, titanium, and Ag/AgCl thin-film layers (biosensor film) for applications in IC, continuous eye monitoring, and telemetry. [20]part from thin-film batteries, micro-batteries with an integrated power source have also been applied in SCL and imbued with flexibility and effectiveness in powering a LED display via eye-tracking. [21]This Li-ion battery comprised different thin films including serpentine electrodes (designed via laser ablation), separated by gel polymer electrolyte containing lithium bis (trifluoromethanesulfonyl)imide (LiTFSI) in methyl methacrylatepolyethylene glycol (MMA-PEG) which was drop casted on the electrodes, all on spin-coated polydimethylsiloxane (PDMS) substrates.The cathode and anode materials contained lithium nickel manganese oxide (LNMO) and lithium titanate (LTO), respectively, which were each mixed with carbon black, N-methyl-2-pyrrolidone (NMP), and polyvinylidene fluoride (PVDF) to make them in a moldable paste.Even though the system tested was very simple and removed other complications involved in real contact lens usage, it was a beneficial proof-of-Figure 1. A) Digital photographs of a human eye model labeling the parts that interact with Smart contact lenses, B) Sample of smart contact lens designed to fit the eye with some of its components labelled. [52]Reproduced with permission from. [52]Copyright 2014, Elsevier.
concept to show the capabilities of using batteries in contact lenses to power or run external systems.It is expected that improvement in the battery capacity can improve its practical utilization, for instance via choosing improved thin-film materials in its components such as electrodes with improved mechanical strength (achievable with graphene) or using an electrolyte with superior ionic conductivity (achievable with ionic liquids [22] ).The battery efficiency in this system, being dependent on its thickness (which cannot be higher than some nm for eye comfort) and surface area (limited to the part of the iris), was limited.It was expected that improvement in this efficiency by using batteries with large surface area can be achieved with strong and transparent functional materials which can cover more parts of the iris as well as the pupil (see parts of the eye in the Figure 1).4a] 3.2.Thin-Film Supercapacitors Carbon-based supercapacitors were synthesized via direct ink writing (DIW) to make soft lenses containing silver-based antennas. [3]

Tear Fluid Electrolytes in SCL
In an interesting report the eye fluid (containing sodium and potassium ions) was employed as electrolyte interacting with electrodes fabricated from humanly safe and biocompatible Prussian Blue analogs (PBA) in thin-film batteries. [23]Such system eliminated the need for perfect sealing of the SCL components since it did not contain any toxic components.This system comprised thin films of PBA in cathode and anode fabricated by ultraviolet polymerization of hydrogels into moulds.
Tear fluid has also been explored as electrolyte for SCLs developed to act as an alarm system triggered by electrochemical changes in the eye due to variation in the composition of ions in the tears. [24]These ions were made to interact with an electrochromic electrode situated in a thin film of Persian blue (PB) in the SCL, created by electrodeposition on an indium tin oxide (ITO) thin film deposited on a PET substrate via e-beam sputtering (conditions in experimental).ITO was patterned before PB deposition.With this SCL persons with hearing disabilities could see signals indicative of danger by voltage variations leading to color change in the SCL prompting them to take the best cause of action.

Smart Sensors
In an attempt to improve on the sensitivity and accuracy of sensors in smart contact lenses which incorporate microelectrodes, [25] a system physically similar to smart contact lenses was developed and used for improved analysis of the dynamics and fate of analyte in tear fluids which are detected in SCLs.Reproduced with permission from. [25]Copyright 2014, Elsevier.A) Digital photographs of the array of microelectrodes, B) Magnified image of one working electrode, and C) magnified image of the counter electrode.
Song et al developed a "power-less" SCL (since the detection process operates passively) potentially capable of simultaneously detecting IOP changes, drug release, and disease detection in the eye. [26]The superior performance of this SCL was achieved utilizing the following thin films: 1) optically transparent and bio-compatible material-anodic aluminium oxide (AAO); 2) PDMS as base or SCL substrate; 3) Al thin film for producing AAO; and 4) components for drug release and sensing.
Thin films of in organic LED along with rectifier circuits, antenna, and interconnects have been utilized in soft SCLs for therapeutic monitoring of eye diseases.Their lightweight and nanosized thickness (added to transparency in some cases) removed obstructions to vision caused by their placement on the eye.The system comprised electrospun AgNF/AgNW antenna deposited on strain-tunable hybrid structures comprising a mechanically reinforced part (called parylene Cu/Si made from parylene film spin-coated on a sacrificial layer of Cu film created by evaporative physical vapor deposition (PVD) on top of a Si wafer) on which the ILED and rectifier circuits were upon, as well as silicon soft layers for the antenna and interconnects imbuing the lenses with the required softness and flexibility (as shown in the Figure 3). [27]The rectifier and antenna enabled wireless operation of the ILED, which was found to work efficiently and offer phototherapeutic effects. [27] Manufacturing Techniques of the SCL SCL fabrication is divided into two parts: film deposition and patterning.For the deposition of conductive thin films (Au, Cu, AgNW etc), physical and chemical vapor deposition are commonly used.[28] The patterning is done using lithography.These processes require expensive facilities such as vacuum and yellow light room.
The choice of commonly used materials as substrate for thinfilm contact lenses like PET and Polyimide [19] is usually guided by aiming for biocompatible and nonhazardous base components upon which the functional and conductive parts can be firmly deposited.Hard polymers could sometimes limit the comfort and flexibility of the lenses and thus hydrogels and siliconbased materials are employed.A schematic representation of the different materials applied specifically in SCLs from the reviewed articles is presented in Figure 4.
The synthesis conditions applied in fabrication of the thin films in SCL affect its properties and usability for its intended application.For instance, conditions applied in the synthesis of cathode of thin-film batteries [19] require high gas purity (e.g., Argon 6.0) under plasma powered sputtering between 300 and 500 °C that led to higher quality films than conventional systems which might be more economical.The energy and overall efficiency resulting from high-quality thin-film synthesis and nanotechnological systems is expected to provide benefits Figure 3. Fabrication of wireless phototherapeutic smart contact lens. [27]Adapted from. [52]The process showed the deposition of sacrificial layer followed by transfer of diode and attachment of LED.Once all the electrode components are transferred the lens material was coated and contact lens was molded and separated.
outweighing these meagre gains.A summary of the different fabrication techniques applied to thin-film components of SCL is presented in Table 1.

State of the Art with Graphene-Based Thin Films in Smart Contact Lenses
14b] In addition, it has excellent EM wave shielding, and graphene in the contact lens partially absorbs the EM wave. [10] single-layer graphene, which has been previously grown on Cu foil is transferred to the contact lens through the solution method, then patterned using lithography. [28]Nevertheless, the lithography process could result in higher fabrication costs. [28]o this end, Tang et al. deposited single-graphene layers on contact lenses using cheaper processes of drop-casting and direct laser interference printing (DLIP). [28]Drop-casting basically involves placing a droplet of graphene solution on a contact lens, followed by vaporization. [29]Then, the DLIP was done by placing the graphene-coated lens between two mirrors, and a laser beam causes interference on the graphene film such that areas where the laser intensity was high had graphene removed. [28]hiou et al. reported the fabrication of a graphene-based thinfilm supercapacitor as an energy storage system for a smart contact lens as shown in Figure 5B.First copper and parylene-C were deposited on silcon wafer as sacrificial layers, then Ti (40 nm) and Au (200 nm) were deposited and patterned as the current collector, then graphene was coated, baked, and patterned.Afterward, PVA-H 3 PO 4 gel electrolyte was coated by drop-casting, followed by the deposition of the parylene-C for electyrolyte insulation by CVD.Finally, the components were released from the silicon wafer and embedded in a standard hydrogel soft contact lens using a cast-molding process. [30]The smart contact lens fabricated this way had stability and flexibility and can serve as a replacement for the RF-based power system.Similarly, Lee et al. (Figure 5C) synthesized a continuous graphene monolayer on a Cu foil using CVD with methane and hydrogen gases, followed by coating with poly(methylmethacrylate) (PMMA).Then, the Cu foil was etched, and the PMMA/ graphene layer was transferred to the contact lens. [10]This PMMA/graphene-coated smart contact lens was then demonstrated to reduce exposure to EM waves and dehydration, thus showing its capability of protecting the eyes from EM and dehydration.Similarly, a Micro-LED was fabricated on graphene electrodes by using a single-layer graphene patterned using photolithography on a contact lens, followed by the connection of prewired micro-LEDs. [10]

Thin Films for Glucose Monitoring Applications
Keum et al. [2a] developed a dual-purpose smart contact lens for glucose measurement and drug delivery for the treatment of diabetic retinopathy.This smart contact lens was mainly made up of five parts namely: a real-time electrochemical biosensor, an on-demand flexible drug delivery system (f-DDS), a resonant inductive wireless energy transfer system, a complementary integrated circuit (IC) based microcontroller chip with a power management unit (PMU), and a remote radio frequency (RF) communication system are shown in Figure 6.The biosensor detects glucose in tears, the f-DDS acts a self-regulated drug release system by remote communication, and wireless communication is made through the resonant inductive coupling.2a] The f-DDS was fabricated by the deposition of SI and SIo2 layers through PECVD, followed by the e-beam evaporation and photolithography of Au and Ti, the loading of the reservoirs with drugs, and then sealing with a flexible PET film.Finally, the power transmission coil consisting of Cu was made by spin-coating PDMS on Cu foil, followed by patterning by photolithography, detachment from the PDMS, and transfer to the contact lens.
A wireless smart contact lens with a highly transparent AgNW strain sensor was developed for continuous IOP monitoring for glaucoma treatment. [31]The contact lens shown in Figure 6A comprised an AgNW IOP sensor, wireless circuits, and an ultralow power application-specific integrated circuit (ASIC) chip for noninvasive, continuous, and wireless IOP monitoring.The transparent sensor was made by AgNW spin-coated, patterned on a parylene C substrate, and annealed.Afterward, a protective layer of paryleneC was coated to prevent the oxidation of AgNWs under exposure to body fluids such as sweat and tears. [31]The resulting AgNW sensor had a compatible sensitivity, high optical transmittance over 90%, high stability, and a high correlation of R 2 = 0.995 between IOP and output signals. [31]

Thin Films for Intraocular Pressure (IOP) Monitoring Applications
Glaucoma is an irreversible chronic ocular disease that requires continuous, long-time medical care. [32]Intraocular pressure (IOP) measurement is one of the ways this disease is observed, and several commercial tonometer exist for the measurement of Figure 5. A) Synthesis of smart contact lens with graphene layer embedded.Reproduced with permission from. [30]Copyright 2017, American Chemical Society.As indicated in the schematics showing deposition of sacrificial layer by CVD, followed by patterning, drop casting, and integrating the chip with the contact lens.B) Applications of graphene in EMI protection and dehydration protection.Reproduced with permission from. [10]Copyright 2017, ACS.(i-v) Stepwise process of fabrication of graphene-coated contact lens.
the IOP.However, these measurements have some limitations such as inconvenience due to time and space constraints and difficulty in collecting large data in a day and monitoring IOP fluctuations.In addition, the current mode of treatment, i.e., usage of eye drops, is challenging as the method and time of treatments have not been followed by patent.32a] This smart contact lens comprises of a gold hollow nanowire (AuHNW) based IOP sensor, made by selective etching of Ag core of Ag@Au core-shell nanowire (Ag@AuNW) in nitric acid and a flexible DDS, made by deposition and patterning of gold on parylene C followed by Figure 6.Schematic illustration of smart contact lens for glucose sensing and diabetic therapy.2a] Copyright 2020, AAAS.A) Components of the smart contact lens embedded with biosensor, B) a wireless power transmission system, C) schematic representation of an application-specific integrated circuit (ASIC) chip, and D) a wireless platform for diagnostic and therapeutic applications.
Figure 7. IOP sensing and glaucoma treatment in smart contact lenses, A) structure and photo of a smart contact lens on a rabbits eye (a) and wireless IOP monitoring and communication by a smart contact lens and Reproduced with permission from. [31]Copyright 2021, American Chemical Society.B) schematic illustration of a smart contact lens embedded AuNHW for IOP sensing and glaucoma treatment.32a] Copyright 2022, Springer Nature.
spin-coating of SU-82 015 on the gold channel and electrode as drug reservoirs into which timolol and PCA were loaded.Both the DDS and IOP sensors were embedded into the smart contact lens along with the wireless circuits and an ultralow power ASIC chip.32a] Although transparent smart contact lenses made from materials such as graphene/metal nanowire hybrid have been successfully fabricated to achieve unobstructed vision and lens-shaped plastic substances, however, issues such as low gas permeability and poor wettability will limit their use in more complex applications. [33,34]Wei et al., synthesized highly porous, gas-permeable, optically transparent smart contact lenses based on metal-coated nanofiber mesh (metal c -NM) as shown in Figure 8. [33] First, gold (110 nm) was sputtered on electrospun PAN nanofiber mesh, then the obtained AU c -NM was attached to a commercial disposable hydrogel-based contact lens to obtain Au c -NM/HyCL.Furthermore, poly(3,4-ethylenedioxythiophene) (PEDOT)/poly(styrene sulfonate) (PSS) electrochemical deposition was carried out to improve the metal-substrate adhesion.The resulting smart lens exhibited high gas permeability, wettability and hydration level as well as excellent optical transparency and mechanical compliance and robustness. [33]

3D-Printed Smart Contact Lenses
Attempts have been made to synthesize quality SCLs in a repeatable manner using vat-photopolymerization-based additive manufacturing, digital light processing 3D printing, techniques. [35]These methods entail computer-aided design before 3D printing of the contact lenses, which would be integrated with sensors for health monitoring.Major challenge here was attaining the desired lens geometry and surface smoothness. [36]The optimization of the 3D printing parameter revealed the superior influence of surface roughness influenced by the synthesis procedure, especially detachment from substrate, in producing the SCL thin film with minimal reduction of light transmittance. [37]The application of post processing treatment led to smoother surface finishing of the SCLs. [38]iocompatible thin films of food-grade colors were applied to tint the SCL surface using DLIP holographic laser ablation. [39]The successfully nanopatterned lenses proved the compatibility of the 3D-printed lenses with sensors and electrodes for smart multifunctional systems.In one of such systems, the 3D-printed lenses were used to fix color blindness issues [40] due to their incorporation of thin films of hydrogel dye holders from which dyes were released on the tinted lens to filter out unrequired wavelengths of light to automatically correct vision. [41]gure 8. Schematic illustration of the synthesis of gas-permeable, irritation-free, transparent hydrogel contact lens bases on Au c -NM and in situ electrochemically deposited PEDOT/PSS.Reproduced with permission from. [33]Copyright 2019, American Chemical Society.A) schematic representation of structure of the device, B) digital photograph of the device, and C) stepwise illustration of the fabrication process.
Overall, while 3D printing presents an easy and controllable means to fabricate smart contact lenses incorporating devices for interesting functionalities, the perfect incorporation, and tests is still limited and thus further studies are required to prove its feasibility for pilot trials before commercialization. [42]It would also be interesting to see how the quality of 3D-printed SCLs compare with those produced from other deposition techniques like electrodeposition [43] and PVD or CVD, especially with the issues of step effect (or defects) which although reportedly improved upon still remains uneliminated. [44]ble 2. Applications of thin films in smart contact lenses and their manufacturing techniques.

Characterization Techniques for Analysis of SCL Thin Films
Nanoindentation [45] and plane-strain buldge tests [46] can be used to characterize the mechanical properties of thin films among other tools used to assess the quality of the synthesized thin films surfaces and interfaces.Three-point method was used to determine mechanical properties of the thin films.Tensile strength, elasticity, and elongation of 3D-printed lenses were also determined. [37]Surface characteristics have also been estimated via scanning electron microscopy (SEM) and atomic force microscopy (AFM) imaging.Gold electrodes patterned on PDMS were characterized using AFM to study effect of plasma treatment on surface roughness reduction of the PDMS substrate. [15]The improvements observed were worthwhile because surface roughness and cracks typically found on PDMS limit the effective adhesion of the gold electrodes on their surface to create stable and well-connected thin film components in the SCL.The adhesion was further improved by using an adhesion layer (another thin film) between the electrode and PDMS, utilizing the etch step of the metal rather than having to resort to the use of a photoresist layer which presents other issues during lift-off.
X-ray diffraction was used to evaluate the crystallinity of the thin-film material.An SCL comprising electrochromic Pursian Blue (PB) electrode was characterized using optical tools [24] such as (transmittance, red-green-blue; RGB data) and electrochemical tools (such as cyclic voltammetry) (Table 2).

Challenges with Smart Contact Lenses and their Thin Films
SCL is a device that people wear directly on the surface of the eye, and thus must be resistive to temperature increase, durable, and biocompatible.Some challenges that must be overcome with smart contact lenses, as well as thin films in them, related to their real-time, optimally effective, and safe utilization in the human eye are: Limited thermal resistivity: When a smart contact lens is operating, electrical currents flow through the devices that are integrated with lens, resulting in Joule heating. [47]ccordingly, the temperature of the wearer's eye may increase. [48]Thus, it is essential to make the choice of thin-film components in smart contact lens operate within an appropriate temperature range after sufficient heat tests have been conducted: 1) limited thermal resistivity: when a smart contact lens is operating, electrical currents flow through the devices that are integrated with lens, resulting in Joule heating.Accordingly, the temperature of the wearer's eye may increase.1a] Possibility of damage to the eyelid or cornea by rigid and brittle materials of the integrated electronic system, e.g., mounted IC chips and rigid interconnects [1a,2c,50] ; 4) microbial associated infections: One challenge associated with the smart contact lenses is the risk of microbial infection to the cornea, because wearer do not exercise proper wear and care habits.1a] 7. Future Aspects of the Thin Films on the Contact Lenses Smart contact lenses will bring advancement in the health monitoring such as monitoring blood sugar level, providing information about ocular diseases, and treating them.Apart from health monitoring, smart contact could help in drug delivery application also.It is expected that the smart contact lenses embedded with wireless sensor will help as aid in vision correction as well as eye medication while in contact.The smart contact could help in the vision of the colorblind patients and help in autocorrection in focusing and other vision correction-related issues.It is expected that in future, the smart contact may learn from the vision and store them to recall in the future, and this could lead into augmented reality interfaces.

Conclusion
Thin films are indeed an essential part of smart contact lenses, and in the improvement of their properties and processability lies the true practical advancement in the utilization of smart contact lenses.Thin films have been used to imbue the SCLs with sensing and storage capabilities, in addition to protection of the eyes from EM radiation and dehydration which have resulted in superior augmented reality systems, noninvasive health monitoring and therapeutic tools, and long-living functional devices, and improved biocompatibility and biosafety for SCL wearers.Challenges remaining with energy storage and eye comfort are expected to be improved on with further research and technological improvements especially with respect to functional 2D nanomaterial thin films and efficient fabrication techniques.

Figure 2 .
Figure2.Gold microelectrodes for sensitive SCL from[25] Reproduced with permission from.[25]Copyright 2014, Elsevier.A) Digital photographs of the array of microelectrodes, B) Magnified image of one working electrode, and C) magnified image of the counter electrode.

Figure 4 .
Figure4.Substrate materials commonly employed in the fabrication of smart contact lenses, summarized from reviewed articles.[53]

Table 1 .
Biomarker in eye used for detecting that disease.