Hardware

The backbone of Eye kNOw and Eye Screen

Eye Screen

An affordable and portable device, providing real-time IOP measurement.

Background

A major risk factor of glaucoma is the elevation of intraocular pressure (IOP). Therefore, measuring IOP is the most important method to predict glaucoma development. However, current IOP detectors are only available in clinics or hospitals and are inefficient due to untimely IOP monitoring. Moreover, these hospital-based IOP measurement systems cause significant inconvenience and inaccessibility to many patients. The current cornerstone of IOP measurement includes both non-contact and contact systems. When receiving non-contact air-puff examination, patients may experience discomfort and eye irritation. As for contact systems, such as Goldmann applanation tonometer and Tonopen tonometer, expensive instruments are required, and some of them are heavy and immobile. Most important of all, these methods could not be done by the patients themselves, all require additional personnel to complete the task. To make IOP detection more accessible, iGEM NCKU Tainan 2020 came up with Eye Screen.

Device overview

Eye Screen is an affordable, portable, and user-friendly ultrasonic tonometer. It measures IOP with air-coupled transducers that transmit ultrasonic waves into the eye and analyze the reflected signal. The result is displayed on the LCD and uploaded onto Eye Cloud, an application that can record the user’s IOP value and inform one’s risk of glaucoma in a real-time manner. As a result, Eye Screen not only provides a convenient and painless IOP measurement method to detect glaucoma but also allows users to monitor their IOP regularly, providing valuable insights on our treatment.

Fig. 2. Eye Screen (illustration/prototype).

Principle

Ultrasonic waves are used to measure the cornea’s elastic properties via acoustic impedance, and research showed that the acoustic impedance has a linear relationship with the elastic modulus of the cornea, which increases with stress^[1]. By exploring the relationship between IOP and acoustic impedance, their results proved the proportional correlation between IOP and amplitude of reflected ultrasound signals from the cornea.

Therefore, we came up with Eye Screen that applies a brand-new measurement method to detect IOP developed on the research. We further found that there is a proportional correlation between IOP and amplitude of the reflected ultrasound signals of the cornea^[2]. Based on this theorem, we transmit ultrasonic waves onto the cornea and analyze the reflected signal. After establishing the relationship between IOP and amplitude of the reflected signal, we are able to get IOP readings immediately.

Design

Transmit

Arduino

We use Arduino mega 2560 as our power supplier, providing most of the components with 5V.

Function generator

Since a typical function generator is heavy and expensive, we decided to make a function generator on our own! We used the AD5930^[3] to implement the function generator. It’s a chip that can generate waveforms with programmable frequency sweep and output burst capability. By using Arduino Mega 2560 programming control, AD5930 can be operated in a variety of modes. For more details, you can see on Contribution.

In our case, intending to drive the 40kHz transducer, we program our function generator to generate a sinusoidal wave with the same frequency of 40kHz and 10 cycles with burst period 10ms (Fig. 6A)

Fig. 6. Waveform generated by function generator (before/after amplified)

Since the output waveform generated by the chip AD5930 has a limit of 0.3V, we need to add an amplifier circuit (Fig. 7) to increase the voltage value enough to drive the transducer. The amplified waveform is shown in Fig. 6B.

Receive

Envelope detector

After receiving the reflected ultrasonic signals, additional processing needs to be done on the signal in order to be read successfully due to the limited 10kHz sampling rate of Arduino. Therefore we added an envelope detector circuit to process these high-frequency signals (Fig 8), reducing signal distortion and improving signal quality.

Arduino

Lastly, Arduino reads the reflected signals from the analog pin to perform calculations and to generate an IOP reading. The generated IOP readings are displayed on Eye Screen’s LCD and transmitted to the Eye Cloud application.

As the above flowchart, Arduino can integrate the function of the buzzer, the button, Bluetooth, LCD and function generator. To read the reflected signals successfully, we first increase the sampling rate of Arduino. Then, users can press the button to tell Arduino to initiate the function generator. We find the maximum amplitude in the signals saved in an array. Applying our formula that got from experiments, we can turn the amplitude into the corresponding IOP value. Finally, the buzzer will turn on to remind users that the test is complete. Also, users can check their IOP value and risk group level on the LCD display and in the app.

The complete code is as follows:

The whole circuit diagram shows below:

Advantages/Comparison

Budget

Components	Quantity	Cost per unit(NTD)	Cost (NTD)	Cost (USD)
AD5930	1	506	506	17.6
Capacitors 0603	11	1.6	17.6	0.61
Tantalum Capacitor	2	21	42	1.46
Resistors	6	1.1	6.6	0.23
PCB Board	1	256	256	8.9
50 MHZ-Oscillator	1	50	50	1.74
Arduino MEGA	1	495	495	17.22
Air-coupling Transducer	42	52	208	7.24
Total			1581	55

Table 1. Eye Screen: device budget.

	Eye Screen	Air-Puff Tonometer	Golamann Applanation Tonometer	iCARE Home Rebound Tonometer
Painless	V	(Air-Puff)	(Probe)	(Probe)
User-friendly	V			V
Accuracy	V	V	V	V
Low Resource-wasting	V	V
Portable	V			V
Affordable	V(55 USD)	(13972 USD)	(10479 USD)	(2794 USD)
Real-time monitor	V			V
Database	V	V		V

Table 2. Eye Screen’s comparison with current tonometers.

Experiment and Result

To validate the function of Eye Screen, we adopted a gravity model to control the IOP of porcine eyeballs using the trocar system via microincision vitrectomy surgery. By adjusting the height of the saline bottle connected to the eyeball, we can measure the IOP by calculating the difference in height of the saline and the eyeball. And we tested whether the amplitude of reflected signals is proportional to the IOP change.

Fig .11. Experimental setup for the gravity model. (A) Real experimental setup (B) Schematic diagram for experimental setup

We used the advanced function generator to trigger signals to the ultrasonic transducer and received reflected signals by the oscilloscope.

The ultrasonic transducer worked at 40kHz with a burst of five cycles. As shown in Fig. 11, there is a positive correlation between IOP and mean amplitude indicates that the IOP changes would lead to an increase in the mean amplitude of reflected signals.

Fig. 12. Mean amplitude of reflected signals from different porcine eyes. (N=4)

Demonstration

Video 1. Demonstration of Eye Screen.

Eye kNOw

The smart contact lens that can detect IOP

Overview

Providing a brand-new detection method for IOP is just our first step of combating glaucoma. After going through studies regarding current glaucoma treatment^[4], we found out that compared with common ocular drugs such as beta-blockers and carbonic anhydrase inhibitors, nitric oxide has less resistance and side effects. However, since nitric oxide is easily degraded into nitrite and nitrate, it cannot be provided in the gaseous state, which is why only NO donors are used for present glaucoma treatment. On top of that, due to the short half-life of nitric oxide, NO donors require multiple administration per day to maintain their efficacy. Thus, we came to realize that if the problem of nitric oxide preservation can be solved, it would be a great advance of glaucoma medication.

To make nitric oxide treatment on site and on time, we came up with Eye kNOw, a pair of contact lenses containing engineered bacteria that produces nitric oxide synthase (NOS) in response to IOP changes. By using synthetic biology, we’re able to release NO at the precise time without worrying about the storage. As for contact lenses, it is a promising therapeutic treatment for increasing ocular bioavailability^[5]. Compared with eye drops, contact lenses are able to keep drug molecules in the post-lens lacrimal film for a longer time, leading to increased flow to the cornea and decreased drainage through the nasal-lacrimal duct. To allow both water and NO to pass through the contact lens, we designed a torus-shaped semipermeable membrane as the chamber for our engineered bacteria, and chose HEMA as the main material of our contact lens. We believe that with Eye kNOw, we’re able to make glaucoma treatment even more effective.

Mold of contact lens

The mold is 3D printed using polypropenyl (PP) with a chamber as shown in the figure below. During early stages, we discovered that our initial 3D printing material choice - polylactide (PLA) is hydrophilic, making it hard to demold our hydrophilic HEMA contact lens. Therefore, we decided to use PP due to its hydrophobic properties for better demolding process.

Material

Contact lens

Hydroxyethylmethacrylate (HEMA) is the main material used for our contact lenses, a popular ingredient used to manufacture today’s contact lenses. We mixed HEMA, EDGMA (ethylene glycol dimethacrylate) - a crosslinking agent, and AIBN (azobisisobutyronitrile) - a thermal initiator, then injected the mixture into our mold. The filled mold is flushed with carbon dioxide to remove excess oxygen and heated at 75°C for 2 hours to form the final hydrogel contact lenses.

Semipermeability

The resulting HEMA hydrogel has a pore size of around 0.48-14.74 microns^[6], making it unable to keep our engineered bacteria, IPTG, L-arginine, and DAP contained. Consequently, we designed a semipermeable compartment to limit them inside the lens and enhance biosecurity measures.

According to a study focused on the nanofiltration of aqueous solutions containing organic compounds^[7], we found out that molecular size is not the only parameter influencing the retention of organic compounds, but also the compound’s hydrophobicity. The researchers included xylose in their study, and showed that the membranes (UTC-20, Desal HL-51, and NF210) can effectively block xylose^[7]. Comparing with xylose, the smallest compound in our contact lens (L-arginine) has a larger molecular weight and is more hydrophilic, confirming that membrane material like poly(piperazine-amide) is a suitable choice for our semipermeable compartment.

Since our bacteria will be filled in a hollow tubular semipermeable compartment containing L-arginine, IPTG, and DAP. To form the compartment, we will use interfacial polymerization to make the thin film composite membranes required to sustain the hollow structure^[8]. Because the nanofiltration membrane needs to be coated on the substrate, we will use the spinning process for the coating step. The schematic of the device designed for the spinning process is shown below.

Fig. 16. Schematic of the semipermeable membrane forming device.

The syringe pump will transfer a hollow fibre into the semipermeable membrane solution, coating the entire length with it, then the coated hollow fibre will be air dried to obtain our semipermeable membranes. Finally, these semipermeable membranes will be shaped into a compartment and embedded inside our contact lens before solidifying the HEMA hydrogel, then our engineered bacteria, IPTG, L-arginine, and DAP will be injected to make the end product - Eye kNOW.

Proof of Concept

In order to verify that our contact lenses can really work on the eyes, we conduct step-by-step experiments in proof of concept.

References

He, X., & Liu, J. (2011). Correlation of corneal acoustic and elastic properties in a canine eye model. Investigative ophthalmology & visual science, 52(2), 731-736.
Zhang, J., Zhang, Y., Li, Y., Chen, R., Shung, K. K., Richter, G., & Zhou, Q. (2017). Correlation of IOP with Corneal Acoustic Impedance in Porcine Eye Model. BioMed research international, 2017.
AD5930 Datasheet and Product Info | Analog Devices. Analog.com. https://www.analog.com/en/products/ad5930.html#product-overview. Published 2014. Accessed October 14, 2020.
Aliancy J, Stamer WD, Wirostko B. A Review of Nitric Oxide for the Treatment of Glaucomatous Disease. Ophthalmology and Therapy. 2017;6(2):221-232.
Holgado MA, Anguiano-Domínguez A, Martín-Banderas L. Contact lenses as drug-delivery systems: a promising therapeutic tool. Archivos de la Sociedad Española de Oftalmología (English Edition). 2020;95(1):24-33.
Yañez F, Gomez-Amoza JL, Magariños B, Concheiro A, Alvarez-Lorenzo C. Hydrogels porosity and bacteria penetration: Where is the pore size threshold? Journal of Membrane Science. 2010;365(1-2):248-255.
Braeken L, Ramaekers R, Zhang Y, Maes G, Bruggen BV der, Vandecasteele C. Influence of hydrophobicity on retention in nanofiltration of aqueous solutions containing organic compounds. Journal of Membrane Science. 2005;252(1-2):195-203.
Wang T, He X, Li Y, Li J. Novel poly(piperazine-amide) (PA) nanofiltration membrane based poly(m-phenylene isophthalamide) (PMIA) hollow fiber substrate for treatment of dye solutions. Chemical Engineering Journal. 2018;351:1013-1026.

Team:NCKU Tainan/Hardware