1. Introduction

Recent developments in electronic contact lenses have generated enormous excitement in both the scientific community and the public domain, as the incorporation of functionality such as LED (Light Emitting Diode) pixels, LCD technology and medical sensors promise technology previously only seen in science fiction [1–3]. In this paper we describe a further development in electronic contact lenses, demonstrating that liquid crystal (LC) technology offers the potential to correct presbyopia, one of the biggest issues facing optometrists today. Presbyopia is the loss of eye’s ability to have variable focusing which usually leads to diminished near vision acuity with age, occurring in effectively 100% of the population aged over 50. Presbyopes typically require an additional lens with a focal power of + 1.00 to + 2.00 Dioptres (D) in order to perform near vision tasks (focal power is defined as the reciprocal of the focal length of the lens, described in D or m⁻¹). There is a growing need for the correction of presbyopia beyond existing approaches such as reading glasses, due to an aging population with a growing near vision demands which expects eyesight correction with minimal compromise [4]. In particular, using contact lenses to correct presbyopia is extremely desirable, but all contact lenses presently available for such correction have associated compromises such as a reduction in visual acuity [5]. Liquid crystal (LC) lenses have long been proposed as a solution for presbyopia correction [6] and switchable LC spectacle lenses have recently been commercialized (PixelOptics Inc., Roanoke, VA, USA). A key property of LC lenses is that they are switchable, with the application of a voltage activating the lens. The variation in optical power occurs via a change in the refractive index of the LC, induced by an applied voltage. An important aspect of the device described in this paper is that the change in focal length occurs by the direct application of a voltage across the LC layer so that operating voltages are lower than in some other LC and electrowetting lenses. In comparison with other work in the field of LC contact lenses, the device takes advantage of a LC layer of non-uniform thickness to correct presbyopia, rather than a uniform thick curved surface, which is more appropriate for display applications [3].

There are many types of LC lens design, including solid lenses [7, 8], refractive gradient index lenses [9–12] and diffractive lenses [13]. However, such devices are not generally appropriate for use in contact lenses, as the lens structures utilize glass substrates, and often suffer from either high driving voltages or complex fabrication methods. In this paper, we report a switchable polymethyl methacrylate (PMMA) LC lens which has all of the characteristics that would be necessary for adaptation into a contact lens. The device described produces an optical switch of + 2.00 ± 0.25 D when powered, perfect for presbyopia correction. Key issues affecting the development of a switchable LC contact lens are the substrate material, geometry, response time and need for low operating voltages; all of these concerns are addressed in the design and construction processes described here.

2. Optical design

The key requirements necessary in a switchable contact lens for the correction of presbyopia include: a lower substrate with a curvature appropriate for placement upon the anterior corneal surface (with an average mm radius of curvature of 7.8mm); a system which allows an increase of the optical power upon activating the lens; the use of plastic materials; and low operating voltages. Solid switchable lenses have been identified as offering the simplest method for providing a change in focal power when using curved substrates [7]. Previous work conducted with converging solid switchable lenses generally comprise a lens-shaped cavity filled with a LC, which experiences a reduction of focal power when switched. However, for a manageable contact lens device, an increase in focal power is required upon voltage application for both efficiency and to provide a fail-safe unpowered state for driving and other distance vision tasks. The design reported in this paper, shown in Fig. 1, uses a balanced optical system to overcome this limitation, with optical power contained in the substrates as well as the LC layer. The design shown in Fig. 1 provides −2.00 D optical power for plane polarized light in the direction of the LC director. The liquid crystal layer inside the device takes the shape of a negative meniscus lens, resulting in negative optical power arising from the liquid crystal layer. When the lens is unpowered, the converging optical power from the substrates is less than the diverging optical power from the LC layer (with refractive index n_e). Upon voltage application across the LC layer, the refractive index reduces towards n_o, and hence the diverging optical power of the LC layer is reduced via the change in refractive index of the LC. The change in optical power of the LC layer induces an increase in the total optical power of the system, and hence the lens is activated.

 

Fig. 1 (a) shows a diagram of the design used for the switchable LC lens. The base curve of the lens is designed to fit on the eye, with a radius of curvature of 7.8 mm appropriate for the average human cornea. The optical power of the lower substrate is + 0.50 D, the optical power of the upper substrate is + 6.00 D, with the optical power of the liquid crystal layer varying between −8.50 D and −6.50 D depending on the polarization direction of light or orientation of liquid crystal director. (b) is a photograph of the device while images shown in (c) are CAD renders of the lens.

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The liquid crystalline optical element used was designed based on the optical properties of the well-known LC 4-cyano-4'-pentylbiphenyl (5CB) to induce a switch between −2.00 D and 0.00 D, which provides an optical switch of ± 2.00 D, perfect for presbyopia correction. The radii of curvature for the device are suitable for placement upon the cornea, with a 4 mm diameter optical zone used to cover the pupil. The thickness of the LC layer varies between 50 µm in the center of the lens and 67 µm at the periphery of the optical area, and the substrates are between 100 µm to 150 µm in thickness, resulting in a total lens thickness of approximately 300 µm. The dimensions of the device are appropriate for contact lens use, with a reduction of thickness readily available for future designs.

3. Construction of the lens

PMMA substrates were selected due to the historical success of this material in the contact lens industry, with its rigidity important for the construction of the lens. The substrates were lathed from ophthalmic grade PMMA blocks using an Optoform 40 contact lens lathe (Sterling, Florida, USA) into the desired lens shape (tolerance was ± 0.5 µm for the prototype device). Prior to construction, the inside surfaces of the PMMA substrates were coated with a transparent Indium Tin Oxide (ITO) layer, allowing electrical switching of the device. The ITO coating process resulted in a 20 nm thick uniform layer of ITO across the substrates which was able to withstand the cleaning processes, addition of an alignment layer and attachment of powering wires. The ITO was unpatterned in this device with no significant variation in the ITO film thickness expected or observed.

The next stage of construction was the treatment of the interior substrate surfaces to provide a uniform planar alignment of the LC layer, which is challenging on curved surfaces. Due to the low glass transition temperatures associated with PMMA, polyvinyl alcohol (PVA) was identified as a suitable alignment layer. The PVA alignment layers were rubbed in a parallel configuration on the ITO-coated substrates. The substrates were placed together, filled with the liquid crystal and then sealed using a two-part epoxy. A finished prototype device is shown in Fig. 1(b).; the wires offer a convenient method of testing the prototype devices, but of course will not be present in a lens designed to be worn in the eye.

4. Optical testing and operation of the lens

An initial assessment of the quality of alignment of a test device was conducted using a polarizing microscope. Figure 2(a) and 2(b) show photographs of the optical textures of the LC layer viewed between crossed polarizers at 100x magnification. Both the bright and dark states are uniform, confirming a high degree of planar LC alignment in the lens. The intensity of transmission through crossed polarizers of a uniform, well-aligned layer of planar liquid crystal is described by $I=I_0{\sin }^2(2\theta ){\sin }^2(\delta /2)$ [14], Eq. (1) where I is the intensity of light, θ is the angle between the polarization of light and the liquid crystal director, and δ is the phase difference between the ordinary and extraordinary rays. Figure 2(c) shows the angular dependence of the light transmitted by the lens in the polarizing microscope, with the fit to the equation indicating the excellent alignment of the LC layer in the lens.

 

Fig. 2 (a) and (b) Polarizing microscopy photographs showing excellent LC alignment in the lens, with incident light polarized at 90° and 45° to the director respectively. (c) shows the light transmission measured with respect to the direction of polarization, with excellent agreement between theoretical and experimental data.

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The focal power of the lens was measured using an optometric focimeter; the optical power was determined to be −2.00 ± 0.25 D when unpowered; the accuracy of the focal power measurement of ± 0.25 D is typical for a rigid contact lens. Upon application of a voltage above threshold (~0.7V_rms for 5CB), the focal power decreases, taking values of 0.00 ± 0.25 D at higher voltages (see Fig. 3). It was not possible to measure the focal power using this approach at voltages just above the threshold voltage for 5CB (i.e. between ~0.8 V_rms and 1.3 V_rms) as defects form as part of the switching process, causing scattering.. The clear change in focal power observed in the lens in response to voltages as low as 2V_rms is ideal for switching between distance and near vision as required for presbyopia correction.

 

Fig. 3 Measurements of focal power with respect to applied voltage, measured using a focimeter. No measurement could be made for voltages between 0.8 - 1.3 V_rms due to scattering.

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The Point Spread Function (PSF) was measured to provide a quantitative measurement of optical performance of the lens. A 632 nm Helium-Neon laser beam passed through a beam expander and was subsequently imaged upon a CCD camera sensor using a lens and the liquid crystal device, with the resulting intensity profile used to measure the PSF as shown in Fig. 4. A neutral density filter was placed in the beam to reduce intensity and avoid saturation of the CCD sensor. The CCD camera was placed upon an optical rail to allow movement and to identify of the focal point. For measurements of the optical quality of the lens when powered and unpowered, two different lenses were used in conjunction with the liquid crystal lens. For analysis of the system when unpowered, a + 4.00 D lens was used, with the focal point approximately 50 cm from the lens. For analysis of the optical performance of the lens when powered, a + 2.00 D lens was used, and when the lens was powered the focal point was also approximately 50 cm from the system. A comparison of the off and on states is shown in Fig. 4, with a Full Width Half Maximum (FWHM) of 137 ± 10 µm for the on state focus and 125 ± 10 µm for the off state focus, suggesting a comparable quality of focus for the LC lens when powered and unpowered. Analysis of the Modulation Transfer Function (MTF) of the liquid crystal lenses yields similar values of MTF50 (the cut-off spatial frequency at 50% modulation and most relevant to vision) to that of empty lenses suggesting that limitations of the lens are due to the non-optimised construction processes rather than the liquid crystal layer. MTF50 values of different LC lenses were typically 50% - 70% of the diffraction limit, with ophthalmic grade rigid lenses typically diffraction limited with an MTF50 of 95% - 100% of the diffraction limit for comparison.

 

Fig. 4 PSF measurements of the LC contact lens in both powered and unpowered states. Application of 1.76 V_rms to the LC contact lens brings the laser beam into focus when using an additional + 2.00 D lens. The light intensity for each image set was normalized independently. The region shown is a 40 by 30 pixel area with a pixel size of ~20 µm.

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The response time of the lens is an important parameter to consider when assessing LC lenses for ophthalmic use. The time taken to bring an image into focus following the application of 1.76 V_rms to the lens was measured as 3.5 ± 0.5 seconds; an image of a light bulb filament being brought into focus is shown in Fig. 5. The response time, t_on, of a planar LC device is:

 

Fig. 5 Images of a light bulb filament with the liquid crystal lens and a + 11.50 D diffraction limited lens. Initially the image is out of focus (a). Upon application of 1.76 V_rms the lens is brought into clear focus after 3.5 ± 0.5 seconds (c) with brief, intense scattering occurring prior to the image formation seen in (b).

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4. Conclusions

A switchable, plastic LC lens has been described, with the geometry and materials employed suitable for contact lens implementation. The device has a very low operating voltage and can provide variable focal power up to + 2.00 D, perfect for presbyopia correction. LC lenses are usually limited by their size, but with the small optical areas associated with contact lens correction, this device offers a rare example of where contact lens correction is advantageous compared to other visual aids such as spectacles. It will be important in future designs to remove polarization sensitivity, which could be achieved using standard approaches employed for other types of LC lenses [16]. Further, the optical quality of the lens, although good, will need to be improved, as some scattering losses are obvious in Fig. 1(b). Finally, for a clinical product, a method of wireless operation is required, with many options available, such as wireless powering or small scale batteries [1]. None of these limitations offer insurmountable difficulties and the device is an important step towards realising switchable contact lenses.