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Having to Wear Reading Glasses As You Get Older May Soon Be a Thing of the Past

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Having to Wear Reading Glasses As You Get Older May Soon Be a Thing of the Past physicsworld.com Feature: Optics iStock/hmproudlove Looking – into the future Having to wear reading glasses as you get older may soon be a thing of the past. Helen Gleeson describes her team’s research into liquid-crystal contact lenses that will be able to switch focus and restore youthful vision We’re often told that with age comes wisdom. Unfor- But imagine if we could magically get rid of these Helen Gleeson is a tunately, this virtue also comes hand-in-hand with vision problems and you never had to wear glasses Cavendish Professor various downsides, notably our bodies no longer again? Is there a way to restore the vision of your of Physics and the functioning as they used to (as much as we may not youth so it is no longer a thing of the past? head of school at the want to admit it). And as we reach middle age and University of Leeds, lose count of the grey hairs, there is the inevitable Imperfect solutions UK, e-mail h.f.gleeson@leeds. prospect of having to put on a pair of glasses. There are several existing solutions for presbyopia, ac.uk Over the course of your life, it becomes ever harder but all are compromises and none fully restores to focus on objects closer to you because the crystal- youthful vision. The most common approach is to line lens in the human eye becomes more rigid. This use spectacles with different focal regions, known means the lens can no longer change shape as readily as bifocals or varifocals. The spectacle lenses are as it used to, so cannot focus on nearby objects – to shaped differently in the regions associated with use the technical term, the lens can’t “accommo- distance compared to those linked with reading, date”. This condition, known formally as presbyopia thereby forming different focal lengths. and informally as not-having-arms-long-enough-to- Bifocal contact lenses also exist and, as with hold-text-far-enough-away-to-see-properly, affects glasses, these work by having different focal regions everybody over the age of 50. A few people don’t in different parts of the lens. Selecting the right part think it has affected them, but in truth their distance of the contact lens relies on the brain distinguishing vision was already imperfect and presbyopia just between in-focus and out-of-focus images, together makes everything a little bit better. with the pupil generally being smaller in conditions Physics World February 2018 31 Feature: Optics physicsworld.com Inside an LCD lenses by virtue of them having a refractive index that varies with applied voltage. At first glance, our unpolarized light unpolarized light approach may seem more rooted in science fiction than reality, but we believe it could offer the eyesight polarizer electrodes of youth to older generations. From TVs to eyes Nowadays, liquid crystals are a massive part of everyday life, as they are used in the flat-panel liq- uid-crystal displays (LCDs) of billions of mobile E=0 E phones, televisions and laptops. These fascinating “soft” materials are characterized by the fact they are ordered fluids. There are many variants of liq- uid crystal because there are many ways of achieving order in a fluid, but it’s the so-called “nematic phase” used in LCDs that is of interest to us. liquid no light The molecules that form nematic liquid crystals polarizer crystal are usually rod-like and orientated such that their long axes point, on average, in the same direction – polarized light this is defined by a unit vector called the “director”. Liquid crystals have long-range orientational order, Liquid-crystal displays (LCDs) contain a thin film of a nematic liquid crystal so many of a nematic liquid crystal’s physical prop- roughly 5 µm thick, held between two transparent electrodes, which are erties, such as its refractive index, are anisotropic, themselves sandwiched between two polarizers. The liquid crystal molecules which means that the value of those properties varies have a specific geometry depending on the type of device, but in the most from one region to another, depending on the local common arrangement, known as a “twisted nematic” device, the “director” – the value of the director. average direction in which the nematic liquid-crystal molecules point – is aligned But because liquid crystals are fluids, the director to the polarizers, which are at 90° to each other. This means the director rotates is extremely responsive to external stimuli, such as through an angle of 90˚ across the layer. In the absence of an electric field (E), electric and magnetic fields, temperature and pres- unpolarized light becomes polarized as it enters the LCD. Its axis of polarization sure. Applying a voltage above a particular value, for gets rotated 90° by the liquid crystal and the light then passes through the example, orients the director either parallel or per- second polarizer, causing a bright state. When a sufficiently large electric field pendicular to the electric field’s direction, depend- is applied across the electrodes, however, the liquid-crystal molecules align ing on whether the dielectric anisotropy is positive or perpendicular to the plane of the device, changing its effective refractive index negative respectively. As the system’s optic axis (i.e. and suppressing the twisted arrangement. The light therefore gets completely the direction of optical symmetry) is defined by the absorbed by the second (crossed) polarizer, creating a dark state. director, it also reacts to the applied voltage. These are the properties any LCD devices rely upon (see box, left). where we read. This clever solution works well for some people, but it is, again, a compromise. Eye on the challenge In particular, reading in the dark is a problem with Based on the voltage-dependent refractive index, bifocal contact lenses. That’s because when it’s dark, researchers have been suggesting liquid-crystal your pupil is wide open to try and let in as much light lenses since the 1970s. But making a liquid-crystal as possible. But the dilation of the pupil exposes the contact lens that can be placed on the eye is not outer edges of the contact lenses – the parts usually easy. The overall device must be curved and less associated with focusing on distant objects. There’s than 300 µm thick – anything chunkier would not be also a lack of contrast as not all of the light passing comfortable to wear. The change in focus must be through the lens is focused on a single image, making around +2.0 dioptres, which is the measure of focus it difficult to distinguish small text. in optometry; most people with presbyopia need an Another common solution is so-called “monovi- additional +1.5 or +2.0 dioptres. The lens needs to sion”. This corrects one eye for good distance vision change focus faster than the eye can blink, meaning and the other for good near vision and can be done less than a second in practice. Ideally, it shouldn’t via contact lenses or laser surgery. Once again, how- be too expensive either, which means that a simple ever, this solution doesn’t work for everyone as some manufacturing process is important. And, somehow, people feel disoriented by having different foci asso- the lens needs to have a source of power so it can ciated with each eye and the approach falls short of change focus. Indeed, to make a device that stands restoring our vision to that of our youth. a chance of being used in the eye, it’s important that We’re working on another possible solution. The the lens can stay powered for at least a day. aim is to develop liquid-crystal contact lenses with We decided to use polymethylmethacrylate variable focus. Our team, which we set up a few (PMMA) as the base for the lens as this is a material years ago with staff at the UK-based firm Ultravi- commonly used for contact lenses. We then came up sion, brings together optometrists and experts in with a simple – and in our view elegant – solution optics and liquid crystals. We wanted to see if liquid based around “balanced optics”. It involved form- crystals could be used to make “switchable” contact ing a contact lens with three layers (figure 1), each of 32 Physics World February 2018 physicsworld.com Feature: Optics There are still some 1 Layered lenses liquid crystal challenges to be overcome director ITO before we get close to a off on top PMMA wire substrate thin layer of PMMA commercial switchable connection contact lens conductive paint lower PMMA substrate wire connection which is a lens in its own right. The bottom layer, a Mylar spacer substrate of PMMA, follows the curvature of the eye The liquid-crystal contact lenses rely on balancing three layers that act as lenses in their and is coated on the surface adjacent to the liquid own right. crystal with a transparent indium-tin-oxide (ITO) electrode and a polymer that aligns the liquid crystal when in the “off” state. Next comes the liquid-crys- wavelength-dependent and leads to different wave- tal layer, topped with another layer of ITO-coated lengths being focused at slightly different points. PMMA. Together, these three lenses are balanced. That’s because the aberration in the lens is less than Their curvature, the PMMA refractive index and the aberration of the cornea and the brain just sorts the “off”-state refractive index of the liquid crystal it out. Second, we don’t need to be concerned about are designed to provide either no vision correction off-axis rays that would experience a different focus or distance vision correction, depending on what is because of the liquid-crystal geometry.
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