Why researchers looked at tears
Tear fluid is a thin film on the front of the eye, and it contains water, salts, proteins, lipids and small amounts of molecules that also appear in blood, such as glucose and lactate. A contact lens already sits in that film for hours, against the eye, without needing a needle. For researchers interested in noninvasive measurement, that combination was appealing: a device people might already wear could, in principle, sample a body fluid continuously.
This article is a history of research projects, not a guide to health care. The idea of monitoring a condition through a lens has produced many papers and prototypes, but most of the sensing described here has not become a proven way to manage a medical condition.
Early sensor ideas
Chemical sensors based on enzymes predate smart lenses by decades. The standard approach to glucose measurement, for example, uses an enzyme that reacts with glucose and produces a small electrical current or a color change. By the early 2000s, researchers in several countries were asking whether such a sensor could be made small and flexible enough to sit on a lens. Early versions tested optical readouts, such as a fluorescent dye whose glow changed with the amount of target molecule, because these did not require any wires or power in the lens.
Other groups worked on electrochemical sensors, which need electrodes and a small circuit. Making them was harder, because a lens is curved, wet, transparent in the center and soft, while conventional electronics are flat, dry, opaque and rigid.
Building electronics for the eye
The main engineering problems fall into a few groups.
- Materials. Metals and silicon are hard, so researchers developed stretchable conductors, thin films, and in some cases transparent materials such as thin carbon-based layers. Work on graphene electronics and similar materials drew attention for this reason, although laboratory success did not guarantee a wearable product.
- Power. A lens cannot carry a normal battery. Prototypes used wireless power from a nearby coil or antenna, tiny energy-harvesting circuits, or very low power designs that depend on being read out from outside.
- Communication. Sending readings to a phone or reader required a small antenna and radio circuit, which had to fit around the edge of the pupil without blocking vision.
- Safety and comfort. Lenses must let oxygen through to the cornea, avoid irritation, and withstand the eye’s moist environment. Putting electronics in the lens makes all of these harder.
Small chips for such systems drew on the same advances in miniaturization behind biochips, which place sensing and processing on a tiny scale for biological samples.
Researchers and projects
In the late 2000s, a group at the University of Washington led by Babak Parviz demonstrated a lens with a small light-emitting element and a wireless power link, and reported tests in animals. Other university teams, including groups in Korea, Europe and the United States, published designs that measured glucose, lactate or other molecules in tears, usually in laboratory settings or in small animals, and in some cases in a handful of volunteers.
In 2014, a large technology company announced a research project on a glucose-sensing lens. It drew wide public attention, and the project was later reported to have ended.
Not all research aimed at glucose. Other targets have included markers of eye disease, dryness and inflammation. Measuring eye pressure, which is related to glaucoma, was taken further than most other ideas, and a lens for that purpose reached the European market around 2009.
The open scientific question
The central difficulty is not only engineering. Tear glucose, for example, tends to follow blood glucose only loosely, with delays and differences between individuals, and its concentration is far smaller than in blood. Tear composition also changes with stimulation, such as irritation or crying, and with the way a sample is collected. For these reasons, the link between a tear measurement and a medical decision remains uncertain, and many researchers have stressed that tears are not a simple substitute for blood.
Other approaches to continuous monitoring, such as small sensors worn on the skin, tied into connected sensors that send readings to phones, advanced faster and are the ones in routine use. These are discussed elsewhere; the lens approach remains mostly in the research phase.
Display and wearable ideas
A parallel line of work asked whether lenses could carry displays or augmented-reality content. The thinking was similar to that for the smartwatch design: put a small computer on the body where it is always available. Prototype lenses with a few light elements existed, but a lens that shows a real image remained a research goal rather than a consumer item.
Where the history stands
The development of the smart contact lens is a story of clever laboratory demonstrations meeting the difficulty of making a reliable, safe and meaningful medical measurement. The research has produced useful advances in flexible electronics and in understanding tears. Whether a lens will ever give measurements reliable enough to guide medical care is still unproven, and anyone with a medical condition should rely on devices that regulators have approved and on advice from a clinician.