How do temperature variations influence the signal in the Triggerfish contact lens sensor?

Variations in the shape of the profile of a 24-h continuous, indirect measurement of intraocular pressure with a contact lens sensor may possibly be influenced by artifacts. We examined the sensitivity of a contact lens sensor to changes in temperature that patients may experience during the measurement period.

Three Triggerfish contact lens sensors (Sensimed, Lausanne, Switzerland) were placed in a container with 0.9 % physiological saline solution. The temperature of the solution was either raised with a thermostat over a period of hours, rapidly cooled with crushed ice, or allowed to cool slowly to room temperature. The temperatures were recorded with two probes; the Triggerfish profiles were shown in millivolt equivalents (mV eq).

All three profiles mainly showed that the mV eq fell when the temperature increased, and rose when the temperature decreased, with a few deviations from this behavior. Rapid temperature change with ice produced pronounced spikes in the profile. The extent of change in mV eq was not uniform with slow changes in temperature but was always slight (between 0.5 and 3 mV eq per degree centigrade), though it was high with rapid cooling.

Our studies with the Triggerfish contact lens sensor showed that the sensor is sensitive to temperature changes. This is not surprising because the gauge that measures strain is made of platinum and titanium, and metals possess electrical and thermal conductivity. Apparently, the temperature sensitivity is not adequately compensated by the Wheatstone bridge used. The possible temperature sensitivity should be taken into consideration in evaluating the pressure profiles.