AnSem is in the business of translating real world problems into ASIC solutions by matching up RF, analog and digital processing elements to transform real world inputs into the required outputs. Medical applications often have particular needs for ultra-low power consumption, high integration, small size and novel powering techniques. The choice of process is often make or break for such a project which is why AnSem works with a number of leading fabs in order to support its customers.
Sensimed AG, a Swiss company specialized in design, development and commercialization of integrated micro-systems for medical devices has developed a soft silicone single use contact lens called Sensimed Triggerfish for Glaucoma management and treatment – see figure 1
Figure 1: The Sensimed Triggerfish is a soft silicone single-use contact lens for Glaucoma management and treatment.
Glaucoma is an eye disease affecting about 4 percent of the population over 40 years of age which can lead to blindness unless treated early. One of the symptoms of glaucoma is an increased intraocular pressure. It has long been recognized that, since intraocular pressure varies during the day, continuous monitoring of glaucoma patients is needed. The static measurements currently performed by ophthalmologists during office hours are not able to detect pressure peak variations.
Passive and active strain gauges embedded in the Sensimed Triggerfish silicone lens monitor fluctuations in intraocular pressure through variations in diameter of the eye. The patient wears the Sensimed Triggerfish for up to 24 hours and undertakes normal activities including sleep periods. When the patient returns to his doctor, the data is transferred from the recorder to the practitioner’s computer via Bluetooth technology for immediate analysis – see figure 2
Figure 2: The lens monitors the fluctuations in intraocular pressure through variations in diameter of the eye.
AnSem was contracted to design the tiny data acquisition ASIC embedded within the lens. The ADC at the heart of the ASIC had to be so sensitive as to detect the change in eye pressure caused by the patients’ heart beat (ocular pulsation). Since there is no room for a battery within the structure of the lens, the system must power itself entirely from a localized magnetic field emitted by an RF antenna worn around the eye and connected to a portable data recorder through a thin flexible Data Cable – see figures 3 and 4
. The RF field provides both power to the sensor and the data back channel to the recorder.
Figure 3: The patch antenna transmits RF power to the lens.
The ASIC was designed to mount directly within the contact lens. Other than the RF antenna; a coil of wire with a specified inductance, and the strain gauge, there are no other electrical components in the system. The bumped die is attached directly onto copper traces within the lens during manufacture, connecting it to the antenna and MEMS array.
Being powered from an RF source meant that the device had to first bridge rectify the AC signal coming from the antenna. The obvious rectification method using schottky diodes would heavily constrain the process choice. Normal PN diodes on the other hand would exhibit too much power loss. Instead, AnSem designed a patent pending RF speed, voltage multiplying, low dropout active rectifier with superior dropout performance even than schottky diodes and that could be implemented on virtually any mainstream 5v tolerant process.