10.1117/2.1201004.002903 Millimeter-scale nearly perpetual sensor system Gregory Chen, Mathew Fojtik, Daeyeon Kim, David Fick, Junsun Park, Mingoo Seok, Mao-Ter Chen, Zhiyoong Foo, David Blaauw, and Dennis Sylvester A 9mm 3 measuring device harvests solar energy and consumes only picowatts of power, allowing collection of environmental data for decades until components wear out. By all accounts, wireless sensing is a growing field, with new technologies enabling sensors to operate in remote locations and under challenging conditions. These advances have created a deluge of emerging applications, of which many are highly vol- ume constrained. For example, doctors can implant millimeter- scale medical devices into the eye through minimally invasive surgery. These sensors can measure eye pressure to track the pro- gression of glaucoma and other ocular diseases. We have proposed a sensor that will deliver continuous pressure and temperature measurements (which represent just two of many possible sensing modalities). The measurements are digitized using capacitance- and time-to-digital converters, and delivered to an on-sensor processor that performs signal processing and logs measurement results into memory. The sys- tem is powered by solar cells and a thin-film lithium battery. Although miniature sensor nodes have been proposed in the past, they have all suffered from short device lifetimes. 1, 2 The root cause of this is that it is impossible to store an apprecia- ble amount of energy on such a tiny sensor node. Even with the most advanced battery technology, a millimeter-scale sys- tem using commercial circuit techniques would only operate for minutes before draining its power supply. To extend lifetime from minutes to decades, our system uses solar-energy harvest- ing to continuously recharge a battery (see Figure 1). 3 Ultralow- power operation enables us to bridge year-long gaps when no solar energy is available. Our system harvests energy under bright indoor- to sunny outdoor-lighting conditions. Solar energy is harvested on two series-connected silicon photovoltaic diodes. The solar cells have an output of 20nA–2A at 1V. However, most common battery chemistries have higher voltages. We have chosen a 3.6V 12Ah Figure 1. The system’s miniature size and nearly infinite lifetime enable operation in volume-constrained and hard-to-access locations, such as inside the human eye. Li: Lithium. SRAM: Static random- access memory. Ah: Ampere hours. ARM: Advanced reduced instruc- tion set computer machine processor architecture. (ampere hour) lithium battery supplied by Cymbet TM for the system, since lithium chemistries have high energy density. To recharge the battery, we use a switched-capacitor network (SCN) to pump up the voltage from the solar cells. Since the solar cur- rent is low because of the small photovoltaic area, this charge pump must be heavily optimized to prevent consumption of more energy than it converts. While solar-energy harvesting generates a nearly infinite power source, its availability is sporadic. Our system must be able to operate, or at least survive, during extended periods when there is no light. In the glaucoma-monitoring application, these zero-light conditions occur when the patient closes his eyes and goes to sleep. Our system uses ultralow-power operation to take sensor measurements without quickly depleting the battery. For ocular-pressure sensing and many other applications, readings taken every 10 or 15 minutes represent a ‘continuous’ measurement. This frequency is very slow on the timescale of the sensing and processing circuits used in the system. Thus, for the Continued on next page