”We’ve made autonomous robots 10,000 times smaller,” said Marc Miskin of the University of Pennsylvania. ”That opens up an entirely new scale for programmable robots,” he explained.
The appearance of such sub-millimeter robots is a robotics milestone in miniaturization. Sized approximately 0.2 × 0.3 × 0.05 millimeters a size common to many microbes each robot combines propulsion systems, sensory systems, computing systems, and energy systems sufficient to let them function for many months on just one penny per copy. Penn-developed robots with computing expertise from the University of Michigan can function for many months at just one penny per robot and perform digitally specified tasks without any external wiring or magnetic control.
Autonomous movement on this scale has been an issue for four decades. At the microscale, drag and viscosity are high. So much so that swimming in water would be akin to swimming in tar. Using limbs to move would be impossible since small components would be weak and difficult to manufacture. The solution devised by the Penn group would involve creating an electrokinetic system of propulsion suited to microscale physics. The robots would produce an electric field to move ions within the liquid it is in. The resulting ions would press against water molecules in its surroundings, causing it to move. The system has no moving parts and is highly robust. A micropipette could be used to transfer it from one sample to another.
This propulsion method takes a cue from other tiny inventions, in which actuators directly converted energy into motion of a fluid. Unlike chemically powered micromotors and magnetically controlled microengines, electrokinetics in these robots need low voltages and current. Moreover, these robots move at a speed of one body length per second, perform complicated patterns, and even move in a coordinated group manner, like a school of fish.
The computing core, which originated in David Blaauw’s lab at U-M, is a record-breakingly compact microcomputer designed for highly constrained power scales. Solar panels covering 97% of its surface area provide only 75 nanowatts of energy not a penny to a smartwatch’s spending budget. To accommodate fully autonomous behavior on such a tight allowance, researchers used ultra-low voltage circuits and a custom Instruction Set Architecture. Complex behaviors were reduced to a handful of specialized instructions, Brock explains.
Each robot is equipped with a processor, memory, temperature sensor, solar cells, and actuator control circuitry. Programs are uploaded using light pulses, each robot having a distinct identifier to enable personalized commands. After programming, the robots run autonomously, reacting to their built-in sensors. Temperature measurement has a discriminability of 0.3°C, allowing it to detect functions such as migration to hotter areas or indicating temperature changes to quantify cellular activity indirectly. Robots convey their data using characteristic motion patterns, much like honeybees’ “waggle dance,” decipherable using a microscope.
Sensing, processing, and propulsion packaged in a sub-millimeter platform can be applied in medicine and industry. Biomedical applications may include micro-fluidic routing, cell viability assessments for individual cells, and therapeutic drug delivery at high spatial resolution. Present designs function in hydrogen peroxide solutions, which are toxic to living cells, although modularity of the system enables substitution of more biocompatible actuators for those used in current designs. In industry, many of these robots can be applied to assemble micro-component parts in manufacture and represent a flexible approach to product production by existing fabrication methods.
The manufacturing technology utilizes the complementary metal-oxide-semiconductor (CMOS) approach that allows the mass production of robots by the hundred on a single chip. Additionally, the low unit cost and the reduced startup complexity that requires only an LED light source and imaging system will contribute to making the robots more accessible. Future versions could potentially offer increased speed, larger memories for more complex software, additional sensors, and the ability to operate in harsher conditions, added Miskin, who further observed, “Once you have that foundation, you can layer on all kinds of intelligence and functionality.” By virtue of having powerful, ion-powered propulsion, highly efficient computation, and strong sensing capabilities, such robots are a valuable platform for a micro-scale autonomous system.