Engineers at UC Berkeley have developed a new technique for making sensors for wearable technology that enables medical researchers to prototype-test new designs much faster and at a far lower cost than existing methods.
The technique replaces photolithography, which is a multistep process used to make computer chips in clean rooms. The new method uses a $200 vinyl cutter, which slashes the time to make small batches of sensors by nearly 90% while cutting costs by almost 75%, said Dr. Renxiao Xu.
“Most researchers working on medical devices have no background in photolithography,” Xu said. “Our method makes it easy and inexpensive for them to change their sensor design on a computer and then send the file to the vinyl cutter to make.”
Wearable sensors are used by researchers to gather medical data from patients over extended periods of time. They range from adhesive bandages on skin to stretchable implants on organs.
These devices consist of flat wires, called interconnects, as well as sensors, power sources, and antennas to communicate data to smartphone apps or other receivers. To maintain full functionality, they must stretch, flex, and twist, with the skin and organs they are mounted on — without generating strains that would compromise their circuitry.
To achieve low-strain flexibility, engineers use an “island-bridge” structure, Xu said. The islands house rigid electronics and sensor components, such as commercial resistors, capacitors, and lab-synthesized components like carbon nanotubes. The bridges link the islands to one another. Their spiral and zigzag shapes stretch like springs to accommodate large deformations.
In the past, researchers have built these island-bridge systems using photolithography, a multistep process that uses light to create patterns on semiconductor wafers.
The new technique is simpler, faster, and more economical, especially when making the one or two dozen samples that medical researchers typically need for testing.
The process starts by attaching an adhesive sheet of polyethylene terephthalate (PET) to a Mylar (biaxially oriented PET) substrate, although other plastics would also work, Xu said.
A vinyl cutter then shapes the structure by using two types of cuts. The first, the tunnel cut, slices through only the top PET layer but leaves the Mylar substrate untouched. The second, the through cut, carves through both layers.
This is enough to produce island-bridge sensors. Tunnel cuts are used in the upper adhesive PET layer to trace the path of the interconnects; the cut PET segments are then peeled off, leaving behind the pattern of interconnects on the exposed Mylar surface.
Next, the entire plastic sheet is coated with gold — or possibly another conductive metal. The remaining top PET layer is peeled away, leaving a Mylar surface with well-defined interconnects, as well as exposed metal openings and contact pads on the islands.
Sensor elements are then attached to the contact pads. For components such as resistors, a conductive paste and a common heat plate are used to secure the bond. Some lab-synthesized components, such as carbon nanotubes, can be applied directly to the pads without any heating.
Once this step is done, the vinyl cutter uses through cuts to carve the sensor’s contours, including spirals, zigzags, and other features.
The researchers produced a variety of stretchable elements and sensors to demonstrate the technique. One mounts under the nose and measures breath based on the tiny changes in temperatures it creates between the front and back of the sensor.
Another prototype consists of an array of water-resistant supercapacitors, which store electrical power like a battery but release it more rapidly.
“We could also make more complex sensors by adding capacitors or electrodes to make electrocardiogram measurements or chip-sized accelerometers and gyroscopes to measure motion,” Xu said.
Size, however, is sensor cutting’s one key limitation. Its smallest features are 200 to 300 micrometers wide, while photolithography can produce features that are tens of micrometers. But most wearable sensors do not require such fine features, said Xu.
The researchers believe this technique could one day become a standard feature in every lab studying wearable sensors or new diseases. Prototypes could be designed using high-powered computer-aided design (CAD) software or simpler apps made especially for vinyl printers.