“The system represents a new direction in personalised health care that will eventually enable diagnostics and therapy on devices that can be worn like a child’s temporary tattoo,” said Dae-Hyeong Kim, chemical and biological engineer at Seoul National University, who led the work.
"While still under development, the technology might someday be useful to sufferers of Parkinson’s disease or other movement disorders," said Massachusetts start-up MC10, which is working with the team to commercialise the underlying stretchable electronics.
This is a proof-of-concept device, with sensors, memory and drug delivery built using transfer printing as three separate assemblies on a single piece of conventional medical hydrocolloid bandage (see photo).
All three make extensive use of meander-line conductors, which are the technology of choice for elastic circuits. The meanders - serpentine shapes - are designed to convert stretching of the substrate into bending, rather than tearing, forces within the wide thin conductors.
Metals are prone to fatigue fracturing when bent so, in general, conductors on elastic substrates are sandwiched between two sheets of a durable polymer - polyimide for example - patterned to match the conductor's meandering route. When the sandwich is bent or stretched, the polyimide keeps the more fragile metal within the structure's 'neutral plane' where it is neither compressed nor stretched.
The sensor array uses resistive strain gauges to measure muscle activity. In this case, the meandering lines are boron-doped silicon (fine lines on right of adjacent picture), fabricated on a silicon wafer with their polyimide supporting layers and then transferred to the elastomer. Chromium and gold are plated over most of the length of each line to increase conductivity, leaving short sections of exposed silicon which changes resistance when bent. Sensing elements are orientated in both x and y directions on the elastomer, and are dispersed across a wide area of the strip.
The memory is built as a 10x10 array of storage locations, each at the intersection of meandering aluminium conductors (visible on the left of the photo, memory cells too small to be seen).
At each location is non-volatile metal-insulator-metal resistive RAM (RRAM).
In this case, the insulator between the aluminium is 130nm thick titanium dioxide. Passing current in one direction through the cell causes oxygen 'traps' to form. These hold electrons, forming a low-resistance conductive path from electrode to electrode, which can be sensed using voltages and currents below the write current. Reversing the write current erases the traps and sets the cell to a high-resistance state. Within the reported structure, a few layers of gold atoms have been deposited half way through the TiO2 layer which affects the traps in such a way to significantly cut operating current.
Operation is stable over 100 cycles of stretching by over 25%, and four-level (two bits/cell) operation was demonstrated.
Drug release relies on combination of a heating element and highly-porous silica particles infused with the drug. The heating element (visible through the sensor and hydrocolloid on the right) doubles as temperature sensors to monitor drug release.
Warming the silica causes bonding between drug and silica to fail, releasing the drug so that it can diffuse through the skin. So far the patch has been demonstrated releasing dye into pig skin, where it was found that higher temperatures drove the dye faster into the skin.
The patch is a combined set of elastic technology demonstrators rather than a complete system. Its three parts do not interact, nor is there processing, a battery, or wireless interface.
However, information taken from the sensors when the patch was exposed to different tremor frequencies has been stored in the memory at two bits per cell. Processing was done externally through a LabView programme (National Instruments).
“Ultimately we will develop a fully-automated system that incorporates these sensors and a memory- and drug-release mechanism together with a microcontroller to deliver automated drug release in an epidermal patch,” said MC10 founder Roozbeh Ghaffari.
Also involved were researchers from: The Korean Center for Nanoparticle Research and the University of Texas at Austin.
The work is described in a comprehensive Nature Nanotechnology paper: 'Multifunctional wearable devices for diagnosis and therapy of movement disorders'.
This is not the first elastomeric circuit for epidermal use.
In 2011, a team from University of Illinois at Urbana-Champaign and Northwestern University connected silicon mosfets, PiN diodes, InGaN LEDs, platinum resistance thermal sensors and loop aerials to make a stretchable epidermal wireless tag.
This showed silicon semiconductors in die form could be integrated onto an elastomeric substrate. It also used a novel form of flexible thin sinuous silicon mosfet.
Last year, US, Korean and Chinese researchers made an elastic battery.
In this case, a huge effort went into designing a serpentined-serpentine copper tracks where strain in the copper only reached 1% - said by the researchers to be the failure point of copper - after 300% elongation. At up to 150% elongation, the copper return completely to its pre-stretched state.
The photo shows the 10x10 memory array (left) and drug delivery array (right). The strain-gauge array is deposited on the back side of the elastomer behind delivery array.