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Posted: Apr 6th, 2011
Human-motion detection - nanotechnology takes interactivity to a new level
(Nanowerk Spotlight) Nanotechnology-enabled electronics of the future will be invisible, i.e. transparent (see "Invisible electronics made with carbon nanotubes") and flexible (see "Stretchy and conductive nanotechnology composite for robot skin and strain sensors"). Instead of rigid semiconductor chips we'll see flexible and stretchable electronic devices that will be embedded in almost any material imaginable. Rather than having dedicated electronic devices like music players, sensors, remote controls, cell phones, computers etc, the functions of these devices will be embedded into items of our daily lives – clothing, protective garments, packaging, furniture; they could be attached to skin or even to organs. Expect to see health monitors printed on T-shirts; intelligent sensors on doctors' surgical gloves; diagnostic devices embedded in your baby's diapers; or human-machine interfaces on data gloves for industrial use as well as computer games.
Many strategies to develop stretchable electronics rely on engineering new constructs from existing materials, e.g. ultrathin, stretchable silicon structures. Another approach is to fabricate ultrathin CMOS circuits on stretchable materials such as polymers (see "Nanotechnology electronics at the tip of your gloved finger").
"Nanotechnology allows a novel route to materials and structures that can be used to develop human-friendly devices with realistic functions and abilities that would not be feasible by mere extension of conventional technology," Kenji Hata, group leader of the Super Growth CNT Team, Nanotube Research Center at AIST in Tsukuba, Japan, tells Nanowerk. "Our research suggests devices that can act as part of human skin or clothing, and can therefore be used ubiquitously. We believe that such devices could eventually find a wide range of applications in recreation, virtual reality, robotics and health care."
Many applications of future electronics will be based on carbon nanomaterials – graphene and nanotubes. New research from Hata's group has now led to a new type of stretchable electric nanomaterial consisting of aligned single-walled carbon nanotube (SWCNT) thin films that deform when stretched in a manner similar to the structural deformation of a string cheese when peeled.
Reporting their findings in the March 27, 2011, online edition of Nature Nanotechnology ("A stretchable carbon nanotube strain sensor for human-motion detection"), first-authored by Takeo Yamada, the team realized a novel strain sensor that can measure and withstand strain up to 280%, with high durability (10,000 cycles at 150% strain), fast response (delay time, 14 ms) and low creep (3.0% at 100% strain).
Key steps in fabricating the SWCNT strain sensor. (Reprinted with permission from Nature Publishing Group)
"These important features allow the material to be used to precisely monitor large-scale and rapid human motion, as was demonstrated by embedding various strain sensors into clothing worn over the skin then using it to detect movement, typing, breathing and speaking," says Hata.
To fabricate their SWCNT film strain sensors, the team first grew vertically aligned and very sparse (3–4% occupancy) SWCNT thin films using chemical vapor deposition. Laying these films side by side, overlapping at the edges, and with the alignment of the SWCNTs arranged perpendicular to the strain axis, they were able to make long films of arbitrary length.
"For each iteration, we wet the film with a droplet of isopropyl alcohol, which flattened the film (thickness, 400 nm) to the substrate in a manner similar to deflating an air mattress," explains Hata. "This allowed the SWCNTs to be packed into a highly densely packed solid form."
This process resulted in the development of a strong van der Waals contact with the substrate, achieved without any additional mechanical pressure. The adhesion strength was measured as ∼12 N cm-2 and was sufficient to bear large strain.
Hata points out that two important advantages of the SWCNT strain sensor include its low creep and fast response. "We cycled the sensor between 5 and 100% strain at a speed of 10.6 mm per second and with a recovery time of 5 seconds," he says. "The response to this strain was fast, with a low overshoot of 3.0% and recovery time of ∼5 seconds. This is markedly different from the 8.8% overshoot and more than 100 seconds recovery time observed for polymer composites with conductive
fillers, even with a three times lower strain speed."
By its nature, the strain sensor is sensitive to temperature, because the expansion/shrinkage of the substrate caused by variation in temperature will be detected as a strain by the sensor. In addition, any environmental effect that influences the conductivity of carbon nanotubes would also influence the strain sensor. For example, exposure to gases is well known to dope carbon nanotubes and change their conductivity. Even breathing at it would register as strain.
"Packaging is therefore important in reducing the sensitivity of the strain sensor to environmental effects and to prevent damage from abrasion" says Hata. "To address this, we sealed the strain sensor with a PDMS coating. When sealed, the sensor behaved similar to when it was unsealed, with only a slight decrease in the gauge factor."
Stretchable wearable devices. Left: bandage strain sensor attached to throat. Right: strain sensors attached to data glove. (Reprinted with permission from Nature Publishing Group)
To demonstrate the potential of the SWCNT films in wearable devices, the researchers fabricated a stretchable human motion detector by connecting stretchable electrodes to the films and assembling them on bandages and clothing. Hata says that there is a wide range of potential applications in human motion detection: "When fixed to the chest, respiration could be monitored by the upward and downward slopes of the relative resistance associated with inhalation and exhalation (chest expansion and contraction). When attached to the throat, the device monitored phonation (speech) by detecting motion of the Adam's apple. Such devices might be useful in a breathing monitor for the early detection of sudden infant death syndrome in sleeping infants."
Another interesting application demonstrated by the team is a data glove made from five independent SWCNT strain sensors assembled on a single glove. "This interactive device could detect the motion of each finger individually and precisely, and the output of each gauge could be measured to assess the hand configuration" says Hata. "This device might be used as a master-hand to control a remote slave robot to remotely perform surgical procedures or to increase safety and speed of mine clearing operations."