Making skin sensors stick

(Nanowerk Spotlight) Research groups around the world are taking big strides towards developing ultrathin and flexible sensor devices that could be attached to the skin, or even organs, and monitor vital body functions.
One recent example is a wearable graphene-based piezoresistive strain sensor to monitor human vital signs. Another example is a wearable, low-cost nanowire sensor to measure skin hydration.
However, the adhesion to skin of many of these sensor patches is weak. A new milestone study on skin adhesives for wearable devices is about to change that.
Reporting their findings in Advanced Materials ("Bioinspired Composite Microfibers for Skin Adhesion and Signal Amplification of Wearable Sensors"), researchers from the Max Planck Institute for Intelligent Systems in Germany show that they can strongly and non-invasively attach soft wearable sensors and other devices to dry or wet skin.
"We have developed a facile method for superior conformation and adhesion of bioinspired composite microfibers to the hierarchical topography of soft and textured skin," Professor Metin Sitti, who led the work, tells Nanowerk. "Our soft and stretchable skin-adhesive micropatterns are composed of poly(dimethylsiloxane) (PDMS) microfibers decorated with conformal and mushroom-shaped vinylsiloxane tips. We show that crosslinking of these viscous tips directly on the skin surface can greatly enhance the skin adhesion through their excellent shape conformation to the multiscale roughness of the skin."
SEM side view image showing fibers of an adhesive film attached to an artificial skin
SEM side view image showing fibers of the adhesive film attached to an artificial skin replica. The red dashed line indicates the interface between the fibers and the skin replica. (Image: Physical Intelligence Department, Max Planck Institute for Intelligent Systems) (click on image to enlarge)
After optimizing the pattern geometries and processing parameters, the skin-adhesive films achieved high adhesion strength of up to 18 kPa.
The team chose vinylsiloxane as skin interfacing material due to its several features that can influence the skin adhesion: it is developed and approved for biomedical applications; as a two-component material possesses much faster cross-linking kinetics than other elastomers; its suitable viscosity enables successful transfer-patterning process and texture/roughness conformation; and it belongs to the family of silicone rubbers and allows covalent bonding with base PDMS microfibers.
As a proof of principle for a wearable device application, the team integrated their skin-adhesive films with wearable strain sensors for respiratory and heart-rate monitoring.
"We found that the signal-to-noise ratio of the strain sensor is significantly improved to 59.7 because of the considerable enhanced signal transfer of microfibrillar skin-adhesive films," says Dr. Dirk-M. Drotlef, the paper's first author.
An issue with the current adhesive films is that they can be reused only a few times. Going forward, the researchers are planning to improve the adhesive's repeatability performance for applications requiring a large number of cycles of skin attachment.
In order to be useful for commercial applications, these microfiber-based adhesives need to be mass-produced. To that end, the team is planning to use roll-to-roll type of continuous large-scale production methods.
"In addition to skin, our proposed composite microfibrillar adhesive films could attach to other surfaces with complex topographies and a wide range of surface roughness length scales under various dry and wet environmental conditions," concludes Sitti.
Michael Berger By – Michael is author of three books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: The Future is Tiny, and
Nanoengineering: The Skills and Tools Making Technology Invisible
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