Journal: Nano Letters
Affiliation: † Institute for Integrative Nanosciences, IFW Dresden, ‡ Leibniz Institute of Polymer Research Dresden, § Department of Materials Science, Fudan University, Material Systems for Nanoelectronics, Chemnitz University of Technology
For years researchers have been talking about flexible computer screens, solar cells, and a plethora of other flexible electronics that could be another revolution in technology. This paper investigates the possibility of stretchable magnetic sensors. These could have applications in biomedical systems and add functionality to the flexible electronic devices already developed.
In order to make a magnetic sensor device one must have a material with giant magnetoresistance (GMR). The researchers made GMR multilayers on Poly(dimethysiloxane) (PDMS) rubber membranes that were attached to silicon chips and they also made the same GMR multilayers directly on the silicon chips. The PDMS/GMR multilayer material could be removed from the silicon chip thereby creating a flexible material.
In order to study the GMR properties of the materials the authors calculated the GMR ratio – “the magnetic field dependent change of the sample’s resistance normalized to the value of resistance when the sample is magnetically saturated.” There was not a significant difference in GMR ratio between the multilayers on the PDMS attached to the silicon wafer, those on the PDMS after it had been peeled off the wafer, and those directly attached to the wafer.
Interestingly, the morphology of the multilayers on PDMS changed quite a bit after they were peeled off the silicon wafer. Using atomic force microscopy (AFM) the authors found a very smooth film on the unpeeled layers that develops wrinkles after it is removed from the silicon wafer. The authors attribute this to the manner in which the material was synthesized. The PDMS rubber was allowed to cool on the silicon wafer during the curing process. As it was already attached to the wafer it was not allowed to shrink at that time; however once removed from the wafer it could and did contract. The wrinkles allow for stretchability.
They also tested the electrical resistance of the GMR material on PDMS as it was stretched. The material showed very little increase in resistance at first, but when the strain went beyond 4% the resistance increased quickly and beyond 4.5% no electrical contact existed.
The change in the GMR was also studied as a function of strain. Very little change in the GMR effect was found with strains up to 4%. The sample could be repeatedly stretched to 1% strain and relaxed with little change in GMR ratio between cycles.
The authors suggest increasing the wrinkle amplitude by mechanically pre-straining the material instead of thermally pre-straining it. This would hopefully increase the ability of the material to stretch and still exhibit the desired GMR properties.