Printable Magnetoresistive Sensors for On-Skin Interactive Electronics

Ultra-portable, imperceptible[1], and shapeable[2] devices are expected to be widespread due to the emergence of flexible electronics as an industrial technology. Printing is an affordable and high throughput method to process electronics in soft substrates that is still to be optimized to deliver electrically and mechanically reliable electronic devices[3].

In particular, printable magnetoresistive pastes have been developed as an alternative single-step fabrication method to obtain magnetic field sensors [4]. These pastes usually consist of composites of magnetic particles embedded in a non-magnetic matrix[5,6]. Particle-based pastes can achieve large magnetoresistance ratios at the expense of high resistivity and noise levels[5-7]. We previously reported magnetoresistive pastes based on microflakes as an alternative to overcome the problems presented in particle-based pastes[8,9]. Magnetoresistive flakes were produced after the delamination of thin-film stacks from a deposited sacrificial layer. With this technology, it was showed that flakes-based Co/Cu printed sensors exhibit low resistance and 37% GMR response at moderate magnetic fields (500 mT)[9].

Despite the advances in printable magnetic sensors, there are no reports of systems that show good sensitivity at low magnetic fields relevant for safe integration into consumer and wearable electronics. Electronics with magnetic components have to perform below the WHO limit of continuous exposure to magnetic fields (<40mT) to comply with this health standard[10]. Especially for on-skin electronics that experience considerable strain, there are not examples of magnetic printed sensors that deliver steady sensing behaviour after stretching.

Here, we will present low-noise printable magnetic field sensors sensitive down to sub-mT, which are mechanically stretchable after printing. We demonstrate the fabrication of printable sensors in ultrathin foils (3-μm-thick Mylar) based on magnetoresistive pastes that can undergo 100 % strain and 16 μm bending radius maintaining stable sensing and mechanical performance. The pastes are composites of poly(styrene-butadiene-styrene) copolymer (SBS) with embedded magnetoresistive microflakes. Using [Py/Cu]30 and [Ta/Py] flakes, we obtained printed giant (GMR)[11] and anisotropic (AMR)[12] magnetoresistive-based sensors, respectively. We address the key role of SBS to enable an enhancement of two orders of magnitude improvement in bendability and sensitivity at low magnetic fields.

Due to the good performance at low fields, reduced noise levels and high compliance, we will show the direct lamination of the printed sensors as an on-skin interactive device for scrolling through documents or digital maps. We envision that the proposed magnetic sensors will enable printing on-demand utilities for physical activity tracking systems or human-machine interfaces that can improve and even expand our sensing capabilities.

[1] M. Melzer et al., Nat. Commun. 6, 6080 (2015)
[2] D. Makarov et al., Appl. Phys. Rev. 3, 011101 (2016)
[3] Q. Huang et al., Adv. Mater. Technol. 4, 1800546 (2019)
[4] D. Makarov et al., ChemPhysChem 14, 1771 (2013)
[5] J. Meyer et al., Smart Mater. Struct. 22, 025032 (2013)
[6] J. L. Mietta et al., Langmuir 28, 6985 (2012)
[7] L. Ding et al., ACS Appl. Mater. Interfaces 12, 20955 (2020)
[8] D. Karnaushenko et al., Adv. Mater. 24, 4518 (2012)
[9] D. Karnaushenko et al., Adv. Mater. 27, 880 (2015)
[10] World Health Organization, Static fields (2006)
[11] M. Ha et al., (2020) [Submitted]
[12] E. S. Oliveros Mata et al., (2020) [Submitted]