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Page 6 of 25 Lin et al. Soft Sci 2023;3:14 https://dx.doi.org/10.20517/ss.2023.05

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structure of the device. Right frames: optical micrography of the e-skin compass. Reproduced with permission from Ref. [41] . Copyright
2018. Springer Nature; (F) An active matrix consisting of micro-origami sensor arrays. Left frame: structure of the device. Inset of the
left frame shows the micrograph of the AMR sensors. Right frame: optical image of an integrated micro-origami magnetic sensor device
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with 8 × 8 pixels. Right frame: magnified view of several pixels. Reproduced with permission from Ref. [87] . Copyright 2022. Springer
Nature; (G) Schematic illustration of the structure and mechanism of a TMR sensor; (H) A flexible TMR sensor. Top frames:
transmission electron microscopy (TEM) images of the MTJ structure. Bottom frame: optical image of MTJs transferred onto nitrile
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glove. Reproduced with permission from Ref. [104] . Copyright 2016. John Wiley and Sons; (I) A film-type strain gauge with the
exchange-biased MTJ. Left frame: schematic illustration of the device. Top right frame: optical image of the motor-driven tensile
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machine for the sample. Bottom right frame: illustration of structure. Reproduced with permission from Ref. [107] . Copyright 2022. AIP
Publishing. AFM: Antiferromagnetic; AMR: anisotropic magnetoresistance; GMR: giant magnetoresistance; MTJs: magnetic tunneling
junctions; PET: polyethylene terephthalate; SCMN: stretchable and conformable matrix network; TMR: tunneling magnetoresistance.
One important direction of soft GMR sensors focuses on investigating the influence of different flexible
substrates on the performance of the GMR device, in order to develop magnetic sensors with desired
sensitivity, flexibility and mechanical endurance for applications in biomedicine or other bio-integrated
systems [75,76] . Figure 2B introduces electronic skins (e-skins) integrated with GMR sensors. Such e-skins
allow wearers to perceive the presence of static or dynamic magnetic field, thereby expanding the sensing
capability of the human body. Here, the highly sensitive GMR sensor elements are on an ultrathin
(thickness: 1.4 μm) polyethylene terephthalate (PET) foil with mechanical properties of light weight and
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high strength. The GMR sensors exhibit high sensitivities of up to 0.25% Oe , identical to their counterparts
on rigid Si/SiO wafer substrates. The e-skins are thin enough to provide an imperceptible feature during
2
[77]
wearing and can withstand cyclic tensile strains (270%, 1000 cycles) without fatigue .
The incorporation of photolithographic techniques into these magnetoelectronic nanomembranes on
ultrathin plastic foils enables the fabrication of devices with accurate patterns over large areas and in a
multiplexed array format. These resulting sensor arrays exhibit flexibility, stretchability, and mechanical
robustness and can integrate with other soft electronics to form a multifunctional system. Figure 2C shows a
skin-inspired, highly stretchable and conformable matrix network (SCMN) that combines multiple
functions, including but not limited to the sensing capabilities of temperature, in-plane strain, humidity,
light intensity, magnetic field, pressure, and proximity . The multilayer design [left frame of Figure 2C]
[78]
separates six different types of sensor units in different layers to avoid complicated wiring. The magnetic
field sensors exploit multilayers of Co/Cu as the GMR elements (MR ratios: 50%), and locate in the middle
of the multilayer stacks. The combination of magnetic field sensors with other devices allows for
simultaneous measurements of various signals induced by the external environment, providing immediate
applications in navigation, touchless control, and human-machine interface.

Other examples of soft electronics based on GMR nanomembranes include printable GMR sensors for low-
cost large-area production and easy integration with wearable devices and textiles [79,80] , highly integrated
magneto-sensitive electronic membranes for extensive applications in the field of interactive
[83]
electronics [81,82] , GMR 3D angular encoders with high angular accuracy in all directions , and many
more [84-86] .

Soft electronics based on AMR effect
Compared with GMR nanomembranes, whose resistance only depends on the intensity of the magnetic
field, the resistance change of AMR sensors depends on both the intensity and the direction of the magnetic
field. The AMR effect is an important physical phenomenon in spintronics, where the angle between the
current flow and magnetization direction determines the resistance of the ferromagnetic material
[Figure 2D]. Compared with GMR sensors, AMR sensors usually have a smaller resistance change under a
magnetic field. However, the capability to distinguish the direction of magnetic field underpins unique