Magnetic field sensors are an integral part of many industrial and biomedical applications, and their utilization continues to grow at a high rate. The development is driven both by new use cases and demand like internet of things as well as by new technologies and capabilities like flexible and stretchable devices. Magnetic field sensors exploit different physical principles for their operation, resulting in different specifications with respect to sensitivity, linearity, field range, power consumption, costs etc. In this review, we will focus on solid state magnetic field sensors that enable miniaturization and are suitable for integrated approaches to satisfy the needs of growing application areas like biosensors, ubiquitous sensor networks, wearables, smart things etc. Such applications require a high sensitivity, low power consumption, flexible substrates and miniaturization. Hence, the sensor types covered in this review are Hall Effect, Giant Magnetoresistance, Tunnel Magnetoresistance, Anisotropic Magnetoresistance and Giant Magnetoimpedance.

The outstanding properties of graphene have initiated myriads of research and development; yet, its economic impact is hampered by the difficulties encountered in production and practical application. Recently discovered laser-induced graphene is generated by a simple printing process on flexible and lightweight polyimide films. Exploiting the electrical features and mechanical pliability of LIG on polyimide, we developed wearable resistive bending sensors that pave the way for many cost-effective measurement systems. The versatile sensors we describe can be utilized in a wide range of configurations, including measurement of force, deflection, and curvature. The deflection induced by different forces and speeds is effectively sensed through a resistance measurement, exploiting the piezoresistance of the printed graphene electrodes. The LIG sensors possess an outstanding range for strain measurements reaching >10% A double-sided electrode concept was developed by printing the same electrodes on both sides of the film and employing difference measurements. This provided a large bidirectional bending response combined with temperature compensation. Versatility in geometry and a simple fabrication process enable the detection of a wide range of flow speeds, forces, and deflections. The sensor response can be easily tuned by geometrical parameters of the bending sensors and the LIG electrodes. As a wearable device, LIG bending sensors were used for tracking body movements. For underwater operation, PDMS-coated LIG bending sensors were integrated with ultra-low power aquatic tags and utilized in underwater animal speed monitoring applications, and a recording of the surface current velocity on a coral reef in the Red Sea.

We report a biomimetic magnetic tactile sensor for reading Braille characters. The sensor consists of magnetic nanocomposite artificial cilia implemented on magnetic micro sensors. The nanocomposite is produced from the highly elastic polydimethylsiloxane and iron nanowires that exhibit a permanent magnetic behavior. This design enables remote operation and does not require an additional magnetic field to magnetize the nanowires. The highly elastic nanocomposite is easy to pattern, corrosion resistant, and thermally stable. The tactile sensors can detect vertical and shear forces, which allows recognizing small changes in surface texture, as in the case of Braille dots. The six dots of a braille cell are read from top to bottom with a tactile sensor array consisting of four elements and 1 mm long nanocomposite cilia.

The detection of small forces is of great interest in any robotic application that involves interaction with the environment (e.g., objects manipulation, physical human-robot interaction, minimally invasive surgery), since it allows the robot to detect the contacts early on and to act accordingly. In this letter, we present a sensor design inspired by the ciliary structure frequently found in nature, consisting of an array of permanently magnetized cylinders (cilia) patterned over a giant magnetoresistance sensor (GMR). When these cylinders are deformed in shape due to applied forces, the stray magnetic field variation will change the GMR sensor resistivity, thus enabling the electrical measurement of the applied force. In this letter, we present two 3 mm × 3 mm prototypes composed of an array of five cilia with 1 mm of height and 120 and 200 μm of diameter for each prototype. A minimum force of 333 μN was measured. A simulation model for determining the magnetized cylinders average stray magnetic field is also presented.

A magnetoresistive tactile sensor is reported, which is capable of working in high temperatures up to 140 °C. Hair-like bioinspired structures, known as cilia, made out of permanent magnetic nanocomposite material on top of spin-valve giant magnetoresistive (GMR) sensors are used for tactile sensing at high temperatures. The magnetic nanocomposite, consisting of iron nanowires incorporated into the polymer polydimethylsiloxane (PDMS), is very flexible, biocompatible, has high remanence, and is also resilient to antagonistic sensing ambient. When the cilia come in contact with a surface, they deflect in compliance with the surface topology. This yields a change of the GMR sensor signal, enabling the detection of extremely fine features. The spin-valve is covered with a passivation layer, which enables adequate performance in spite of harsh environmental conditions, as demonstrated in this paper for high temperature.