Sensors and actuators based on magnetic materials for haptic interfaces

Abstract

Haptic interfaces have attracted unprecedented interest and stand as a vital link between humans, computers, and robots for haptic information interaction. These interfaces rely on sensors and actuators to collect and output various haptic data, such as the coding of texture, roughness, vibration, shape, pressure, temperature or stiffness. Thus, tactile sensors and actuators based on different functional materials have been successively reported. Among electro-elastomer materials, thermoelectric materials, piezoelectric materials and magnetic materials, magnetic materials show new potential for the design of high-performance devices for haptic interfaces due to the rich coupling effects of magnetism with thermal, electrical, and mechanical properties, together with the excellent advantages of high adjustability, stability, wearability, rapid response time and low cost. In this perspective article, we provide an up-to-date overview of haptic interfaces utilizing magnetic materials. Firstly, the fundamental logical topology of haptic information interaction interfaces is introduced. Secondly, the coupling effects of magnetic materials for haptic device designs are described, and then, the structure and application of haptic sensors and actuators under different magnetic effects are summarized and discussed in categories. Finally, we delve into the developmental challenges faced, describe potential opportunities in this field, and summarize future directions for devices based on magnetic materials in haptic interfaces, aiming to summarize and enhance the collection of data available for the generation of future haptic systems.

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Yunlei Zhou

Yunlei Zhou received his PhD degree in Materials Science and Engineering from Nanjing University in 2021. Following his doctoral studies, he conducted postdoctoral research in Mechanical Engineering at Huazhong University of Science and Technology (HUST). Currently, he serves as an Associate Professor at the Hangzhou Institute of Technology, Xidian University, China. His research focuses on flexible sensors, stretchable circuits, and biomedical electronics, with an emphasis on developing innovative technologies for wearable and healthcare applications.

1 Introduction

Over the past decade, with rapid developments in computer graphics, smart materials, and integrated circuit manufacturing techniques, the haptic interface has become a dynamic emerging interactive feature of today's pioneering applications such as virtual reality, augmented reality, brain–computer interfaces, neurofeedback, intelligent robots, medical monitoring, health rehabilitation, and information accessibility.^1–7 The study of haptic interfaces with tactile sensors and actuators focuses on the tactile and kinesthetic sense/feedback through human interaction with the environment via touch.⁸ A recent review of haptic devices based on piezoelectric materials, magnetorheological fluids, shape memory alloys, liquid crystal elastomer polymeric compounds, and electroactive polymers for the visually impaired was introduced by Bhatnagar and colleagues.⁹ Chun et al. reported a T-skin film based on conductive particles in an elastic polymer matrix with piezoresistive and piezoelectric properties that can respond selectively to external pressure and vibration.¹⁰ Yu et al. presented a skin-integrated wireless multimodal haptic interface composed of complex actuator arrays that offer mechanical stimulation, electro-tactile stimulation and thermal stimulation as haptic feedback.¹¹ These studies confirm that advanced functional materials and device fabrication play key roles in the innovation of haptic interfaces, enabling tactile information to be both collected and produced.

The tactile information afforded by haptic interfaces is traditionally used in the coding of shape, vibration, pressure, texture, temperature, roughness, stiffness, and kinesthesis information, which can be sensed and generated by haptic sensors and actuators, respectively. For the tactile information perception function of haptic interface, many haptic sensors based on different transduction principles have been put forward, including capacitive, piezoresistive, thermosensitive, magnetically sensitive, thermoelectric, and triboelectric effects.^12,13 For example, nanowire-based strain sensors featuring resistive or capacitive mechanisms have been investigated for human motion detection.¹⁴ Liquid metals in elastomer-based microfluidic channels with piezoresistive response can be used for pressure detection.¹⁵ Rubber/carbon nanotubes were demonstrated to detect weak and large deformations, as well as human body temperature.¹⁶ For haptic sensors, the response range, sensitivity, multimode capabilities, and response time are important parameters that are directly related to the performance of the haptic interface. For the tactile information feedback of haptic interfaces, haptic actuators can provide vivid tactile information in an immediate, intuitive manner that is perceived by the user's skin.^17,18 Vibrational feedback from mobile devices is a common haptic technology for realistic interaction with virtual objects.¹⁹ Other technologies, including ultrasound and electrostatic forces, pneumatic, piezoelectric, and shape memory actuators, and micro-electromechanical systems, have also been reported that can change the surface friction, adhesion, temperature, viscoelasticity and morphology of a material.^20–23 Electro-tactile stimulation entails the use of an electrical current applied to the skin to arouse sensory nerves.²⁴ Organic stimuli-responsive materials, such as polymers and composites, are a class of materials that can change their oxidation state, conductivity, shape, and rheological properties, and thus might be useful in future haptic technologies.¹⁹ Sub-millimeter thickness, flexible hydraulically amplified electrostatic actuators are capable of both out-of-plane and in-plane motion, providing normal and shear forces to the user's fingertip, hand, or arm.²⁵ For haptic actuators, the stability, response time, controllability, stimulation safety, and comfort are important factors for developing high-quality haptic interfaces.

A range of haptic sensors and actuators based on piezoelectric materials, thermoelectric materials, shape memory alloy, and electro-elastomer materials have been fabricated and employed in haptic interfaces. However, compared with the human tactile perception ability in space and time, haptic sensors and actuators still face challenges such as low sensitivity, limited response range, low durability, and high cost, for the accurate measurement and generation of tactile information for haptic interaction.²⁶ Therefore, identifying and developing suitable smart materials to build high-performance haptic sensors and actuators remains an urgent task in the field of haptic interfaces.

Recent findings indicate that emerging magnetic materials offer distinct advantages for the design of haptic sensors and actuators for high-performance haptic interfaces. This is because magnetic materials exert rich magnetic effects and offer excellent physical properties such as high adjustability, low energy consumption, and wearability, while being relatively cost-effective. Examples of such materials are Sm–Co and Nd–Fe–B permanent magnetic materials.^27,28 Researchers are increasingly recognizing that magnetic materials under different magnetic effects have great potential for the design of high-quality haptic sensors and actuators. For instance, magneto-sensitive e-skins can enable flexible and soft human–machine interfaces that function under magnetic fields. Magnetic sensing can serve as a touchless detection platform that can unlock diverse interaction scenarios.^13,28,29 Magnetic actuators offer more controllable parameters compared with other actuation techniques (e.g., heat, chemicals, and pressure). This is because the actuating magnetic fields can be defined not only by their intensity but also by their directions and spatial gradients. Notably, magnetic fields present a safe and efficient means of manipulation for biomedical applications. These applications usually demand remote actuation within enclosed and restricted spaces.³⁰ For example, a shape-adaptive and reversible robot gripper skin featuring a magnetorheological effect can grasp different objects without inflicting any damage on them.³¹ Soft haptic actuators with electromagnetic effects have been realized that offer a vibrotactile feedback display.³² Our previous works on tactile actuators have also highlighted the advantages of magnetic materials with high response speed, small size, long-term endurance and low power consumption.^33,34 Based on the above analysis, we summarize the logical topology of haptic interfaces based on smart functional materials, as shown in Fig. 1.

Fig. 1 Logical topology of haptic interfaces based on smart materials. (a) Pioneering applications of haptic interfaces in virtual reality (VR), augmented reality (AR), brain–computer interface, intelligent robots, health monitoring, and information accessibility for the visually impaired etc.; (b) haptic sensors and actuators with the function to perceive and reproduce tactile information for haptic interactions, in the coding of shape, vibration, pressure, texture, temperature, roughness, and stiffness etc.; (c) smart materials, such as piezoelectric materials, thermoelectric materials, magnetic materials, shape memory alloys, and electroactive polymers etc. The magnetic materials are the key materials introduced in this paper.

Given that magnetic materials with versatile effects are being increasingly used in haptic interface applications, this article presents an overview of state-of-the-art haptic interfaces with magnetic materials. In Section 2, magnetic properties coupled with other physical fields (mechanics, electricity, and calorifics) are introduced, and the existing research and application scenarios of magnetic materials, classified according to magnetic effects, are also briefly summarized. In Section 3, we give a detailed review of the sensing and actuating devices under different magnetic effect principles. Various sensing mechanisms encompassing piezoresistive and magnetoelastic effects, capacitance changes and magnetoresistance are described in Section 3.1. Various actuating mechanisms, including electromagnetic force, shape programmable, shape memory, magnetic anisotropy and magnetic thermal effects are described in Section 3.2. Finally, in Section 4, we discuss the exceptional challenges and opportunities to achieve seamless compatibility between haptic sensors and actuators. We address the outlined challenges and describe the specific research directions that we anticipate will lead to the development of more sophisticated and immersive haptic interfaces based on magnetic materials.

2 Versatile performances of magnetic materials

As illustrated by recent progress, the functional applications of magnetic materials continue to expand, fostering innovation and exploration in various fields.^35–37 Recent advances in the integration of magnetic materials into objects have endowed them with diverse functionalities, including mechanical reinforcement, controlled movement, enhanced efficiency, deformation detection, and heat generation. This has ignited a growing interest in magnetic materials and their applications.³⁸ For instance, utilizing a magnetic field as a non-contact operation source allows for the orientation of magnetic fillers inside composites, resulting in anisotropic mechanical, electrical, thermal, and optical properties.^39,40 The wide interest in magnetic materials in haptic interfaces is mainly because magnetic properties are usually coupled with other physical fields (mechanics, electricity, and calorifics) to produce a variety of magnetic effects.

The coupling of magnetism and mechanics results in magnetorheological (MR) and magnetostrictive (MS) effects. Materials that exhibit MR effects possess the remarkable capability to transition between pliant elastomeric and semi-solid states, contingent upon the applied external magnetic field.^41–44 MR materials are promising candidates for constructing tactile interfaces designed to reproduce 3D shapes detectable by touch.⁴⁵ Materials featuring MS effects exhibit a capacity to contract or expand under an external homogeneous magnetic field.^46–48 MS sensors enable noncontact measurements, finding wide utility in vibration detection within aerospace, chemical, petroleum, engineering, and mechanical industries. MS actuators, boasting higher energy density and intrinsic robustness, could potentially surpass piezoelectric actuators in the realm of smart materials and devices.^49–51 Materials exhibiting magnetoelectric (ME) properties induce the emergence of an electric field across their structure when a magnetic field is applied.⁵² In ME materials, the magnetic and ferroelectric order parameters are mutually dependent. The potential applications of magnetoelectric multiferroics are substantial, encompassing functional devices such as magnetic sensors, power transfer mechanisms, magnetic memories, and spintronics. For instance, ME devices transform an externally applied magnetic field into a generated electric field via strain-mediated coupling, facilitating wireless power transfer.^53–58 Materials featuring magnetocaloric (MC) properties heat up when subjected to a magnetic field and cool down when the magnetic field is removed.⁵⁹ The magnetocaloric effect is based on the change in entropy of a material within a magnetic field and is derived from an intimate connection between the crystallographic structure and magnetism. This interplay induces concurrent changes in both magnetic and lattice entropies. With spin entropy as the principal contributor, the spin-dependent Seebeck effect responds to magnetic-field scaling, offering promising avenues for effective solid cooling and magnetocaloric therapy.^60,61

To sum up, magnetism can intricately intertwine with various physical fields, encompassing force, heat, electricity, and more, unveiling an array of magnetic properties, including magnetocaloric, magnetoelectric, magnetooptical, magnetoelastic, and magnetorheological effects.^62,63 These magnetic phenomena have found widespread application in sensing and actuating devices designed for haptic interfaces.^64,65 The main magnetic materials in these areas of research include carbonyl iron (CI), NdFeB, FeNi, FeCo, Ni, and metallic ferrite. These materials are blended with PVDF, PDMS, Ecoflex and other polymers to obtain compositions that greatly expand the multi-function and multi-purpose of the magnetic materials. Here, we give a summary of these magnetic effects for haptic interface applications, as shown in Fig. 2. Examples of magnetic materials that exhibit these magnetic effects are shown in Fig. 2(a)–(f), together with the corresponding components (shown in the orange circle). Detailed descriptions are provided in Section 3.

Fig. 2 Magnetic effects and magnetic materials for the exchange of tactile information. The examples of magnetic materials for haptic sensors are based on (a) magnetoresistance effects. Reproduced from ref. 66 with permission from Nature, copyright 2019. (b) Capacitive effects. Reproduced from ref. 67 with permission from Wiley, copyright 2020. (c) Magnetoelastic effects. Reproduced from ref. 68 with permission from Nature, copyright 2021. (d) Piezoresistive effects. Reproduced from ref. 69 with permission from Springer, copyright 2023. The magnetic materials for haptic actuators are based on (e) magnetocaloric effects. Reproduced from ref. 70 with permission from Wiley, copyright 2019. (f) Magnetic shape memory effects. Reproduced from ref. 71 with permission from Wiley, copyright 2019. (g) Program effects. Reproduced from ref. 72 with permission from Wiley, copyright 2021. (h) Anisotropy effects. Reproduced from ref. 73 with permission from Nature, copyright 2011. (i) Electromagnetic effects. Reproduced from ref. 32 with permission from Wiley, copyright 2018.

Magnetic materials showing different magnetic properties and different multi-physical field coupling properties offer obvious advantages for the collection and feedback of tactile information. The increasingly diverse array of haptic devices constructed from magnetic materials offers more precise sensing and actuating functions for haptic interfaces. In the next section, we discuss haptic sensors and actuators of magnetic materials and summarize recent research progress on haptic sensors and actuators based on magnetic materials as classified by device mechanism.

3 Haptic sensors and actuators of magnetic materials

3.1 Haptic sensors

Haptic sensors based on magnetic materials have demonstrated capabilities in detecting strain, pressure, hardness and magnetic field. The hardness is usually realized by controlling the vibration frequency. The coupling of magnetic properties with the mechanical and electrical attributes of magnetic materials provides new research avenues and performance breakthroughs for research on tactile sensing devices. The combination of magnetic and polymer materials gives increased flexibility, allowing the device to better fit the human body and facilitate the transfer of tactile information. These sensors can respond to changes in external pressure, stress, magnetic field, forward and shear forces, and output the corresponding resistance, current and capacitance. Most of these devices have response times in the millisecond range, which is their biggest advantage; however, the large response range of such devices and the high response sensitivity explain why magnetic materials are being widely explored. In addition, the magnetoelastic power generation and magnetoelectronic resistance effects of magnetic materials are unique properties that other functional materials do not possess; these properties can be used to overcome challenges and provide new uses for tactile sensors. In particular, the application of magnetoresistive effects in tactile sensors has enhanced the perception range of human skin. It opens new paths for the advancement of non-inductive electronic magnetic skin. Different preparation methods, including molding, spin coating, magnetron sputtering, and etching, also provide many possibilities for the structural design of devices. Various sensing mechanisms are involved in this endeavor, encompassing piezoresistive and magnetoelastic effects, and capacitance and magnetoresistance changes, as listed in Table 1. These sensors are integrated and used to measure physiological signals and monitor movement, and can even endow robots with a sense of touch.

Table 1 Examples of haptic sensors based on magnetic materials

Principle of device	Materials	Stimuli	Response variable	Sensitivity	Response time	Range	Manufacturing method	Applications	Ref.
Magnetically assisted piezoresistive sensor	CI flax fiber/PDMS/silver nanowires	Strain	ΔR/R0	—	—	—	Mold	Artificial skins and soft robotics	74
	Nickel-coated carbon fibers/PDMS	Pressure	ΔI/I0	15 525 kPa^−1	30 ms	50 Pa–100 kPa	—	Multiscenario healthcare monitoring, multiscale pressure spatial distribution, and human−machine interfacing	75
	Hair-like NdFeB/PDMS	Strain magnetic field	ΔR/R0	—	—	21 to 170 mT	—	Human–electronics interface devices and artificial electronic skin	76
	PDMS/graphene/Co	Pressure	ΔR/R0	0.4 kPa^−1		0.9 Pa–100 kPa	Coating	Healthcare, robotics, e-skin and smart surgical tools,	77
	PVA/GaInSn–Ni	Strain	ΔI/I0	—	—	—	—	Intelligent wearable devices	69
	CI/PDMS	Stain	ΔR/R0	—	—	—	Mold	Strain and pressure sensors	78
		Pressure	ΔR/R0	3.8 kPa^−1	—	0–160 kPa	Template method and reduction	Health monitoring and human-machine interaction	79
	Multi-walled carbon nanotubes/graphene/silicone rubber/Fe3O4	Strain	ΔR/R0	0.16%	100 ms	Up to 160%	Spin-coat	Pose recognition, non-contact sensing	80
Magnetoelastic sensor	Co-based ferromagnetic microwires	Force	Voltage	6.9 N^−1	—	0.1 mN to 25 N	—	Intelligent robot and sensing in harsh environments	81
	NdFeB@PDMS/Co-based amorphous wires	Strain	Impedance	0.0005%	100–200 ms	—	Mold	Health monitoring and interactive electronics	82
	NdFeB/Ecoflex	Pressure	Voltage	—	—	0–450 kPa	Extrusion processing	Wearable electronics	83
			Voltage	0.27 mA kPa^−1	—	0–6.5 kPa	Mold	Electronic textiles for energy, sensing and therapeutic applications	84
			—	—	3 ms	3.5 Pa–2000 kPa	Mold	Self-powered biomonitoring	85
	NdFeB/PDMS	Pressure	—	6.6% kPa^−1	40 ms	0 to 100 kPa	Mold	Monitoring hand rehabilitation	86
Magnetically assisted capacitive sensor	CI/NdFeB/PDMS	Pressure	Capacitive	0.314 kPa^−1	—	0 to 1000 kPa	Mold	Healthcare monitoring	87
	CI/PDMS	Pressure magnetic field	Capacitive	0.301 kPa^−1	300 ms	5 Pa–200 kPa	Micro-engraving technique	Health monitoring and body motion feedback	88
	Ag@Ni/PDMS	Pressure	Capacitive	—	50 ms	0–145 kPa	Spin-coat	Human health monitoring and intelligent soft robotics	67
	Ni–silicone rubber	Pressure	Capacitive	460 kPa^−1	0.4 s	0–200 kPa	Mold	Flexible tactile sensors	89
Magnetoresistance sensor	Micro-pyramid NdF eB/PDMS	Pressure	ΔR/R0	—	—	240 Pa–42.5 kPa	Wet etching; magnetron sputter deposition;	Augmented reality, robotics and medical applications	66
	Co/Cu/NiFe	Strain	ΔR/R0	—	—	—	Radiofrequency sputtering	Wearable bioelectronics	90
	Co/Cu	Stain	ΔR/R0	0.25%/Oe	—	Up to 270% strain	Ultraviolet lithography, magnetron sputter deposition	Healthcare monitoring, consumer electronics and electronic skin devices.	91
	Fe81Ni19/PDMS/Au	Magnetic field	Voltage	40–60 μT	—	Up to 1 mT	Spin-coat; photolithography	Electronic-skin compass	92
	NdFeB/PDMS	Pressure	Impedance	120 N^−1 (4.4 kPa^−1)	—	0 to ∼1 kPa	Mold	Tactile sensor for smart prosthetics	93
	Magnetic composite + Hall sensor	Normal force and shear force	Magnetic flux density	—	—	—	Mold	Adaptive grasping, dexterous manipulation and human-robot interaction	94
		External force position and magnitude	Magnetic flux density	—	—	—	—	Soft tactile sensors	95

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3.1.1 Piezoresistive-based sensors. The application of piezoresistive sensors based on magnetorheological elastomers (MREs) for detecting human motion has been demonstrated, and excellent performance has been achieved. The approach is distinguished by the extremely small drift or hysteresis during the overall motion detection process, specifically between dynamic loading and sensor responsiveness. Together with PDMS/CI, the relative resistance of an MRE sensor can reach 200% of its original value by applying a magnetic field of about 428 mT.⁷⁸ The mechanic–electric–magnetic coupling properties were investigated for conductive MRE, which can function as strain and pressure sensors.⁷⁸

A non-tensile coaxial piezoresistive fiber sensor using silver nanowires and CI flax fiber/PDMS MRE as a packaging shell was developed, as shown in Fig. 3(a).⁷⁴ The fiber demonstrates exceptional sensing performance, making it suitable for monitoring the movements of various human joints. Further devices based on biomimetic structure design have been reported. The nickel-coated carbon fibers/PDMS pillar forests can be used as an ultrasensitive multifunctional sensor, as shown in Fig. 3(b).⁷⁶ These forests are self-formed under the combined action of a magnetic field and loading pressure, which has outstanding resistance response and repeatability, and the resistance response varies in synchronization under an external magnetic field. This sensor can measure shear angles of less than 1°. The sensitivity coefficient reached 1965 under 50–60% compressive strain, and the magnetic field sensitivity was 240% T⁻¹ from 21 to 170 mT. Drawing inspiration from the structure and function of biological cilia, a flexible and dual-mode DMS/graphene/Co electronic cilia with P was constructed, as shown in Fig. 3(c).⁷⁷ PDMS serves as the matrix, imparting flexibility to the artificial cilia. Meanwhile, Co particles are utilized to endow the cilia with magnetic properties, and a graphene coating is applied to render the cilia conductive. This sensor shows a high sensitivity of 0.4% Pa⁻¹ for pressure from 0100 Pa and a low detection limit of 0.9 Pa. A magnetic field sensitivity of 12.08 T⁻¹ for magnetic field intensities from 150 to 160 mT was recorded. High sensitivity is closely related to the structure of the device. Sensors with biomimetic structures are effective examples of drawing wisdom from nature.

Fig. 3 Piezoresistive haptic sensors based on magnetic materials. (a) Coaxial fibers with a magnetoactive shell. Reproduced from ref. 74 with permission from Elsevier, copyright 2021; (b) hair-like magnetization-induced pillar forests. Reproduced from ref. 76 with permission from Wiley, copyright 2020; (c) electronic and magnetic cilia. Reproduced from ref. 77 with permission from Royal Society of Chemistry, copyright 2018; (d) PVA/GaInSn–Ni liquid metal hydrogel. Reproduced from ref. 69 with permission from Springer, copyright 2023.

Self-healing properties have also been researched for piezoresistive sensors. As shown in Fig. 3(d), the PVA/GaInSn–Ni composite can function as a strain sensor to detect body movements and as a signature sensor, which responds promptly and sensitively to diverse external stimuli.⁶⁹ When attached to the wrist, the PVA/liquid metal hydrogel shows increased current during wrist bending, with the curve of the current versus time exhibiting remarkable stability and reproducibility. MREs are an ideal research area for bionics and self-healing research on haptic piezoresistive sensors.

Piezoresistive fiber sensors, with a non-tensile coaxial structure, and ultrasensitive multifunctional sensors, with magnetization-induced pillar forests, further broaden the landscape of flexible and responsive sensing technologies. These wide-ranging innovations in sensors show great potential for stable and reproducible monitoring of joints, environmental stimuli, and body motions.

3.1.2 Magnetoelastic-based sensors. The magnetoelastic effect refers to the change in a material's magnetic characteristics when mechanical stress is applied. The magnetoelastic effects that emerge from the alignment of magnetic dipoles in soft magnetic fibers exhibit a stronger magnetomechanical coupling compared to those of traditional rigid metal alloys.^78,96 Among the soft MEG based on SrFe₁₂O₁₉, Fe₃O₄ and NdFeB micromagnets, the soft system with NdFeB micromagnets shows the largest magnetomechanical coupling factor of 7.19 × 10⁻⁸ T Pa⁻¹, which is four times that of their rigid counterparts.⁶⁸ This innovative approach opens new avenues for biocompatible self-powered bioelectronics, with applications in sensing and therapeutics using human-body-centered energy.^97–99

When external pressure is exerted on a textile MEG, its magnetic flux density is significantly changed. This change leads to the induction of a current in the textile coil. The MEG NdFeB/silicone matrix is expected to unlock broader application prospects within the bioelectronics domain. The Ecoflex/NdFeB MEG sensor is shown in Fig. 4(a). The electrical output of the MEG indicates that textile MEGs can be used to detect subtle pressures with a range of 0–6.5 kPa. Both subtle pulse motions and coughing can be detected, implying the possible applications of MEG sensors for midlevel motion detection. Meanwhile, 1D soft Ecoflex/NdFeB fibers with conductive yarns⁸³ can be used to detect pressures as low as 0.05 kPa, with a monitoring range spanning 0 to 450 kPa and a maximum magnetomechanical coupling factor of 1.05 × 10⁻⁷ T Pa⁻¹, as shown in Fig. 4(b). Notably, the NdFeB/SiO₂/Ecoflex MEG sensor can successfully detect pressures as low as 3.5 Pa with a large monitoring range of 3.5 Pa–2000 kPa; thus, cough assessment capabilities are possible due to the high sensitivity, as shown in Fig. 4(c).⁸⁵ Moreover, self-powered sensor arrays for human–machine interactions have been developed, which feature good wearability as well as water resistance.^100,101 The combination of the magnetoelastic effect and magnetic induction for electricity generation also suggests that the magnetoelastic effect can open doors to many other fields, such as coupling with magnetocaloric or magneto-optic effects.⁸³

Fig. 4 Magnetoelastic effect haptic sensors based on magnetic materials. (a) Textile Ecoflex/NdFeB for electricity generation from tiny pressures. Reproduced from ref. 84 with permission from Elsevier, copyright 2021. (b) Soft Ecoflex/NdFeB fibers with magnetoelasticity for wearable electronics. Reproduced from ref. 83 with permission from Nature, copyright 2021. (c) NdFeB/SiO₂/Ecoflex MEG sensor for monitoring coughing. Reproduced from ref. 85 with permission from American Chemical Society, copyright 2022.

3.1.3 Capacitive-based sensors. Of the various sensor types, capacitive sensors stand out because they offer advantages such as a straightforward structure, low energy consumption, and straightforward signal collection. However, conventional capacitive tactile sensors face restricted sensitivity using a solid dielectric material placed between electrodes due to the low compressibility of the solid dielectric. To enhance sensitivity, a proven strategy is to design a micro-structured dielectric with higher compressibility. Various microstructures, including micro-pillars, micro-pyramids, micro-domes, and micro-cilia, together with multi-layer structures, have been explored extensively.^87,102 The introduction of magnetic materials has made the response conditions of pressure devices more diverse and their applications more extensive. Drawing inspiration from the high tactile resolution of natural sensing tissues such as skin, some novel capacitive tactile sensors have been introduced.

As shown in Fig. 5(a), a CI/NdFeB/PDMS micro-cilia array has been used as an active layer.⁸⁷ With a high sensitivity of 0.314 kPa⁻¹ and a linearity range extending up to 1000 kPa, this system can satisfy the requirements for detecting physiological signals such as respiratory rates, artery pulses, and knee flexions, and can monitor changes during walking and running. Similarly, a sensor with a magnetic tilted CI/PDMS micropillar array as a dielectric layer exhibits a high pressure sensitivity of 0.301 kPa⁻¹ (0–2 kPa) with an ultra-low detection limit of 1.2 Pa, as shown in Fig. 5(b).⁸⁸ Featuring an optimal structure, the dual-mode sensor enables real-time tactile and touchless sensing, showing high sensitivity in magnetic field sensing (−0.69 T⁻¹ within the range of 25–400 mT) with fast response times (∼300 ms). Furthermore, flexible capacitive sensors with Ag@Ni/PDMS have been fabricated (Fig. 5(c)).⁶⁷ By adjusting the intensity and angle of curing, the formation of microneedles can be regulated. This enables the sensor to achieve a broad pressure sensing range (0–145 kPa), an extremely fast response time (50 ms), and an astonishingly low detection limit of 1.9 Pa.⁶⁷

Fig. 5 Haptic capacitive response sensors based on magnetic materials. (a) Bio-inspired hybrid dielectric tactile sensors. Reproduced from ref. 87 with permission from Wiley, copyright 2021. (b) A capacitive sensor based on magnetic tilted micropillar array structures. Reproduced from ref. 88 with permission from Elsevier, copyright 2020. (c) A capacitive-type flexible pressure sensor for finger bending. Reproduced from ref. 67 with permission from Wiley, copyright 2020.

These magnetic composite materials provide the sensor with double stimulation of pressure and magnetic field, which provides a design scheme for the multi-mode response of the sensor. With ciliated bionic design, both sensitivity and response time are significant advantages, providing opportunities for the application of capacitive sensors. Moreover, due to the MR effect, the fraction of the local volume can be increased under an external magnetic field; thus, particle rearrangement is enhanced during compression, resulting in improved sensitivity in capacitive devices with magnetic conductive polymers.⁸⁹

3.1.4 Magnetoresistance-based sensors. The development of electronic skins with the capability to sense magnetic fields holds profound significance as it enables the emulation of magnetoreception, a navigational and orientational skill observed in certain animals and insects.^103–105 When magnetic sensors are incorporated into haptic interfaces, they can dependably offer information regarding relative positions, distances, pressure, and motion. This functionality makes them valuable in a wide range of applications. For example, they are utilized in the development of soft robots, wearable electronics, and smart textiles. Additionally, they are applied in contactless human–machine interfaces, triggering systems, and are also employed for nondestructive material testing across different industries.^106,107 A variety of flexible magnetic sensors characterized by high sensitivity, minimal hysteresis, and high stability, have previously been constructed, based on giant magnetoresistance, tunnel magnetoresistance, spin valves, anisotropic magnetoresistance, Hall effects, and magnetoimpedance.^108–110

Taking inspiration from the functional multilayer structure of human skin, Yan et al. designed a tactile sensor in a sandwich structure for sensing external forces, as shown in Fig. 6(a).⁹⁴ When subjected to an external force, this sensor exhibits changes in magnetic flux, showcasing super-resolution and self-decoupling abilities comparable to human skin. Hu et al. introduce a wireless, flexible magnetic tactile sensor that has a multidirectionally magnetized flexible film and a non-contact Hall sensor, as shown in Fig. 6(b).⁹⁵ Composed of NdFeB and soft silicone elastomer microparticles, this sensor enables the clear conversion of the position and magnitude of an external force into magnetic signals.⁹⁵ Makarov et al. introduced a new magnetoreceptive platform designed for on-skin touchless interactive electronics.^66,111 This sensor, which is based on flexible spin valve switches with sensitivity to out-of-plane magnetic fields, can transduce both tactile (via mechanical pressure) and touchless (via magnetic fields) stimulations simultaneously. Highly flexible bismuth Hall sensors on polymeric foils have been fabricated that can be bent around the wrist or positioned on the finger to realize an interactive pointing device for wearable electronics.¹¹² The sensor has potential for diverse applications, including on-skin touchless interactive electronics.

Fig. 6 Haptic sensors based on a magnetoresistance response mechanism. (a) A soft tactile sensor that has self-decoupling and super-resolution capabilities. Reproduced from ref. 94 with permission from Science, copyright 2021. (b) A haptic sensor using flexible magnetic films based on the Hall effect. Reproduced from ref. 95 with permission from American Chemical Society, copyright 2022. (c) Imperceptible electronic skins Co/Cu giant magnetoresistance multilayer. Reproduced from ref. 91 with permission from Nature, copyright 2015. (d) Electronic-skin compass system based on the anisotropic magnetoresistance effect. Reproduced from ref. 92 with permission from Nature, copyright 2018.

Highly sensitive giant magnetoresistive sensor elements on a polyethylene terephthalate foil are shown in Fig. 6(c).⁹¹ Due to the lightweight, flexible, and durable nature of the magneto-electronic foil, this sensor can be electrically connected to thin copper wires with conductive silver paste and easily worn directly on the palm of the hand. An electronic-skin compass system based on anisotropic magnetoresistance effects with compliant and mechanical resilience is also presented in Fig. 6(d).⁹² The compass is constructed on a polymeric foil with a thickness of 6 μm and contains magnetic field sensors. Through the arrangement of these sensors in a Wheatstone bridge configuration, the highest sensitivity near the Earth's magnetic field can be achieved. This system attains linear responses and peak sensitivity close to the Earth's magnetic field. It paves the way for interactive devices in virtual and augmented reality applications, as demonstrated by the touchless control of virtual units within a game engine.

The development of electronic skins (e-skins) equipped with magnetic sensing capabilities represents a significant breakthrough. Additionally, combining magnetoelectronics with ultrathin functional components such as solar cells, transistors, light-emitting diodes and tactile sensor arrays,¹¹³ could lead to the development of self-sufficient and multi-functional smart systems equipped with a wide range of sensing and actuation capabilities. Thus, the future of electronic skins appears promising, offering innovative solutions for delicate grasping, human–robot interaction, flexible manipulation, and touchless control in diverse applications.

3.2 Haptic actuators

Haptic actuators based on magnetic materials have a demonstrated capability to offer users a diverse array of stimuli, encompassing vibration, temperature, texture, hardness, and roughness. Magnetic actuators stand out for their precise and rapid response to input signals, and they can be controlled wirelessly in confined spaces.^114,115 The intricate structure of our skin, with innervation densities reaching thousands of sensory neurons per square centimeter, poses challenges for haptic perception,^116,117 but contributes to the skin's remarkable spatial and temporal tactile acuity.¹¹⁸ Haptic actuators play a crucial role in mobile devices by providing discreet and private feedback channels, generating touch sensations that enhance realism and depth in the interactions between humans and machines.^119,120 The underlying principles of magnetic materials in haptic actuators encompass electromagnetic, shape programmable, shape memory, magnetic anisotropy and magnetic thermal effects, as listed in Table 2, providing a versatile foundation for creating a rich and nuanced haptic experience.²⁰ These devices can achieve changes in shape, temperature, etc. through the application of electric and magnetic fields and generate tactile displays. The deformable magnetic actuators can be used as intelligent robot grippers. Variations in interface shapes can offer novel visual and haptic experiences. These devices, through their innovative designs in terms of principle implementation, material composition, and device form, have significantly enhanced the interactivity of the haptic interface. These advances have enabled haptic interfaces to become increasingly effective in applications ranging from rehabilitation and training to teleoperation and navigation controls, significantly enhancing user mobility and overall experience.

Table 2 Examples of haptic actuators based on magnetic materials

Principle of device	Materials	Control	Output	Main methodology	Applications	Ref.
Electromagnetic actuator	NdFeB	Cu coil	Vibrotactile	Mold	Vibrotactile feedback display	121
		Air-coil	Vibrotactile	Mold	Vibrotactile feedback display	122
	NdFeB/PDMS	LM coil	Vibrotactile	Roller coating and stencil printing process; photolithography and wet-etched	Vibrotactile feedback and soft robotic gripper	32
Shape programmable actuator	NdFeB	Magnetic field	Shape change	3D print	Actuator or robots	123
	CI/PDMS	Magnetic field	Shape change	3D print	Soft robots	124
		Voltage	Shape change	Mold	Intelligent devices and bionic robots	125
	NdFeB/Ecoflex	Magnetic field	Shape change	—	Flexible electronics and soft robotics	126
Magnetic shape memory actuator	NdFeB/Fe3O4/SMP	Magnetic field	Shape change	Reprogrammed	Actuator or robots	127
	PLA/TPU/Fe3O4	Magnetic field	Shape change	Single-screw extruder	Actuator or robots	128
Magnetic anisotropy actuator	Colloidal nanocrystal clusters	Magnetic field	Shape change	Spin coat	Multi-directional movement controls	73
	FeCo magnetic nanocubes	Magnetic field	Shape change	Mold	Sub-millimeter actuator	129
Magnetocaloric actuator	O-GaIn	Alternating magnetic field	Temperature	Painting	Hyperthermia therapy	70
	Fe3O4/Au	Microwave	Temperature	—	Microwave dynamic therapy	130
	Zn ferrite	Magnetic field	Temperature	Thermal decomposition; magnetic separation	Localized hyperthermia therapy	131

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3.2.1 Electromagnetic actuators. Electromagnetic actuators, relying on electromagnetic fields for electromechanical energy conversion, are prevalent in the market, with eccentric rotary mass actuators and linear resonant actuators being typical.^132–134 In these actuators, vibrotactile rendering stands out as an effective method for conveying natural, intuitive, and rich information, offering a balance between complexity and cost. The generated cues not only supply objective information such as the timing and intensity of a contact event,^135,136 but may also trigger an emotional response by adjusting the waveform, rhythm, frequency and amplitude of the signal.¹³⁷ Researchers have attempted to substitute copper wires with liquid–metal coils or replace the supporting hard shells with flexible PDMS substrates. This renders linear resonant actuators as promising candidates for soft robotic and wearable devices.^33,138,139 Numerous experimental prototypes of Braille display devices that utilize electromagnetic actuators have been put forward.^33,140–144 Such tactile devices are anticipated to find applications in large-sized texture displays, multimodal tactile displays, Braille displays and VR/AR. However, challenges related to complex designs, limited deformability, and insufficient durability persist.^32,145

To address these challenges, Visell et al. presented a fabrication strategy for miniature soft electromagnetic actuators founded on the Lorentz force principle using the liquid–metal alloy EGaIn, silicone polymer and NdFeB magnetic powder, as shown in Fig. 7(a).³² These intrinsically deformable actuators demonstrate high-frequency linear motion and bending capabilities, showcasing applications in vibrotactile feedback displays and miniature soft robotic grippers. Jung et al. introduce a lightweight, flexible technology for displaying vibrotactile patterns across large skin areas, offering a reduction in size and weight compared to previous systems (Fig. 7(b)).¹⁴⁶ The approach can potentially provide full-body coverage at densities that meet or exceed two-point discrimination thresholds at nearly all anatomical locations. Similarly, a vibrotactile actuator and e-skin haptic interface with a diameter of 5 mm have been designed, allowing the construction of a 3  ×  3 actuation array that can generate cutaneous stimuli normal to the fingertip (Fig. 7(c)).¹⁴⁷ During the user's interaction with virtual objects, the vibrotactile stimulus, along with visual and audio feedback, was produced in real-time on the user's fingertip. Furthermore, a soft elastic sheath integrated with a rigid magnet was employed to transmit expressive signals exceptionally well, allowing users to engage in various tasks while wearing the device, as shown in Fig. 7(d).¹²² In the realm of haptic displays, vibrotactile actuators currently dominate due to their widespread use and clearer sensorial stimuli, surpassing other approaches that modulate sensations like static pressure, skin stretch, or friction.¹³⁶

Fig. 7 Vibrotactile actuators based on electromagnetic effects. (a) Miniature soft electromagnetic actuators. Reproduced from ref. 32 with permission from Wiley, copyright 2018. (b) Schematic of the haptic interface with electronic components, battery bank, haptic actuators and encapsulation structures. Reproduced from ref. 146 with permission from Nature, copyright 2022. (c) Exploded-view and top-view schematic of a mini electromagnetic actuator. Reproduced from ref. 147 with permission from Nature, copyright 2021. (d) A user wearing a magnetic sheath with an attached air-coil to feel realistic vibrotactile cues in virtual reality. Reproduced from ref. 122 with permission from Mary Ann Liebert, copyright 2023.

Haptic interface devices employing Lorentz force magnetic levitation represent a notable advance in interactive displays. The application of electromagnetic forces in these devices allows coils within the display to directly exert forces on a permanent magnet in the stylus or attached to the fingertip.^148,149 Adel et al. developed a position-based impedance electromagnetic haptic interface with 50 mN electromagnetic force.¹⁵⁰ These magnetic levitation haptic devices offer distinct advantages, characterized by the absence of mechanical friction and the potential to deliver high-precision haptic feedback.^151,152 A haptic interface with controllable stiffness was designed and fabricated by leveraging the advantages of MR fluids. Some recently developed haptic interfaces and rehabilitation devices based on MR fluids in medical applications and healthcare have been reported.^153,154 These encounter-type haptic interfaces, employing MR fluid, are designed and evaluated for surgical simulation, showcasing the potential for transformative applications in the medical field.

Vibrotactile rendering has proved to be effective in conveying natural and rich information, striking a balance between complexity and cost, and is prevalent in the market. Lorentz force magnetic levitation represents a notable advancement, offering high-precision haptic feedback without mechanical friction. These innovations open avenues for transformative applications in areas such as virtual reality, augmented reality, and medical simulations. Continued research and development in vibrotactile technologies and electromagnetic haptic interfaces are expected to overcome existing challenges and unlock new possibilities for interactive displays and medical applications.

3.2.2 Magnetic shape programmable actuators. With advances in magnetic field control,¹⁵⁵ soft active machines that can change shape and move in reaction to external stimuli show great potential in various fields such as miniature surgical devices,¹⁵⁶ actuators,¹⁵⁷ soft robots,¹⁵⁸ and flexible electronics.¹⁵⁹ In particular, magnetic actuation is extensively utilized owing to its wireless control, rapid response, and wide penetration range.^160,161 Therefore, shape-programmable materials can present integrated multifunctional shape adjustments and sequential activation, which include reprogrammable, wireless and reversible shape changes. They can respond to external stimuli such as heat, light, or a magnetic field, with the latter varying particle loadings for heating purposes.^162,163 Soft materials with magnetic responsiveness have evolved that can produce nonuniform magnetization profiles in polymeric sheets.

3D printing is an efficient method for crafting responsive materials with rapid prototyping capabilities. With an externally applied field, magnetized NdFeB particles can realign in the direction of the field. Kim et al. reported that, through 3D printing, programmed ferromagnetic domains could be created in soft materials with complex 3D shapes and transformed by magnetic actuation (Fig. 8(a)).¹²³ A new type of magneto-capillary 3D-printed soft actuator is presented in Fig. 8(b).¹²⁴ These actuators consist of silicone–iron particle composite filaments. The reversible, shape-dependent response can be used for multifunctional soft robots, such as a soft extending grabber that could stretch out and seize an object floating on the surface of water. The applications demonstrate the use of magnetic shape programmable actuators as intelligent soft actuators and as robots interacting with objects on water surfaces but also open new avenues for the design and rapid fabrication of programmable materials with exotic properties.¹⁶⁴

Fig. 8 Shape programmable actuators based on magnetic materials. (a) Design of the ferromagnetic domain within soft materials produced by 3D-printing. Reproduced from ref. 123 with permission from Nature, copyright 2018; (b) 3D-printed extendible surface grabber activated by turning on a magnetic field. Reproduced from ref. 124 with permission from Wiley, copyright 2019; (c) geometry, magnetization distribution, and operational principle of a magnetic microgripper based on NdFeB/UV resin. Reproduced from ref. 115 with permission from Science, copyright 2019; (d) reconfigurable soft magnetic actuators based on Ecoflex/NdFeB/PEG. Reproduced from ref. 126 with permission from American Chemical Society, copyright 2020.

One research direction of shape programmable actuators is to achieve the actuation of flexible magnetic films by patterning magnetic particles in polymer films through magnetic rewriting. A four-arm NdFeB/UV resin magnetic gripper, constructed by molding with programmable three-dimensional magnetization, is shown in Fig. 8(c).¹¹⁵ The time required to fabricate these magnetic microgrippers, with different numbers of arms to target various cargo shapes, can be less than 20 min. Song et al. synthesized a new soft magnetic composite material, Ecoflex/NdFeB/PEG, as shown in Fig. 9(d).¹²⁶ By controlling the polymer phase transition, the magnetization profiles of these materials can be rewritten by physically realigning the ferromagnetic particles. This innovation enables the development of diverse magnetic actuators with reprogrammable magnetization profiles.

Fig. 9 Actuators based on magnetic shape memory polymers. (a) Magnetic shape memory polymers embedded with NdFeB and Fe₃O₄ particles. Reproduced from ref. 127 with permission from Wiley, copyright 2020. (b) Sequential shape memory recovery process of a bionic flower structure based on PLA/TPU/Fe₃O₄. Reproduced from ref. 128 with permission from Elsevier, copyright 2023.

Integrated multifunctional shape manipulations are further exploited for applications including soft magnetic grippers with large grabbing force, reconfigurable antennas, and sequential logic for computing. As advancements in magnetic field control and 3D printing continue, the development of magnetically responsive soft materials holds tremendous promise for shaping the future of soft robotics, actuation, and flexible electronics.

3.2.3 Magnetic shape memory actuators. Magnetic shape memory polymers (SMPs) are part of the shape memory material category. These materials can return to their pre-set “remembered” shapes once they are subjected to an external magnetic stimulus,^165,166 distinguishing them from other materials activated by temperature, light, pH, or electricity. Magnetically activated shape memory materials have attracted significant attention in the biomedical field. Their key advantage lies in the fact that they do not rely on heat for activation and can be controlled remotely. This characteristic renders them safer and less invasive when introduced into the human body, thus opening promising applications in various medical procedures and treatments.^167,168 Magneto-responsive SMPs, consisting of an SMP polymer matrix with embedded magnetic particles, can be effectively actuated by a contactless and harmless magnetic field. The magnetically induced shape-memory effect will extend the concept of noncontact triggering significantly.

Fe₃O₄ particles can generate inductive heating under a high-frequency alternating current magnetic field. This property allows them to be used for shape locking and unlocking. The NdFeB particles are magnetized for programmable deformation when an actuation magnetic field is applied.¹²⁷ Embedded with NdFeB and Fe₃O₄ particles, magnetic shape memory polymer cantilevers can be magnetized with a desired magnetic profile, as shown in Fig. 9(a). The matrix can undergo rapid and reversible shape changes through the manipulation of embedded magnetic particles, resulting in a locked configuration upon cooling.

The shape memory mechanism is also observed in polylactic acid/thermoplastic polyurethane/Fe₃O₄ (PLA/TPU/Fe₃O₄) composites, as shown in Fig. 9(b).¹²⁸ When the temperature was higher than the glass transition temperature (T_g) of PLA, the amorphous region of PLA transitions from a glassy state to a temporary shape. Upon cooling, the composite reverts to a rigid state, fixing the deformed shape and storing elastic energy within the blend matrix. The shape memory effect was experimentally validated using a magnetic induction collar, capturing a sequential blooming process.

Furthermore, the temperature-dependent stiffness of SMP, coupled with pre-stress control through magnetic torque, enables a single SMP system to exhibit untethered, reversible, and reprogrammable deformations with shape locking, showcasing the potential for advanced applications in diverse fields.¹⁶⁹ These innovative materials for magnetic shape memory actuators hold great potential for advanced applications across diverse fields, combining the benefits of magnetically induced shape-memory effects with the flexibility and programmability of shape memory polymers, such as noncontact actuation, safety in biomedical applications, and remotely programmable deformations. However, challenges related to complex fabrication, limited magnetostrictive strain, material cost, and temperature sensitivity should be addressed for widespread and efficient implementation in various applications.

3.2.4 Magnetic anisotropy actuators. A magnetically anisotropic material endeavors to orient its magnetic moment along one of the easy axes, which represent the directions in which magnetization is energetically most favorable. Superparamagnetic materials, characterized by aligning magnetic moments with an external magnetic field without spontaneous magnetization or hysteresis, enable unprecedented versatility in microactuator design. This novel approach facilitates diverse movements within a microactuator, as each part gains distinct magnetic anisotropy, responding uniquely to external magnetic fields.

A magnetic actuator reported by Kim et al. was fabricated from superparamagnetic colloidal nanocrystal clusters and a photocurable resin monomer solution,⁷³ as shown in Fig. 10(a). Without an external magnetic field, the nanoparticles are randomly dispersed in a photocurable liquid resin. Upon the application of an external magnetic field, these superparamagnetic nanoparticles undergo self-assembly, forming chain-like nanostructures along the magnetic field lines to minimize the system's magnetic dipole interaction energy. Remarkably, when the magnetic field direction is changed, all the chains rapidly rotate or realign along the changed magnetic field direction. Chen et al. reported magnetic microactuation using nanostructure assemblies (Fig. 10(b)).¹²⁹ The actuator demonstrates exceptional transduction efficiency, achieving visible deformations such as bending and S-shaped twisting modes with an applied field of less than 400 Oe. Fig. 10(c) shows magnetic chains formed by magnetic field-directed self-assembly for applications in soft robotics, such as an innovative “accordion” structure, capable of both compression and extension.¹⁷⁰ Moreover, anisotropic MnBi/NdFeB hybrid bonded magnets have potential applications as magnetic anisotropy actuators with an improved coercivity temperature coefficient compared to that of pure NdFeB.¹⁷¹

Fig. 10 Magnetic microactuators based on magnetic anisotropy. (a) The movement of a magnetic actuator using superparamagnetic colloidal nanocrystal clusters/photocurable resin. Reproduced from ref. 73 with permission from Nature, copyright 2011; (b) the actuation of an elastomer using FeCo nanochains/Ecoflex. Reproduced from ref. 129 with permission from Wiley, copyright 2023; (c) the directionally controlled actuation of soft robots using chained iron microparticles/thermoplastic polyurethane. Reproduced from ref. 170 with permission from American Chemical Society, copyright 2017.

The self-assembly of superparamagnetic nanoparticles into chain-like nanostructures, guided by an external magnetic field, enables rapid and reversible deformations in the actuator. The utilization of superparamagnetic materials in the microactuator design presents a groundbreaking approach, allowing unprecedented versatility in movements at the microscale. These developments not only enhance our fundamental understanding of magnetic interactions at the nanostructure level but also pave the way for the design of highly efficient, reconfigurable microactuators with diverse applications in robotics and beyond.

3.2.5 Magnetocaloric actuators. Magnetic materials can produce heat, derived from the magnetocaloric effect, which is a magneto-thermodynamic phenomenon. In the presence of an alternating magnetic field, a magnetic material experiences a temperature change. The progress in magnetic materials and nanotechnology spurred the development of magnetic-based hyperthermia, magnetic refrigeration technology and biomedical imaging.¹⁷² With the biomedical characteristics of biosafety, softness, conformability, stretchability, self-healing ability, and high electroconductivity, gallium-based liquid metals have recently received considerable attention for a group of biomedical applications, such as health monitoring, biosensing, and tumor treatment.^173,174 Due to their excellent magnetic characteristics, metals such as Fe, Co, Ni, Mn, Zn, Mg, Gd, and their alloys, oxides (CoFe₂O₄, NiFe₂O₄, ZnFe₂O₄, CuFe₂O₄, MnFe₂O₄, Gd-doped Zn and Mn, Fe-doped Au, and Zn–Mn-doped iron oxide) have been examined as MNPs for hyperthermia.¹⁷⁵

Clinical trials have extensively explored the therapeutic potential of magnetic hyperthermia therapy for treating different tumor types. In these trials, biocompatible magnetic iron oxide nanoparticles were used as magnetocaloric agents.^176,177 The heating system facilitated by magnetic nanoparticles was developed into an efficient method for precisely regulating biological systems. This includes controlling high-temperature conditions and cell signal transmissions in a precisely time- and space-regulated way. Researchers selected appropriate magnetothermal agents and adjusted the intensity and frequency of the alternating magnetic fields. As a result, they have been able to utilize magnetic fields as a potent and non-invasive means for tumor treatment. Gang et al. report a new type of robust self-healing magnetic double-network hydrogel. This hydrogel was formed through multiple interactions between bondable magnetic Fe₃O₄ and a chitosan-polyolefin matrix. Moreover, it displays remarkable magnetocaloric effects and can be clearly imaged in MR scans.¹⁷⁸

Wang et al. introduce a biomedical application of alternating magnetic field mediated liquid–metal e-skin with a bioelectromagnetic thermal effect to spatiotemporally control wireless multisite tumor treatment (Fig. 11(a)).⁷⁰ Oxidized GaIn liquid–metal material, boasting super thermoconductivity, electroconductivity and biosafety, was directly applied in customized patterns onto the skin surface. Notably, it can be used as a conformable bioelectrode to cover an abnormal tumor.⁷⁰ Microwave thermal therapy combined with microwave dynamic therapy is a promising option for treating bacterial infection. As shown in Fig. 11(b), Fe₃O₄/Au nanoparticles act as the nuclear component, and the macrophage membrane serves as the outer envelope.¹³⁰ Under microwave irradiation, the nuclear component can generate a large amount of ˙O²⁻ and heat. This, in turn, enhances the thermal sensitivity of the targeted bacteria and the permeability of the bacterial cell membranes. These results demonstrate that Fe₃O₄/Au nanoparticles exhibit a high microwave thermal responsive effect in vivo.

Fig. 11 Actuators based on the magneto-thermodynamic phenomenon. (a) Oxidized GaIn thermal effect-enabled bioelectrode under an alternating magnetic field. Reproduced from ref. 70 with permission from Wiley, copyright 2019. (b) Microwave catalysis of Fe₃O₄ nanoparticles and Fe₃O₄/Au nanoparticles under microwave irradiation. Reproduced from ref. 130 with permission from Wiley, copyright 2021. (c) Schematic of a magnetic nanoemulsion hydrogel for heating. Reproduced from ref. 131 with permission from Royal Society of Chemistry, copyright 2017.

Based on the unique behaviors of magnetic nanoemulsion hydrogel within tumors, a body-temperature induced gelation strategy has been developed to achieve localized and accurate magnetic regression of tumors, as shown in Fig. 11(c).¹³¹ The Zn ferrite is evenly distributed within the aqueous continuous phase. By applying an alternating current magnetic field, it is anticipated that the tumors will undergo coagulative necrosis through thermal ablation induced by magnetic nanoparticles. This study highlights the substantial potential to boost the efficiency of magnetic hyperthermia and the accuracy of the injection site during the localized treatment of tumors. Giant MCE materials have a strong coupling between crystallographic structure and magnetism.⁵⁹ The fractional temperature change dT = τ × df, where τ is the maximum temperature change assuming the sample experienced a complete transition and df is the change of austenite fraction. The dT can reach up to 10 K at a low field of 10 mT for the In₁₃Co₅ sample, which is very promising to be applied in the field of tactile thermal feedback.

Driven by the magnetocaloric effect, the utilization of magnetic materials in biomedical applications has witnessed significant progress. This magneto-thermodynamic phenomenon has led to the development of magnetic-based hyperthermia, magnetic refrigeration technology and biomedical imaging. Additionally, magnetocaloric sensitive hydrogels can be heated by the alternating magnetic field and undergo significant volume shrinkage under high temperature for applications in biomimetics, soft robotics, and biomedical engineering.¹⁷⁹

Natural skin sensory systems, which are furnished with thermoreceptors and mechanoreceptors, can perceive and distinguish intricate stimuli originating from the surrounding environment, including vibration, pressure, strain and temperature.^180,181 However, these methods have limitations in influencing texture, tackiness, friction, viscoelasticity, softness, and moisture. These near-surface properties are of crucial importance since nearly all objects in natural environments possess them.¹⁸² Through the design of a layer-by-layer geometry, multimodal sensing functions have been achieved by incorporating various sensors into a single sensing network. Nevertheless, multimodal sensors and actuators that consist of diverse functional sensory layers inevitably display mutual interference and, at the same time, demand complex device integration.^183,184 Consequently, a favorable approach to creating multimodal sensors with strong interference resistance is to combine multiple sensing principles within a single sensory unit. The contact and contactless (strain and magnetism) sensing capacities are visually demonstrated in the carbon fiber aerogel/Fe₃O₄/silicone composite sensor.¹⁸⁵ A stretchable chromotropic ionic skin based on ferroferric oxide–carbon core–shell magnetic nanoparticles was designed in a rational way to detect multiple stimuli simultaneously and separate different signals, including temperature, strain and pressure.¹⁸⁶ Integration of multi-principle functional devices may contribute to the future development of interactive e-skin systems.

4 Conclusion and outlook

Research on haptic interfaces is still in its infancy, and many challenges remain; however, exceptional opportunities exist to develop sophisticated haptic technologies. The advances in magnetic materials have opened new possibilities for creating advanced haptic sensors and actuators, marking a significant leap forward in the evolution of haptic interfaces, impacting applications ranging from virtual reality to medical rehabilitation, and reshaping the landscape of haptic devices. To better understand the tactile devices based on different functional mechanisms of magnetic materials, we summarized and compared the merits and limitations of these devices, as shown in Table 3. Making full use of the characteristics of haptic sensors and actuators to find suitable application scenarios is an important consideration for achieving tactile interactions.

Table 3 Comparison of haptic sensors and actuators based on magnetic materials

	Types	Merits	Limitations
Sensors	Magnetically assisted piezoresistive sensor	Dual-mode;	Incapable of measuring static or low-frequency signals;
		Fast response time;	Requiring a charge amplifier for weak output signal
		Easy to prepare;
		Self-powered
	Magnetoelastic sensor	Suitable for harsh environments;	Weak output signal
		Wide measurement range;
		Self-powered
	Magnetically assisted capacitive sensor	Dual-mode;	Susceptible to temperature influence;
		High sensitivity and good linearity;	Large parasitic capacitance interference;
		Wide dynamic response range;	High processing accuracy requirements
		Capable of measuring both static and dynamic pressure;
	Magnetoresistance sensor	Dual-mode;	Vulnerable to strong magnetic field interference;
		Extremely high sensitivity;	Requiring the accurate installation position;
		Fast response speed;	Limited measurement range
		Low power consumption;
		Good temperature stability
Actuators	Electromagnetic actuator	Fast response speed;	High power consumption;
		High control accuracy;	Significant heat generation;
		Wide output force range	Large in size
	Shape programmable actuator	Complex 3D deformations;	Slow response speed;
		Lightweight for flexible electronics	Small driving load;
			Requiring continuous input of external energy
	Magnetic shape memory actuator	Fast response speed;	Requiring rare earth permanent magnets;
		Magnetic control reversible;	Requiring high uniformity of the magnetic field
		No current heat loss
	Magnetic anisotropy actuator	Simple structure;	Small output force;
		Customized deformation mode;	Limited deformation;
		Low power consumption	Requiring high magnetic field shielding
	Magnetocaloric actuator	Non-contact drive;	Limited response speed;
		Low heat loss and high energy efficiency;	Requiring alternating magnetic field generator
		Combined with biocompatible materials

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More systematic and focused research needs to be carried out to clarify the relationship between the magnetic material families and these haptic devices, which should also result in improved device performance. Regarding the improvement of devices, the size of devices, and their applications, the key issues that are generally of concern in this field have been summarized.

4.1 Micro- and nano-haptic devices based on magnetic materials

Drawing inspiration from the bionics of the hairs on human skin, researchers have developed a flexible tactile sensor founded on a magnetic cilia array that exhibits remarkably high sensitivity and stability, boasting a resolution of 0.2 mN within a working range of 19.5 mN.¹⁸⁷ To achieve devices that can imitate distributed, high-speed, high-resolution and high dynamic range mechanical signals, new material technologies are required, including highly sensitive magnetic materials¹⁸⁸ and highly reliable device structure designs that can connect the electronic and mechanical fields on the tiniest scale. The increased exchange interaction between Fe, Co, Ni atoms with Dy, Tb or Gd doping, can be used to construct various advanced magnetic materials for device design.¹⁸⁹ However, the response speed and miniaturization degree of tactile actuators still lag real-time requirements of interactive interfaces. Based on the above research, optimizing the material composition and structure by combining high-sensitivity magnetic materials with micro-nano structure designs can further enhance device performance.

4.2 Flexible and wearable haptic devices based on magnetic materials

Haptic devices designed for immersive VR/AR need to be wearable and unnoticeable until they are activated.¹⁹⁰ These devices should also be able to adapt to the complex and continually changing shape of the human body. A practical approach is to use actuators that are either micrometer or millimeter thick, fabricated from magnetic soft materials, and can cover large surface areas.^32,191 Devices made of flexible and elastic magnetic materials can bring about a more natural VR/AR experience. In the field of haptic interaction, 3D printing technology assisted by external fields will become an effective means of preparing magnetic materials, and may even be used to directly print devices with special structures.

4.3 AI for magnetic materials used in multimodal response

Multi-parameter response is also an important factor for accurate tactile recognition of various objects, because human haptic perception is multimodal and bidirectional. An interesting and meaningful research direction is the use of AI to achieve multi-parameter fusion and interaction of sensors and actuators based on magnetic materials. For example, the accuracy of a sensing-actuation system based on soft magnetoelasticity can achieve 94.68% with the assistance of machine learning algorithms.¹⁹² Tactile sensing arrays can be used for tactile perception with machine learning and deep neural networks.^193–196 AI for tactile devices can further address some limitations of tactile devices themselves and accelerate the practical application of tactile interactions. In addition, by integrating high-throughput computing with neural network models, rapid prediction of the composition–performance relationships of new composite materials (such as magnetoelastic materials) can be achieved. This promotes tactile interfaces from traditional experience-driven to precise intelligent manufacturing. It is another effective way for AI to empower haptic devices.

4.4 Remote operation haptic devices based on magnetic materials

Benefiting from the advances in material science, processing technology, and robotic control techniques, small-scale flexible robots have developed rapidly in recent years and offer the prospect for a wide range of applications, including targeted drug delivery, minimally invasive surgery, small-scale manipulations, and microfluidic devices. A miniature soft robot constructed from magnetic gelatin hydrogel has been designed with a legged structure resembling insect claws, which can carry out grasping and cargo-transportation tasks.¹⁹⁷ Magnetic robot drug delivery systems require a balance between magnetocontrol efficiency and biological safety, and it is necessary to integrate multimodal imaging navigation (such as MRI/ultrasound fusion) with intelligent drug release systems to accelerate the transformation towards precision medical scenarios.

The field of haptic interaction provides abundant research prospects in materials science and device engineering. In addition to the development of high-end sensors and actuators, the field also encompasses every other vital part of these systems, ranging from integrated electronics, data communication components, and power sources to skin interfaces. Research on magnetic materials in haptic devices is gaining attention. We envision that future research on haptic sensors will focus on increasing the sensitivity and sensing range of devices and incorporating multi-sensing systems. Research on haptic actuators will focus on miniaturization and achieving higher densities and higher quality simulation devices. With the advantages of millisecond-level response, external field drive, and non-contact interaction, haptic interfaces based on magnetic materials will present distinctive advantages as core perception mediums connecting the virtual and the real, as well as the human–machine and the environment, and play a key role in the future intelligent society.

Author contributions

Liuxia Ruan (conceptualization: lead; investigation: lead; writing – original draft: lead; writing – review & editing: lead); Jiahong Wen (writing – original draft: lead; writing – review & editing: lead); Yaxin Wang (conceptualization: lead; writing – review & editing: lead); Xiaoyu Zhao (conceptualization: lead; funding acquisition: lead; investigation: equal; writing – review & editing: equal); Fei Li and Yongjun Zhang (investigation: equal; writing – original draft: equal); Chang Liu (investigation: equal; writing – original draft: supporting); Wentao Tao (investigation: equal; validation: equal; writing – original draft: supporting); Hongbo Liang (investigation: equal; writing – original draft: equal); Xianmin Zhang, Yunlei Zhou, Chengxuan Tang, Xiqiang Zhong, Shaoqi He (investigation: equal; writing – review & editing: equal); Wenzhen Yang (conceptualization: lead; funding acquisition: lead; investigation: equal; methodology: lead; writing – original draft: supporting; writing – review & editing: lead).

Conflicts of interest

The authors declare no conflict of interest.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Acknowledgements

This paper is supported by the National Key Research and Development Program of China (no. 2023YFC3604103, 2021YFF0600203); the Key Research Project of the Zhejiang Lab (no. 2022MG0AC04); the National Natural Science Foundation of China (no. 62006209, 12374345, U24A20103); the Key R&D project of Zhejiang Province (no. 2024C03258) and the Fundamental Research Funds for the Provincial Universities of Zhejiang (no. GK239909299001-007).

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Footnote

† These authors contributed equally: Liuxia Ruan, Jiahong Wen, Yaxin Wang.

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