Flexible Linear Magnets, Rewritten with Light
Published in Bioengineering & Biotechnology and Materials

Familiar to every child, yet powerful enough to shape the world
Magnets are usually one of the first scientific tools many of us encounter as children—simple yet magical objects that attract or repel each other through invisible forces. Traditional magnets, like the ones many of us used in school experiments, are rigid and fixed in shape, typically made from hard materials like metal. They have two magnetic poles—north and south—and a fixed magnetization direction that cannot be easily changed once the magnet is made. Historically, magnets have played transformative roles—think of the compass, which helped guide explorers across oceans during the Age of Discovery. Beyond childhood curiosity, magnets are continuously powering the next generation of advanced technologies—from soft robotics to wearable electronics, microfluidics, and minimally invasive medical tools.
Fundamental problem
In most magnet-based systems, performance relies heavily on the magnetic fields each unit produces for remote interaction and how it interacts with external magnetic fields to generate forces and torques that drive motion or shape-morphing for actuation. These magnetic forces, torques, and fields are determined by two key factors: the direction each magnetic unit is magnetized and how those units are aligned. However, local magnetization direction adjustment usually requires strict conditions (temperatures higher than the Curie point and fields stronger than the coercive field) to overcome the high coercivity of the micro-magnets or suffers from repeated procedures to adjust the conformal assembly on the microscale with low efficiency or poor spatial resolution.
Bio-inspired design
In our latest work, we introduce a linear magnet composed of a transparent hydrogel matrix and discrete phase-change-material (PCM) cells containing NdFeB magnetic microparticles (~5 μm). Our inspiration comes from nature, specifically from Magnetospirillum magneticum (AMB-1), a type of magnetotactic bacteria which contains magnetite-rich magnetosome chains within the hydrogel-like cell membrane. The hydrogel-like cell membrane enables the bacteria to undergo conformal or active shape-morphing, and the intrinsic magnetic dipole moment arising from the aligned magnetosome chains endows the bacteria with the ability to respond to external magnetic fields. However, the orienting directions of magnetosome chains are immutable once assembled, making the magnetotactic bacteria only move along a fixed magnetic axis correlated to the geomagnetic field. This limitation inspired us to ask what if we could create an artificial, flexible version of this system—one that retains a strong magnetic response, but with programmable directionality. The PCM (i.e., eicosane, melting point ~36 ℃) is thus assembled surrounding these magnetic particles inside the hydrogel.
Artful mechanism and manufacturing technology
Fabricating a linear hydrogel structure that integrates multiple materials with microscale precision is no easy task—but that’s where microfluidic technology comes in. Using this technique, we embed tiny, evenly spaced magnetic cells into a hydrogel matrix. Each cell contains a PCM loaded with NdFeB magnetic microparticles (~5 μm). By using a focused near-infrared laser, we can locally heat selected cells to around 40 °C, melting the PCM just enough to let the magnetic particles rotate freely. With the help of a mild external magnetic field (~30 mT), we reorient the alignment of those particles. Once cooled, the PCM solidifies again and locks the magnetic particles in a new direction. This simple light-triggered switch allows us to rewrite the magnetization direction of each cell, one by one, with high precision (~150 μm). The entire magnet is also highly stretchable (up to 80% strain), making it ideal for conforming to various curved surfaces. This linear magnet is totally different from the magnets in our childhood memories, which were rigid and with a single fixed magnetic direction.
Versatile functions for various applications
By wrapping the linear magnet into a surface or weaving it into wearables, we created localized magnetic fields tailored to specific areas. This could enable magnetically responsive materials or interactive wearables. In addition, the linear magnet could also work as programmable actuators. Under external fields, the linear-magnet-based devices exhibited customized deformation modes depending on the programmed magnetic cells. This is ideal for soft robotic or minimally invasive tools that need shape-morphing behavior on command. Besides, the linear magnet can act as a magnetic encoder, with each unit carrying a distinct field signature. Using simple Hall sensors, the signals representing position or speed could be decoded, supporting applications in motion tracking or programmable electrical devices.
Perspectives
Looking ahead, coupling this linear magnet with advanced 3D printing techniques could enable the fabrication of complex 3D architectures with precisely defined, spatially programmable magnetization. Its miniaturized scale, biocompatible hydrogel matrix, and stretchable design make it a highly versatile platform—well-suited for integration into wearable or implantable devices, where embedded magnetic microsensors can monitor subtle movements or physiological signals like heart rate. Moreover, its soft, reconfigurable nature opens new possibilities in minimally invasive medicine, such as magnetic microrobots for targeted diagnosis and drug delivery. Extending this concept beyond linear structures to 2D sheets or 3D volumes could ultimately enable programmable magnetic metamaterials, adaptive surfaces, and bio-integrated artificial tissues with embedded intelligent magnetic functionalities.
This work, titled “Linear magnet with fluid-solid-switchable cells for flexible devices,” was published in Nature Communications.
Quick to find us
First author: Qiyu Deng
Corresponding authors: Liqiu Wang Xiaobo Yin Wei Li Xin Tang
Fig. 1. Magnetospirillum magneticum-inspired linear magnet with fluid-solid-switchable cells. a Schematics of the programmable linear magnet which is an alginate hydrogel string hosting equidistantly distributed phase-change-material (PCM) microspheres (i.e., micro cells) containing NdFeB microparticles. The inset TEM image on the lower right shows that the Magnetospirillum magneticum (AMB-1) possesses magnetosome chains that are rich in magnetite within the hydrogel-like cell membrane. Scale bar: 1 μm. b Optical microscopy images showing a section of the linear magnet. Scale bars: 3 mm (left), 500 μm (right). c Schematics of the programming strategy for the linear magnet. Upon near-infrared (NIR) laser irradiation, the PCM melts into the fluid state because of the intrinsic photothermal property of NdFeB microparticles. The boundary constraint on the NdFeB microparticles is removed, allowing their realignment in a mild programming magnetic field and temperature. By switching off the laser, the PCM solidifies into a solid state, and the programmed magnetization is preserved as the NdFeB microparticles are kinematically fixed. The insets on the right show that the magnetization direction of a cell is programmed from one direction to another. Scale bar: 100 μm. d Comparison of different magnet-based systems. Although the linear magnet’s coercivity is stronger than that of a ferrofluid and a fluidic magnet like a conventional solid magnet, its fluid-solid-switchable cells make it simpler to change the direction of magnetization. Additionally, the hydrogel’s softness of the linear magnet also makes it easier to be conformably assembled, like ferrofluid and fluidic magnets. e Selective programming of the 4th cell of a 7-cell linear magnet. Scale bar: 500 μm. f The reprogrammable shape-morphing of the linear magnet in a fixed magnetic field (30 mT in the vertical direction). Scale bar: 10 mm. g A surface with 3D-printed character grooves conformally assembled with linear magnets and a piece of fabric woven with linear magnets. Scale bars: 5 mm.
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