Imagine how exciting it would be if we could accurately perform tasks in the microcosm and become the architects of this tiny world! This is what microactuators give us the ability to precisely control the movement of fine structures, driving the application and development of microelectromechanical systems (MEMS) in microrobotics, biomedical devices, integrated electronics, and more.
To precisely control these tiny structures, scientists employ a variety of non-contact control methods, including magnetic, temperature, light, and sound control. These control technologies often rely on a complex array of smart materials, such as stimuli-responsive hydrogels, liquid crystal elastomers, and piezoelectric materials, that enable micro-drive structures to perform rich functions in a variety of environments. Often, the introduction of specific anisotropic features in these smart materials, such as special domain programming or specific molecular spatial orientation of liquid crystal elastomers, can enable specific motion patterns or enhance driving forces. However, the precise programming and processing of these materials at the micro- and nanoscales remains a challenge. In addition, once microactuators are programmed, their movement patterns are fixed, limiting their versatility and adaptability. Recently, researchers such as Professor Metin Sitti and Dr. Mingchao Zhang of the Max Planck Institute for Intelligent Systems in Germany were inspired by the goosebumps phenomenon during the erection of animal hair, and reported a three-dimensional sensor based on light stimulus response. A novel networked liquid crystal elastomer (LCE) artificial drive method (Fig. 1). This artificial ** can produce micron-sized artificial goosebumps under the precise stimulation of femtosecond lasers, which in turn can drive delicate structures on them, such as artificial hair, to achieve high degrees of freedom of motion control. By precisely programming the laser, the researchers explored various applications of this actuation method in the field of micromechanics, including precise control of the angle of the micromirror and fixed-point recovery of capillary assembly structures to enable new methods of information storage. and controllable switching based on the interaction forces between microstructures. The study was published in the latest issue of the journal Nature Materials, titled "Artificial-goosebump-driven microactuation".
A new microactuator inspired by the phenomenon of goosebumps during the erection of animal hair.
Goosebumps are caused by contraction of the small muscles (erector piloids) in the hair follicle. When these muscles contract, small bumps appear on the surface, forming so-called goosebumps, during which hairs (hairs) stand up (Figure 1a). Goosebumps in animals are a natural physiological response, and the main causes include temperature regulation, emotional responses, etc. In cold environments, goosebumps cause hairs to stand on end, forming a layer of air in the upper layer. Since the thermal conductivity of air is lower than that of hair itself, it is difficult to conduct heat from itself to keep warm. In addition, animals get goosebumps when they feel fear or excitement. The erected fur can make the animal look larger and more threatening, thus scaring away potential enemies, such as frightened cats. For humans, goosebumps are a relic of evolution. In the early days, humans and animals had more body hair, and goosebumps played a role in keeping warm and intimidating. Despite the fact that modern humans have lost their body hair, this reaction is still preserved. For example, humans experience intense cold or intense emotional experiences, including panic, ecstasy, and emotionality. Inspired by the natural phenomenon of goosebumps, the researchers proposed a micromanipulation system consisting of two-photon 3D printed fine structures (such as artificial hair) and LCE artificial**. They printed commercially available photoresists (IP-S) directly on LCE artificial ** to make artificial hairs (Figure 1) and combined them with femtosecond laser programming (e.g., laser intensity, scan speed, path, etc.) to achieve these subtle structures with a high degree of freedom of motion control. The local heating generated by the laser triggers the local vertical expansion of the LCE epidermis (the point changes from an ordered nematic phase to an unordered molecular chain orientation), resulting in goosebumps and precise manipulation of microstructures.
However, the precise control of this local drive is challenging due to the difficulty of precisely concentrating the generated heat at a target point on the LCE**. The high thermal conductivity of the LCE artificial** ordered arrangement of molecular chains (guides) makes it difficult for heat to be concentrated in specific locations, resulting in the formation of broader bulges that in turn trigger unintentional drives of other microstructures (Figure 1F). To overcome this challenge, by designing and optimizing the LCE network components, the researchers succeeded in achieving a sharp and localized goosebump morphology and accurately driving the target microstructure (Figure 1G).
LCE artificial** produces localized goosebumps
The preparation of LCE artificial** involves a classical two-step thiol-Michael chemical reaction process. The researchers doped a mobile liquid crystal small molecule (4-CYANO-4) in an LCE dual network'-Pentylbiphen, 5cb), which eventually forms a coupled three-network structure. The addition of liquid crystal small molecules reduces the driving temperature of LCE artificial ** and significantly limits the spatial distribution range of artificial goosebumps (achieving a high degree of localization). The production of this highly localized artificial goosebump is attributed to the active liquid crystal small molecule, which results in a significant decrease in the thermal conductivity of the LCE network in all directions (Figure 2). These small molecules are easily rearranged when heated, hindering thermal diffusion.
Application exploration of microscale drive system based on artificial goosebumps.
Based on this artificial goosebump-driven microscale manipulation system, researchers are exploring various potential applications in the field of micromechanics. Among them, they fabricated an angle-adjustable microreflector to precisely control the angle of light reflection by adjusting the tilt angle of the mirror with artificial goosebumps caused by laser stimulation (Figure 3A). In addition, the microdrive system can be used to restore the self-assembled microstructure caused by capillary forces. Capillary force-induced self-assembly is a common phenomenon in MEMS systems and often leads to deformation, distortion, and even destruction of fabricated micro-nano structures, especially those with large aspect ratios (Figure 3E). Once these micro- and nanostructures are glued together under capillary action, it becomes extremely difficult to recover them effectively due to the inherent challenges of manipulating small-sized structures. This microoperating system provides a solution for the targeted recovery of these self-assembled microstructures. By using a laser to create artificial goosebumps on LCE artificial**, it can introduce interference with the structure of these assemblies. When these disturbances exceed the binding forces between the assembled structures, these assembled structures can be released to achieve the restoration of the original design structure.
Application exploration of microscale drive system based on artificial goosebumps.
To demonstrate the effectiveness of the microdrive, the researchers used mushroom-shaped micromirrors as the basic assembly unit. When free and upright, these mirrors reflect the incoming light back, giving them a bright appearance. However, when these micromirrors self-assemble, their inclined surfaces reflect incident light in other directions, so they appear dull under the microscope (Figure 3i). By scanning the laser on the surface of the LCE, the resulting goosebump perturbation releases these assembled structures. After laser treatment, the assembled structure changed from a dark state (Figure 3l and 3m) to a light state (Figure 3n).
Based on this controllable switching between low-light microstructured pixels, the researchers demonstrated that this micromanipulation method is a novel means of data storage (Figure 4). Before selectively writing information, it is first necessary to implement a large-area uniform array of dual-self-assembled structures. In the dual-assembled structure of these pixels, the researchers demonstrated that they could release these pixels locally using a programmable laser. Since artificial goosebumps can be highly localized on LCE artificial**, only pixels of interest can be turned on without disturbing the surrounding assembled pixels (Figure 4G). By programming a variety of laser parameters, researchers can encode information into these double-assembled pixels, including writing complex patterns such as numbers, letters, and QR codes. This method not only demonstrates the highly localized and precise control ability of the artificial goosebump drive system, but also opens up new possibilities for microscale data storage.
Controllable capillary assembly structure to achieve new information storage application demonstration.
Summary
Conventional microscale drive systems are usually fabricated directly by micro-nano fabrication of smart materials, and the resulting microactuators directly perform specific movements under external control. There are many challenges in the preparation of microscale fabrication in this method: firstly, many smart materials are no longer suitable for common micro-nano processing technologies (such as UV masking**, two-photon 3D printing, etc.); Second, once the structure of a microscale driver is programmed after fabrication, its motion pattern is fixed, limiting its application in a variety of scenarios. The artificial goosebumps reported in this paper act indirectly on the prepared passive microstructures to achieve specific movements of these microstructures. Common microfabrication techniques can be used to process a variety of complex microstructures directly on commercial photoresists, so this method has a wide applicability. In short, inspired by the phenomenon of goosebumps in animals, this study succeeded in developing a miniature actuator based on light stimulation that can precisely control the movement of delicate structures. This not only provides a new method of micromanipulation, but also brings new perspectives and possibilities for the development of micromechanics, biomedical devices, and information storage.
*The corresponding author is Professor Metin Sitti of the Max Planck Institute for Intelligent Systems, Germany (currently the president of Kochi University and newly elected to the American Academy of Engineering in 2024). The first author is Dr. Mingchao Zhang. Co-authors include Dr. Aniket Pal, PhD student Xianglong Lu, and Dr. Yingdan Wu. The research was funded by the Max Planck Institute, the Humboldt Society, and the European Research Council. The authors would like to express their gratitude to Prof. Hamed Shahs**an, Dr. Han Jie, Dr. Zheng Zhiqiang, Wang Fan, Dr. Cem Balda Daman, Dr. Ren Haoshun, Dr. Ugur Bozuyuk, Dr. Gaur** Gardi, and Nima for Mahkam for his assistance in this study.