Toward Practical Applications of Engineered Living Materials with Advanced Fabrication Techniques

Engineered Living Materials (ELMs) are materials composed of or incorporating living cells as essential functional units. These materials can be created using bottom-up approaches, where engineered cells spontaneously form well-defined aggregates. Alternatively, top-down methods employ advanced materials science techniques to integrate cells with various kinds of materials, creating hybrids where cells and materials are intricately combined. ELMs blend synthetic biology with materials science, allowing for dynamic responses to environmental stimuli such as stress, pH, humidity, temperature, and light. These materials exhibit unique "living" properties, including self-healing, self-replication, and environmental adaptability, making them highly suitable for a wide range of applications in medicine, environmental conservation, and manufacturing. Their inherent biocompatibility and ability to undergo genetic modifications allow for customized functionalities and prolonged sustainability. This review highlights the transformative impact of ELMs over recent decades, particularly in healthcare and environmental protection. We discuss current preparation methods, including the use of endogenous and exogenous scaffolds, living assembly, 3D bioprinting, and electrospinning. Emphasis is placed on ongoing research and technological advancements necessary to enhance the safety, functionality, and practical applicability of ELMs in real-world contexts.

Biohybrid Soft Robots Powered by Myocyte: Current Progress and Future Perspectives

Myocyte-driven robots, a type of biological actuator that combines myocytes with abiotic systems, have gained significant attention due to their high energy efficiency, sensitivity, biocompatibility, and self-healing capabilities. These robots have a unique advantage in simulating the structure and function of human tissues and organs. This review covers the research progress in this field, detailing the benefits of myocyte-driven robots over traditional methods, the materials used in their fabrication (including myocytes and extracellular materials), and their properties and manufacturing techniques. Additionally, the review explores various control methods, robot structures, and motion types. Lastly, the potential applications and key challenges faced by myocyte-driven robots are discussed and summarized.

Mimicking the Tendon Microenvironment to Enhance Skeletal Muscle Adhesion and Longevity in a Functional Microcantilever Platform

Microcantilever platforms are functional models for studying skeletal muscle force dynamics in vitro. However, the contractile force generated by the myotubes can cause them to detach from the cantilevers, especially during long-term experiments, thus impeding the chronic investigations of skeletal muscles for drug efficacy and toxicity. To improve the integration of myotubes with microcantilevers, we drew inspiration from the elastomeric proteins, elastin and resilin, that are present in the animal and insect worlds, respectively. The spring action of these proteins plays a critical role in force dampening in vivo. In animals, elastin is present in the collagenous matrix of the tendon which is the attachment point of muscles to bones. The tendon microenvironment consists of elastin, collagen, and an aqueous jelly-like mass of proteoglycans. In an attempt to mimic this tendon microenvironment, elastin, collagen, heparan sulfate proteoglycan, and hyaluronic acid were deposited on a positively charged silane substrate. This enabled the long-term survival of mechanically active myotubes on glass and silicon microcantilevers for over 28 days. The skeletal muscle cultures were derived from both primary and induced pluripotent stem cell (iPSC)-derived human skeletal muscles. Both types of myoblasts formed myotubes which survived for five weeks. Primary skeletal muscles and iPSC-derived skeletal muscles also showed a similar trend in fatigue index values. Upon integration with the microcantilever system, the primary muscle and iPSC-derived myotubes were tested successively over a one month period, thus paving the way for long-term chronic experiments on these systems for both drug efficacy and toxicity studies.

Biohybrid robots: recent progress, challenges, and perspectives

The past ten years have seen the rapid expansion of the field of biohybrid robotics. By combining engineered, synthetic components with living biological materials, new robotics solutions have been developed that harness the adaptability of living muscles, the sensitivity of living sensory cells, and even the computational abilities of living neurons. Biohybrid robotics has taken the popular and scientific media by storm with advances in the field, moving biohybrid robotics out of science fiction and into real science and engineering. So how did we get here, and where should the field of biohybrid robotics go next? In this perspective, we first provide the historical context of crucial subareas of biohybrid robotics by reviewing the past 10+ years of advances in microorganism-bots and sperm-bots, cyborgs, and tissue-based robots. We then present critical challenges facing the field and provide our perspectives on the vital future steps toward creating autonomous living machines.

Biohybrid soft robots with self-stimulating skeletons

Bioinspired hybrid soft robots that combine living and synthetic components are an emerging field in the development of advanced actuators and other robotic platforms (i.e., swimmers, crawlers, and walkers). The integration of biological components offers unique characteristics that artificial materials cannot precisely replicate, such as adaptability and response to external stimuli. Here, we present a skeletal muscle-based swimming biobot with a three-dimensional (3D)-printed serpentine spring skeleton that provides mechanical integrity and self-stimulation during the cell maturation process. The restoring force inherent to the spring system allows a dynamic skeleton compliance upon spontaneous muscle contraction, leading to a cyclic mechanical stimulation process that improves the muscle force output without external stimuli. Optimization of the 3D-printed skeletons is carried out by studying the geometrical stiffnesses of different designs via finite element analysis. Upon electrical actuation of the muscle tissue, two types of motion mechanisms are experimentally observed: directional swimming when the biobot is at the liquid-air interface and coasting motion when it is near the bottom surface. The integrated compliant skeleton provides both the mechanical self-stimulation and the required asymmetry for directional motion, displaying its maximum velocity at 5 hertz (800 micrometers per second, 3 body lengths per second). This skeletal muscle-based biohybrid swimmer attains speeds comparable with those of cardiac-based biohybrid robots and outperforms other muscle-based swimmers. The integration of serpentine-like structures in hybrid robotic systems allows self-stimulation processes that could lead to higher force outputs in current and future biomimetic robotic platforms.

Recent progress in engineering functional biohybrid robots actuated by living cells

Living cells are highly scalable biological actuators found in nature, and they are efficient technological solutions to actuate robotic systems. Recent advancements in biofabrication and tissue engineering have bridged the gap to interface muscle cells with artificial technology. In this review, we summarize the recent progress in engineering the attributes of individual components for the development of fully functional biohybrid robots. First, we address the fabrication of biological actuators for biohybrid robots with muscle cells and tissues, including cardiomyocytes, skeletal muscles, insect tissues, and neuromuscular tissues, in well-organized pattern of 2D sheets and 3D constructs. Next, we discuss the performance of biohybrid robots for various biomimetic tasks such as swimming, walking, gripping, and pumping. Finally, the challenges and future directions in the development of biohybrid robots are described from different viewpoints of living material engineering, multiscale modeling, 3D printing for manufacturing, and multifunctional robotic system development.

Biohybrid robot with skeletal muscle tissue covered with a collagen structure for moving in air

Biohybrid robots composed of biological and synthetic components have been introduced to reconstruct biological functions in mechanical systems and obtain better understanding of biological designs. For example, biohybrid robots powered by skeletal muscle tissue have already succeeded in performing various movements. However, it has been difficult for the conventional biohybrid robots to actuate in air, as the skeletal muscle tissue often dries out in air and is damaged. To overcome this limitation, we propose a biohybrid robot in which the skeletal muscle tissue is encapsulated in a collagen structure to maintain the required humidity conditions when operated in air. As the skeletal muscle tissue maintains high cell viability and contractility, even after encapsulation within the collagen structure, the biohybrid robot can move in air through contractions of the skeletal muscle tissue. To demonstrate the applicability of the developed biohybrid robot, we demonstrate its use in object manipulation. In addition, to prove its capability of functionality enhancement, we show that the biohybrid robot can actuate for a long term when perfusable tubes are set inside the collagen structure; it can actuate even while culturing cells on its surface. The developed biohybrid robot composed of skeletal muscle tissue and collagen structure can be employed within platforms used to replicate various motions of land animals.

Living Materials Herald a New Era in Soft Robotics

Living beings have an unsurpassed range of ways to manipulate objects and interact with them. They can make autonomous decisions and can heal themselves. So far, a conventional robot cannot mimic this complexity even remotely. Classical robots are often used to help with lifting and gripping and thus to alleviate the effects of menial tasks. Sensors can render robots responsive, and artificial intelligence aims at enabling autonomous responses. Inanimate soft robots are a step in this direction, but it will only be in combination with living systems that full complexity will be achievable. The field of biohybrid soft robotics provides entirely new concepts to address current challenges, for example the ability to self-heal, enable a soft touch, or to show situational versatility. Therefore, "living materials" are at the heart of this review. Similarly to biological taxonomy, there is a recent effort for taxonomy of biohybrid soft robotics. Here, an expansion is proposed to take into account not only function and origin of biohybrid soft robotic components, but also the materials. This materials taxonomy key demonstrates visually that materials science will drive the development of the field of soft biohybrid robotics.

Engineered skeletal muscle tissue for soft robotics: fabrication strategies, current applications, and future challenges

Skeletal muscle is a scalable actuator system used throughout nature from the millimeter to meter length scales and over a wide range of frequencies and force regimes. This adaptability has spurred interest in using engineered skeletal muscle to power soft robotics devices and in biotechnology and medical applications. However, the challenges to doing this are similar to those facing the tissue engineering and regenerative medicine fields; specifically, how do we translate our understanding of myogenesis in vivo to the engineering of muscle constructs in vitro to achieve functional integration with devices. To do this researchers are developing a number of ways to engineer the cellular microenvironment to guide skeletal muscle tissue formation. This includes understanding the role of substrate stiffness and the mechanical environment, engineering the spatial organization of biochemical and physical cues to guide muscle alignment, and developing bioreactors for mechanical and electrical conditioning. Examples of engineered skeletal muscle that can potentially be used in soft robotics include 2D cantilever-based skeletal muscle actuators and 3D skeletal muscle tissues engineered using scaffolds or directed self-organization. Integration into devices has led to basic muscle-powered devices such as grippers and pumps as well as more sophisticated muscle-powered soft robots that walk and swim. Looking forward, current, and future challenges include identifying the best source of muscle precursor cells to expand and differentiate into myotubes, replacing cardiomyocytes with skeletal muscle tissue as the bio-actuator of choice for soft robots, and vascularization and innervation to enable control and nourishment of larger muscle tissue constructs.

Voluntary movement controlled by the surface EMG signal for tissue-engineered skeletal muscle on a gripping tool

We have developed a living prosthesis consisting of a living muscle-powered device, which is controlled by neuronal signals to recover some of the functions of a lost extremity. A tissue-engineered skeletal muscle was fabricated with two anchorage points from a primary rat myoblast cultured in a collagen Matrigel mixed gel. Differentiation to the skeletal muscle was confirmed in the tissue-engineered skeletal muscle, and the contraction force increased with increasing frequency of electric stimulation. Then, the tissue-engineered skeletal muscle was assembled into a gripper-type microhand. The tissue-engineered skeletal muscle of the microhand was stimulated electrically, which was then followed by the voluntary movement of the subject's hand. The signal of the surface electromyogram from a subject was processed to mimic the firing spikes of a neuromuscular junction to control the contraction of the tissue-engineered skeletal muscle. The tele-operation of the microhand was demonstrated by optical microscope observations.

Contracting cardiomyocytes in hydrophobic room-temperature ionic liquid

Room-temperature ionic liquids (RTILs) are drawing attention as a new class of nonaqueous solvents to replace organic and aqueous solvents for chemical processes in the liquid phase at room temperature. The RTILs are notable for their characteristics of nonvolatility, extremely low vapor pressure, electric conductivity, and incombustibility. These distinguished properties of RTILs have brought attention to them in applications with biological cells and tissue in vacuum environment for scanning electron microscopy, and in microfluidic devices for micro-total analysis system (micro-TAS). Habitable RTILs could increase capability of nonaqueous micro-TAS for living cells. Some RTILs seemed to have the capability to replace water in biological applications. However, these RTILs had been applied to just supplemental additives for biocompatible test, to fixed cells as a substitute for an aqueous solution, and to simple molecules. None of RTILs in which directly soaks a living cell culture. Therefore, we demonstrated the design of RTILs for a living cell culture and a liquid electrolyte to stimulate contracting cardiomyocytes using the RTILs. We assessed the effect of RTILs on the cardiomyocytes using the beating lifetime to compare the applicability of RTILs for biological applications. Frequent spontaneous contractions of cardiomyocytes were confirmed in amino acid anion RTILs [P(8,8,8,8)][Leu] and [P(8,8,8,8)][Ala], phosphoric acid derivatives [P(8,8,8,8)][MeO(H)PO(2)], and [P(8,8,8,8)][C(7)CO(2)]. The anion type of RTILs had influence on applicable characteristics for the contracting cardiomyocyte. This result suggested the possibility for biocompatible design of hydrophobic group RTILs to achieve biological applications with living cells.