A robot is an artifact exhibiting intelligent behaviors by sensing-processing actuating cycles. In other words, a robot requires the facilities of sensors, actuators and intelligence as well as a body to integrate these facilities. Is it possible to develop a compartment, sensors, actuators and intelligence in molecules? If it is possible, what is different from an ordinary mechanical robot such as a humanoid robot? What kinds of technologies can be applied for molecular robotics? What is the ultimate goal of molecular robotics? How can we reach this goal? These are fundamental questions discussed in the Molecular Robotics Research Group, which was established in the Systems and Information Division of the Society of Instrument and Control Engineers (SICE) in 2010.

Nowadays, almost all robots are developed in a top-down fashion. Robots are mechanical systems designed for specific purposes. A robot’s body, arms and legs are assembled by electronic and mechanical parts by considering the balance of performance and costs. The energy driving the robot must be supplied directly through its power plugs or batteries. The mechanical parts are replaced when broken. However, in this sense, even a state-of-the-art humanoid robot is no different from an electric kettle, no matter how well the robot mimics human behavior.

Molecular robots extend beyond these conventional definitions of robots and even reshapes the concept of artifacts. This new paradigm allows robots with more flexible structures and introduces more dynamic relationships involving exchange of matter between the robot and its environment. In other words, the robot’s structure will not be fixed in advance; instead, it will be ever-changing like Play-Dough. Also, the robot parts are not assembled in a factory but are autonomously assembled into a robot structure, which adapts to its environment.

Until recently, systems possessing such properties were thought to be limited to living beings. That is to say, the ability to maintain their own systems in a changing environment, the ability to self-replicate, and the ability to evolve. Our strategy is to borrow the mechanisms found at the molecular level of living systems and modify them to bring life to our robots. Our goal is not only to realize the sensing-processing-actuating functions at the molecular level, but also to create systems that possess the flexibility and adaptability that are thought to be characteristic of living beings.

In 2012, our proposal “Development of Molecular Robots Equipped with Sensors and Intelligence” was selected as one of  Grant-in-Aid for Scientific Research on Innovative Areas by MEXT (Ministry of Education, Culture, Sports, Science and Technology, JAPAN). This scheme not only allows us to do core projects but also allows us to accept applications of related research projects from the public. This should be used as an opportunity for especially young scientists wishing to pursue a research career in this field. Therefore, it is crucially important at this moment to clarify the concept and research direction of molecular robotics. For this purpose, here, we present a comprehensive view of molecular robotics based on the discussions held in the Molecular Robotics Research Group.

In molecular robotics, the notion of self is a key concept. In life, a self is expressed through a unit called “individuals.” An individual contains all the information necessary for developing, maintaining and reproducing the self. The goal of molecular robotics is to construct an artifact containing a description of such a self. More specifically, the goal is to realize the above self-X properties, such as self-reconfiguration, self-assembly, self-repairing and self-replication, according to the description of the self. To address these issues, a new way of thinking that differs from conventional approaches in robotics and mechanical engineering, is required. The difficulty here is how to describe self-X properties in an artifact. In conventional Molecular Robotics: A New Paradigm for Artifacts engineering, an artifact consists of objects whose behavior can be expressed by a mathematical model such as differential equations. However, in order to realize the self-X properties, one has to take into account that the model must have an ability to describe the change of the model in itself. In other words, in order to realize these self-X properties, we must devise a methodology encoding the description of the self within the system, like reflective programming in computer science. In general, such a description is very difficult, especially when the system possesses a certain level of complexity. Living systems solved this problem by separating the representation of the self into genotypes and phenotypes connected by bottom-up processes, the so-called “self-organization.” Instead of describing the self directly, living things encode the physical properties of the system in DNA sequences as genotypes. Phenotypes, i.e., the physical properties, emerge when living things mature by means of self-organization processes at various levels, such as self-assembly of molecular complex, controlled cell replication, differentiation and apoptosis. As a result, description of the self becomes very simple: it is a one-dimensional base sequence on DNA which can be easily replicated by complementarity of DNA. In this sense, we strongly believe that the “bottom-up” system development is a key concept in molecular robotics to realize self-X properties.

The paradigm of bottom-up development allows a robot to be more flexible in structure and to increase its ability to adapt to its environment. In other words, the robot structure will be perpetually reconfigured, and the robot parts are not assembled in a factory but are assembled autonomously. Such robots may differ from what we usually imagine, but we can still call them “robots” as long as they execute a cycle of sensing-processing-actuating. Until recently, living systems were the only ones which had the property of bottom-up development. In some sense, the bottom-up development is a source of “life”-like systems; the ability to maintain their own systems to a changing environment (adaptability), ability to self-replicate, and the aforementioned abilities. It is this ability that most man-made artifacts have not achieved yet. Rapid progresses in molecular biology, or rather in genome science, have revealed mechanisms at the molecular level since the discovery of the DNA double-helix structure in the mid-20th century. Our strategy is to apply the mechanisms found in living systems to artificial robots in system design ranging from the level of individual molecular device designs, such as DNA sequence designs, to the level of molecular robotic systems.

Chemistry has made amazing strides in the last few decades. Much progress has been made in various technologies to design and utilize molecular devices based on biochemistry, organic metal chemistry, supra-molecules, polymer chemistry, inorganic chemistry, and so on and so forth. In parallel, a new technology called “molecular programming” that is a technology to create molecular devices by designing the base sequences of DNA and RNA evolves at a rapid pace. Altogether, the ever-increasing inventory of available molecular devices allows us to make one more step toward creating “systems” including many molecular devices. This trend is most evident in DNA-based molecular devices. According to Winfree, the complexity of DNA-based molecular systems in terms of the number of bases has doubled every three years since the first DNA nanostructure was created by Seeman in 1983. This means that similar to Moore’s law in the semiconductor industry, an exponential innovation is happening in the world of DNA nanotechnology as well. We think it is important to envision the future of molecular systems at the early stage of development. As mentioned before, the goal of molecular robotics is to derive new paradigms for artifacts by learning from living systems. To be concrete, we can take the frame of reference from the evolutionary process of life. The evolution of organisms is a natural process shaped by various environmental factors. Similarly, the evolution of artifacts is also swayed by various factors, such as technological limitations and social needs at a given time. In this vein, the section below will explore one conceivable evolutionary scenario for molecular robotics. Similar to the many epochs in the evolution of living organisms, the development of molecular robotics faces many technological hurdles. When these hurdles are overcome, new possibilities will arise.