Biohybrid robotics: From the nanoscale to the macroscale

Biohybrid robotics is a field in which biological entities are combined with artificial materials in order to obtain improved performance or features that are difficult to mimic with hand-made materials. Three main level of integration can be envisioned depending on the complexity of the biological entity, ranging from the nanoscale to the macroscale. At the nanoscale, enzymes that catalyze biocompatible reactions can be used as power sources for self-propelled nanoparticles of different geometries and compositions, obtaining rather interesting active matter systems that acquire importance in the biomedical field as drug delivery systems. At the microscale, single enzymes are substituted by complete cells, such as bacteria or spermatozoa, whose self-propelling capabilities can be used to transport cargo and can also be used as drug delivery systems, for in vitro fertilization practices or for biofilm removal. Finally, at the macroscale, the combinations of millions of cells forming tissues can be used to power biorobotic devices or bioactuators by using muscle cells. Both cardiac and skeletal muscle tissue have been part of remarkable examples of untethered biorobots that can crawl or swim due to the contractions of the tissue and current developments aim at the integration of several types of tissue to obtain more realistic biomimetic devices, which could lead to the next generation of hybrid robotics. Tethered bioactuators, however, result in excellent candidates for tissue models for drug screening purposes or the study of muscle myopathies due to their three-dimensional architecture

Biohybrid swimmers at low Reynolds number powered by tissue-engineered neuromuscular units

Biohybrid machines are engineered systems which are built by integrating biological cells with synthetic materials and components. Development of biohybrid machines utilizes the classical engineering modalities of design, modeling, prototype fabrication, testing, and iteration, but also draws from a toolbox that includes biological cells and materials. This enables a range of exciting possibilities since biological systems can develop via self-organization, function autonomously, and monitor and adapt to their environments. Pioneering studies on biohybrid machines have demonstrated the development of devices powered by muscle cells, capable of locomotion, pumping, and micromanipulation. A currently emerging frontier in the field is the integration of neuronal control. A wide range of complex animal behaviors are orchestrated by the nervous system which interfaces the body with the environment through sensing, information processing, and coordinating motor activity. Hence, the integration of neurons may enable the development of autonomous biohybrid machines capable of higher-level functionalities such as sensing, memory, and adaptation. The focus of this dissertation is on the implementation of neuronal actuation in muscle powered biohybrid machines. Firstly, we develop an experimental bioactuator platform to study the in vitro development of neuromuscular units. Engineered skeletal muscle tissues, anchored to compliant pillars, are co-cultured on the platform with optogenetic stem cell-derived neuronal clusters containing motor neurons. The motor neurons extend axons and innervate the muscle fibers, forming functional neuromuscular units. Our study illustrates several outcomes of synergistic interactions between the muscles and neurons. Muscles co-cultured with neurons exhibit significantly higher contraction force and cytoskeletal maturation compared to muscles cultured alone. Neurons self-organize into networks which generate synchronous bursting patterns, the development of which is facilitated by muscle-secreted soluble factors. Next, we implement our neuron-muscle co-culture approach on a free-standing compliant scaffold containing slender flagella, to demonstrate the first example of a biohybrid swimmer powered by neuromuscular units. Optogenetic stimulation of motor neurons evokes periodic muscle contractions, and the swimmer is driven by the resulting time-irreversible deformations of the flagella, a common mechanism of propulsion at low Reynolds number. Lastly, we investigate potential design strategies for improving swimming performance, assisted by analytical and computational models. Our models predict that the swimming speed of our initial prototype can be improved by up to two orders of magnitude by redesigning the swimmer scaffold to reduce drag and increase actuation amplitude