Nature has evolved in a hierarchical manner to achieve outstanding material properties and complex organismal behaviours. Hierarchy is a ubiquitous organizing and functional principle of natural systems, which are controlled in size, shape and pattern. The properties of skeletal muscles mainly rely on their hierarchical structure, composed of fibrous actuators organized in different levels of aggregation (from cm down to nm). This strong hierarchical organization gives muscles their remarkable time-dependent viscoelastic properties, and their efficient hierarchical actuating organization at different levels, from the single sarcomere to the muscle fascicle. 

Muscles are an optimized mechanism over a million years of evolution, and have inspired artificial actuators [1]. Despite that, current actuation technologies are not able to recapitulate the features of natural skeletal muscles. The possibility to grow fully functional living muscles in vitro and use them to power soft machines has the potential to revolutionize robotics, in the long term.

Biohybrid actuation is born as a new and extremely promising actuation method, which combines a live entity, like muscle cells, with an artificial substrate to produce a mechanical movement generated by the contraction of muscle cells. Attractive advantages feature this kind of biological actuator as performance invariance with scalability (from nm to mm), high transduction efficiency, and high power-to-weight ratio. Biohybrid actuators have the potential to deliver remarkable performance with life-like movements at the macroscale to artificial devices. Biohybrid actuators usually rely on a bottom-up approach and include systems based on cardiomyocytes, insect self-contractile tissues, and engineered skeletal muscle tissues (from both mammals and insects), which can convert chemical energy from the environment into mechanical energy. Skeletal muscle cells are the muscle cell types that have generated the most interest because they may be regulated to contract in response to specific stimuli, such as chemical, electrical, or optical signals, following suitable optogenetic changes [2]. There are several examples of bio-hybrid actuators and robots in literature, as the bio-bots can walk or swim [3]. Despite that, the use of a bio-hybrid actuator to actuate a medical device as a catheter has never been explored, so far.

Catheterization is one of the most promising approaches for the targeted treatment of several diseases, including cancer. Indeed, the local administration of medications or chemicals in the area of interest may maximize the therapy efficacy, reducing side effects. Here, we report the concept and an early investigation of a novel intravascular steerable catheter powered by an on-board biohybrid actuator on its tip to navigate deep and tortuous regions within the cardiovascular systems. The catheter performance has been analyzed by analytical and numerical analyses, assessing the influence of the catheter geometrical (e.g., inner and outer radii, wall thickness) and physical (e.g., stiffness) parameters, the actuator geometrical parameters (e.g., length and positioning over the catheter tip) and contractile force. The findings demonstrate the necessity to lower the outer diameters and decrease wall thickness to maximize the catheter deflection. Also, we assessed the influence of the positioning of the biohybrid actuator as the applied forces. These preliminary results hold a lot of promise in light of future experiments using this type of actuation to drive microcatheters through the cardiovascular network, even though the performance of this concept still needs to be improved to match relevant anatomical targets, as the radius of curvature of the inner branches of organs.