Actively Increasing Force Transmission of a Millimeter-Scale Soft Robot for Tissue Interactions in Minimally Invasive Surgery

Interest in soft robotics is rapidly growing in large part due to the ability to safely interact with humans. The use of soft materials allows for scalable fabrication processes that can combine various degrees of freedom into monolithic structures. Miniaturization enables soft robots to navigate in tortuous, narrow, and difficult-to-reach areas, allowing for broad applications ranging from exploration to minimally invasive surgery. However, force output scales with the robot’s cross-sectional area, thus hampering significant robotic tasks and interactions with the surrounding environment at smaller scales.<br/>Here, we present a 3.5 mm diameter soft robot that increases force transmission and, consequently, broadens the potential interactions that may be achieved within similarly scaled environments. These capabilities are accomplished by designing three fluidic-actuated, independent degrees of freedom. The proposed robot consists of a continuum body constructed out of two polymers with differing elasticity and an attached 3 mm linear bellows actuator. The continuum body contains actuation mechanisms for steering via a bending degree of freedom and for stabilization via a radially-expansive actuator. Fabrication of the continuum body is performed through a series of molding processes in which the stiffer of the two polymers, DragonSkin 10, forms the base of the body. Dowel pins run axially along the body for alignment and masking the cross-sectional off-center bending chamber. A 3D masking technique combined with the molded shape of the base are utilized for the creation of the radial stabilization actuator through the over-molding of the less stiff polymer, Ecoflex 00-30. The linear bellows actuator utilizes a layer-by-layer fabrication technology in which the application of heat and pressure bonds each bellow at predetermined locations. The bellows actuator is adhered to the tip of the continuum body to act as an end-effector deployment mechanism enabling the robot to utilize any increase in force transmission provided by the anchoring of the stabilization mechanism.<br/>The increase in force transmission is measured through mechanical characterizations of the robot in states with stabilization activated and unactivated. An increase in the effective stiffness of the system of about five times is measured as a result of stabilization, demonstrating that, for displacements less than 2 mm, the robot can transmit up to 0.75 N of force to the deployment mechanism located at its tip. Characterization of the deployment mechanism further confirms the need for stabilization with blocked forces measuring up to 1.1 N. Results of both stabilization and deployment force measurements validate models that guided the robot design. To characterize dexterity of the robot, bending of the continuum body is measured displaying full retroflexion with a maximum bending angle of 196 degrees. Linear expansion of the deployment mechanism is also measured showing a range of 9.3 mm. The deployment mechanism is assembled with a needle to demonstrate the feasibility of the robot to be used as a platform for minimally invasive tissue biopsy in bronchoscopy procedures. Compared with typical surgical forces, characterizations of the robot verify its ability to puncture stiffer tumor tissues while maintaining safe levels of force on healthy tissue. An in-vitro setup places the robot within a lung model to demonstrate its ability to steer into a desired lung branch, stabilize within the chosen branch, and deploy a needle for tissue biopsy. The proposed soft robot demonstrates the potential benefits of more forceful interactions between miniaturized soft robots and their environments paving the way for the creation of soft robots as advanced surgical tools.