## 3 Soft Robot Design Overcoming Challenges in Sensor Integration
### Considerations for the Design of Soft Surgical Robots Integrated with Sensory Capabilities
Soft robots require soft, deformable sensors that measure bending rather than rotation, stretch rather than translation, tactile contact pressure distribution rather than force. As one of the 'holy grails' of soft robotics, much work has been directed into this area, but despite a degree of progress [26; 27], reliable and robust sensors are few and far between. Integrative Robot Design and Fabrication  
Moving from rigid robot technologies to soft robotic solutions within the realm of MIS brings with it a host of challenges. This is because these technologies are fundamentally different, not just in terms of materials, but in the way they operate. Soft robots have fused parts rather than rigid components linked by discrete joints--motion is therefore achieved through deformation of the body material. As a consequence, the design and fabrication of soft robots is entirely different from that of their rigid relatives as shown in Figure 3.  
In the STIFF-FLOP project, the aim was to create a soft robot manipulator capable of complex motions including bending and elongation (Figure 3B). Fluidic actuation was chosen as the ideal medium for an MIS tool [29], an incidental benefit being its compatibility with magnetic resonance imaging (MRI). The principle behind fluidic actuation is the transfer of a pressurised fluid into an actuation chamber that expands in response. As the pressure on the elastic material of the robot's body increases, there is a corresponding amount of resultant distention in the body. By altering the geometry of the chamber and the robot's body, and by using materials of varying degrees of stiffness and flexibility, one can produce an almost infinite array of poses. Research in this field shows that from just a handful of primary motions (including contraction, elongation, bending and twisting--see Figure 4) an almost infinite range of complex motions can be achieved [30; 31; 32]. Figure 3: (**A**) Schematic showing the fabrication stages for a STIFF-FLOP arm segment made of fibre-reinforced pneumatic chambers in a cylindrical silicone structure [28], (**B**) STIFF-FLOP robot arm prototype comprising two segments and a camera integrated at the tip. Although fluidic actuation has its merits, it also presents challenges. The STIFF-FLOP manipulator was designed as a modular device, each module providing three degrees of freedom (2D bending and elongation). Stacking two modules potentially allowed for 6 degrees of freedom, while increasing the area within which the device could operate. Three longitudinal pneumatic chambers were embedded into the cylindrical body of each module and arranged symmetrically around its main axis (Figure 5A). Pressurisation of one of the chambers, while the other two remain 'at rest', caused the module to bend away from the pressurised chamber. Actuation of all three chambers resulted in elongation, and any combination of pressures resulted in a combined movement. The problem we experienced upon manufacture of the first prototypes was that although expansion produced the requisite elongation, it also led to ballooning or radial expansion (see Figure 5B). An initial solution was to counter that effect by reinforcing the device in such a way as to reduce radial expansion but allow the requisite elongation. An external braided sheath was therefore applied, the result of which was that ballooning was no longer apparent and bending motion was enhanced, see Figure 6.  
Figure 4: Primary motions in soft robotics. (**A**) Soft linear actuator elongating due to increasing pressure being supplied [33]; (**B**) Soft robot segment bending due to strain-limiting material being used on one side of the actuator [30]; (**C**) Soft robot segment rotating due to its toroidal geometry [31].