Robotic technology has been used in the surgical sector for the past 20 years during the search for improved patient outcomes. With the introduction of robots, surgeons can manipulate tissue more precisely, using benefits such as motion scaling and tremor reduction, while working through smaller, less-invasive incisions into the body. Currently, robotic surgery is available for a range of operations, with the desire to expand further the number of procedures that are candidates for this type of technology.
Current surgical robots are impressive pieces of technology, but they have large footprints, usually several orders of magnitude than their effective operating area. This presents a challenge for the surgeons and healthcare professionals who must share a relatively small operating room with them. A further complication resulting from large system size is the integration into current surgical workflows. Each procedure has an established order in which the different steps of an operation take place. Handling large pieces of equipment with intricate set-up routines can be cumbersome in space-constrained environments.
Making surgical robots smaller would provide significant benefits. By reducing the required space for access, smaller robots could be more easily integrated into existing procedures. It would also open up the possibility of using robotic technology in a wider range of procedures. But how do you make smaller robots?
The most common reason these systems are large is that the robots are manipulating long and rigid instruments going into the body through a small incision in the body wall. This arrangement is like a lever in which small motions at the tip of the instrument can translate into large robotic motions at the external end of the tool. Robots also need to be strong enough to counteract the forces exerted on the tool by the body wall and to support their own weight. Creating structures rigid enough to deal with these effects can result in a large surgical robot. One way to reduce in the overall size of surgical robots is to replace traditional lever arm mechanisms with cable-driven flexible instruments that can achieve their required motion range and forces inside the body. This is particularly true for procedures in which the anatomical structure of interest is not large. As the motion takes place inside the body, there is no more lever motion and no large motions outside the body. The robot now does not need to counteract body wall forces. This approach leads to smaller actuators as the force requirements imposed on the system are minimized. Flexible instruments enable smaller systems, which then facilitate integration into existing surgical workflows and operating rooms. Designing minimally invasive, flexible instruments, however, is difficult.
One of the most challenging aspects of miniaturization is the intended size of these systems. Designing and developing flexible manipulators at the millimeter scale brings technical difficulties in all aspects – from design to material selection and right through to manufacturing. At this scale, actuation is usually done through cable-driven mechanisms. Actuation cables require routing through the components, anchoring points, and clearances to minimize interference with their motion. The tools they carry can also require working channels, such as for fluid management or electrosurgery elements. We can look at a manipulator instrument we designed to investigate this possibility of robotic ophthalmic surgery (see Figure 1). The flexible arm of this instrument is 1.8 mm in diameter, with a 1.0 mm diameter central working channel. The actuation cables are roughly 110 microns in diameter, and run through channels with a diameter of 150 microns. Each manipulator arm requires four of these cables, making it necessary to incorporate a large number of features in a very tight space.
This creates a challenge similar to building a very detailed model ship inside a glass bottle. As robots shrink in size, minimizing friction between mechanical components becomes increasingly important. For flexible cable-driven instruments, the complete actuation mechanism (from flexure to actuators) should be designed to minimize friction between the actuator and the instrument tip. Achieving low friction is necessary to produce a fast-actuating, highly responsive system. This is particularly important in surgical robotics, as the system must be able to keep up with the motions of the surgeon’s hands. If the system does not have enough bandwidth to faithfully recreate the movements of the surgeon, its operation will be slow and confusing to use.
One solution to low-friction flexures we identified is to use rolling elements, like pin joints, to build the flexure instead of sliding ones (see Figure 2). This is because one component rolling upon another will resist motion significantly less than if they were sliding against each other. However, using rolling elements can come at the cost of losing the linear behavior of the flexure mechanism, especially if the design of the rolling curve is limited by manufacturing techniques. This means cable pairs used to articulate each of the steering degrees of freedom of the flexure need to be pulled and pushed at different rates because the total cable length varies throughout the range of motion. In order to maintain internal cable tension, the cable length needs to be compensated in some way. One approach is to use one actuator per cable, which results in two actuators per degree of freedom, but this increases the size and cost of the system.
Solving this non-linear cable lengthening problem begins with analyzing the kinematics of the flexure and the requirements imposed by this non-linear behavior. This is the first step in designing a mechanism that produces a linear approximation of the cables’ required motion profile. Numerical simulation and optimization techniques can be employed to optimize the geometry of the mechanism until the error between the linear and non-linear motions is reduced to an acceptable level. We used this approach to optimize a gimbal-type mechanism employed to drive the actuation cables of the flexible arm shown above.
Making precision components at this scale in a cost-effective manner is also a challenge. Techniques like precision micromachining can achieve and maintain the required tolerances of the
different features of these components. Precision comes at a price however, so engineers must look for ways in which these processes can be modified to minimize these costs.
Materials choice is also an important consideration for miniaturization of robotic mechanisms. For a cable-driven mechanism, it would be natural to assume a metal wire, like stainless steel, titanium, or tungsten, would be the best choice. However, at the small scale, forces behave differently so that a 100-micron steel wire which might seem pretty flexible at a macro scale, will be stiff enough to disrupt the smooth motion of the flexure mechanism. Furthermore, having metal-to-metal contact is not the best approach to minimize friction. Polymer threads can be a useful alternative here. Materials like gel-spun polyethylene offer compelling weight, strength, friction, and fatigue properties while maintaining excellent flexibility. But it also comes with another set of challenges. Since the thread is spun from many smaller fibers, it frays easily, making it complicated to handle, especially during assembly. In addition, its low cohesive and friction properties make it difficult to find an adhesive system that can effectively bond it to a grasper or other mechanism.
Assembly is another difficult part of small-scale robotics, beginning with threading cables that are only 110 microns in diameter through a series of holes only 40 microns wider. Although working with such tight tolerances often requires a good deal of manual assembly, properly designed assembly jigs and guides can help simplify the task while promoting consistency and preventing damage to small, delicate components. For example, keeping a thread taut during assembly prevents fraying and protects adhesives while drying. (A good deal of patience and perseverance helps, too.)
As miniaturization continues, we envision more advanced forms of assembly techniques, like micro-automation and design for micro-assembly. Lastly, miniaturization is all about keeping the robot size as small as possible. This means we are trying to pack components into a tight space while maintaining room for cable management and motion clearances. All of these factors, combined with the size scale, make assembly of the devices one of the most challenging parts of these projects.
Although miniaturization of surgical robotics requires new levels of ingenuity, investment, and innovation, we believe it will be a defining trend in medical robotic development. The challenges and complexity of making these robots smaller will quickly be outweighed by the lower cost and ease of use they can offer. Systems at this scale have the potential to expand the range of procedures that could be considered candidates for robotic technology. And, as we have seen, it is possible to design and build robots at this scale by following a sound, well-thought-out engineering approach. What is important to remember is that, although these technologies offer improvements in access and precision of movement, at all times there is a surgeon behind the robot. The objective and intention has, and should always be, to provide better tools for the surgeon – to augment their capabilities and allow them to increase the safety in their procedures and enable better, more consistent outcomes.
Dr. Rodrigo Zapiain Rodrigo is a principal engineer in the medical technology division at Cambridge Consultants. He specializes in the design and development of electromechanical systems, robotics, and bespoke automation instrumentation for medical and life sciences applications, from initial concept to commercial implementation. He holds an engineering doctorate and an MSc, both from the University of Warwick in the UK, and the degree of BS Major in mechanical and electrical engineering from the Instituto Tecnológico y de Estudios Superiores de Monterrey (ITESM-CEM) in Mexico.