The field of robotics has slowly but steadily been seeping its way into surgical applications in medicine for the past 20 years. This has allowed surgeons to manipulate tissue and explore motion scaling and tremor reduction. Robots have been used, for example, for knee surgery, where they can reportedly cut recovery times in half owing to speed and precision.
Now, surgical robotics is further evolving. Miniaturisation allows for more efficient and safer invasive surgeries, omitting the chances of human error.
One such innovation in miniature robotics is the Axsis robot, developed by product design and development firm Cambridge Consultants. The company has created a demonstration of cataract surgery as an example to showcase the proficiency that miniature robotics can bring.
Chris Wagner, head of advanced surgical systems at Cambridge Consultants, says: “Cataract surgery is performed by hand, under a microscope, with tools that are about 2mm in diameter. It’s the world’s most common surgery, yet there are still critical complications that can result due to the small size and delicate nature of the eye, and the experience and skill of the surgeon.”
The Axsis robot can provide minimally invasive access and highly accurate navigation, says the company. The system is tele-operated and uses a master-slave configuration, whereby the robot arms follow the motions that a surgeon inputs using two 3D motion controller devices.
Two grippers are attached to the instrument arms as end effector tools, with the possibility of attaching other suitable surgical tools on the arms. These arms are mounted at the end of an actuation pack that has the motors and mechanisms needed to achieve the full motion range.
The arms use a series of rolling rings made out of stainless steel, forming a flexure assembly. These rings are arranged in a configuration that allows the arm to be steered into the desired direction. In addition, the rings have a working channel in the centre to allow for fluid management. Each arm has three degrees of freedom: up/down, left/right, and in/out directions.
The use of a rolling joint provides low friction and allows for fast actuation of the flexure arm. In addition, making the rings out of steel gives the flexure a high degree of stiffness.
Rodrigo Zapiain, a principal engineer in the medical technology division at Cambridge Consultants, says that fast actuation and high stiffness are particularly important in surgical robotics, as the systems must have the ability to keep up with the motions of the surgeon’s hands. If the system cannot recreate the movements of the surgeon, its operation will be complicated and potentially confusing to use, limiting the ability to perform surgical procedures effectively. Zapiain adds that the approach makes possible the use of haptic feedback features in the system.
“The system controller reads the position commanded by the surgeon as they operate two 3D motion controllers, one for each arm,” says Zapiain. “As the surgeon moves the controllers, the system is continuously updating the position demand on the actuators in order to drive the arm to the desired location.
“Furthermore, as we are monitoring the surgeon’s movements, we can implement some of the benefits of robotic surgery, such as motion scaling and tremor reduction, enabling an increase in the precision with which the tissue is handled.”
However, developing the Axsis robot did not come without its challenges. Zapiain says that when surgical robots are miniaturised it creates many obstacles that must be overcome. “Designing and developing such a complex system at this scale brings challenges in all aspects, from design to material selection right up to manufacturing,” he adds.
The complete instrument arm is 1.8mm in diameter, and has a 1mm diameter central channel in it. This leaves only a small amount of space to accommodate all the requirements and components of the flexure, such as actuation cables, rolling elements, and stiffness requirements with as low friction as possible.
Zapiain says the use of rolling elements instead of sliding ones, such as pin joints, allows lower friction in the flexure. However, the cost of losing the linear behaviour of the flexure actuation meant that the cable pairs used to articulate each of the two steering degrees of freedom of the flexure needed to be operated at different rates, potentially increasing the size and cost of the system.
But numerical simulation and optimisation techniques were employed to optimise the geometry of the mechanism until the error between the linear and non-linear motions was small enough and at an acceptable level.
“The cables were chosen because of their fantastic strength and friction properties,” says Zapiain. “However, the low-friction properties of these cables made them difficult to assemble and to attach them securely within the arms.
“Lastly, keeping the robot size as small as possible meant that we were trying to pack a lot of components in a very tight space while maintaining room for cable management and motion clearances.”
The engineering team behind Axsis at Cambridge Consultants does not plan to develop it into a medical product on its own. But they are open to partnering with a client who would be interested in developing it into a complete medical device.
Zapiain says: “We wanted to build the Axsis as a technology demonstrator, as a platform to show what it is possible to achieve in the robotic surgery space, using a particular ophthalmic procedure as an example area. The system demonstrates that it is possible to build robots at a small enough scale for this kind of procedure.”
Traditional surgical robots have mostly been large by design, owing to the need to control long and straight instruments that pass through small incisions made in the patient. Another reason for the robotic devices being large has been because of the need to be strong enough to counteract the forces exerted on the tool by the incision and to support their own weight during surgery. This can put a strain on surgeons and healthcare professionals who have to work within a space-limited operating theatre.
The Axsis prototype uses flexible instruments instead of straight instruments to overcome this limitation and to achieve miniaturisation of the device. As the motion of the Axsis takes place inside the body, the robot does not need to counteract forces from the incision in a patient’s body wall.
This allows the system to be made smaller, making its introduction to operating theatres and incorporation into surgical workflows easier, according to Zapiain.
Miniature robotics can be cost, time and space effective. They can allow for less complex algorithms and modules. In orthopaedics, for example, the use of miniature robotics can provide better accuracy and precision in the preparation of bone surfaces, and more consistent and reproducible results. Bones can be separated and fixed rigidly, which makes it simpler to attach robotic devices to the bones for surgery.
As part of the annual Hamlyn Surgical Robotics Challenge, two robots with similar technology to the Axsis were highlighted in 2015. The winner of the challenge was a concentric tube robot, described as being “snake-like,” that would slither around inside the body to perform a surgical procedure.
Concentric tube robots have an advantage over conventional surgical robots that use motors and tendons because they can be made to have a very small diameter. The winning robot uses a series of curved nitinol tubes that pass through one another. By pushing, pulling and rotating each tube with computer control, it is possible for the tip of the robot to move to any position in 3D space.
Another entry put forward was a miniature robot that can detect cancer. The robot takes the form of a minimally invasive tool that can be used deep within the body with a pressure sensor array that can feel for tumours, and an ultrasound device that gives a visual representation of the underlying tissue. This allows the surgeon to make a diagnosis with greater confidence, according to the team involved in the development.
However, some critics say there are significant disadvantages to miniaturising robots. The biggest drawback raised is that there have been no known human trials using such devices, as this is relatively new technology. The advantages are so far only theoretical, and it will be years before the technology can be put into practice and more substantial results established.
Another disadvantage is the initial cost of developing and implementing the technology within hospitals and other medical facilities. Hospitals and health organisations must be willing to pay the cost of upgrading their systems to achieve compatibility with the miniature robots.
Despite these considerations, miniaturisation of robotics can also be useful for other applications. In space, miniaturisation of robotic devices allows for gains in mass, volume and power. Robots operating on a nanoscale can allow for quicker and safer mobility on space vessels. Reducing size and weight can optimise the capabilities of the device.
A miniature robotic arm scratching out a cloudy lens during cataract surgery may not become a reality in the immediate future. However, the rapid development of prototypes for the technology promises that great strides will be made in the surgical sector.
Clearly, there is much to be gained by making painful, elaborate surgical procedures safer and more efficient for patients and surgeons. This should see such technologies prioritised and propelled to completion in the medical engineering sector.