Rigid instruments can be used for some types of minimally invasive surgery, and robot systems tend to use rigid instruments. A good deal of ingenuity has been applied to allow these rigid instruments to reach different parts of the body through appropriate natural or created orifices for surgery.
But flexible endoscopes and instruments must be used when the surgical site cannot be reached by rigid devices, or open surgery may be the only option. The challenge of soft robotics is to combine the controllability of rigid robotics with the access capabilities of flexible instruments.
This article examines the various scenarios that demand flexible medical sensors and the benefits these sensors bring to the operating room.
Through the keyhole
The introduction of minimally invasive surgery, also known as keyhole surgery, has brought many benefits to both doctor and patient. By operating through a small opening, or a natural orifice, scarring is minimised. This modern surgery technology reduces blood loss and the risk of infection compared to open surgery.
A thin rod with a lens, light source and camera – an endoscope – gives the surgeon a magnified view inside the body. The surgeon can access the body with instruments passed through the opening. A variety of medical sensors relay back information and actuators perform operations – moving, holding, cutting, injecting.
Such procedures have become commonplace for operations on joints, such as cartilage removal or repair. The use of minimally invasive techniques is widespread for surgeries such as appendectomy and hysterectomy, and for taking biopsies. Pain and tissue damage are reduced and recovery times are shorter.
Further progress in these techniques has been made by introducing robots to augment surgeon’s capabilities. Robot-assisted surgery and minimally invasive robotic surgery (MIRS) are on the rise. Here doctors use a console to operate instruments with mechanical arms. Typically, one arm will carry a camera as the primary sensor to provide an image of the working area. Two others will act as extensions of the surgeon’s hands to manipulate and control the actuators, whilst a fourth may be available to remove blockages. The vision systems provide high definition, magnified 3D images. Other medical sensors can be incorporated. The mechanical arms with flexible electronics translate the surgeon’s hand movements into smaller, more precise movements of tiny surgical instruments. They can filter out tremors in the surgeon’s hands, and make working near to major organs safer.
Robot-assisted modern surgery technology offers more accuracy and precision in complex and delicate work such as spine and neurosurgery.
Retinal microsurgery is one example of why robotics is the future of surgery. To carry out retinal vein cannulation, the surgeon must insert a needle through the eye to inject therapeutics into the tiny veins at the back of the eyeball. These retinal veins are perhaps 150 microns wide, about twice the thickness of a human hair. The hand control for such a task would be beyond any human operator. But a mechanical arm can scale down natural hand movement. Instruments can be manoeuvred at speeds of a fraction of a millimetre per second, and positioned with sub-millimetre accuracy. Together with magnified imaging it is possible for an operator to successfully locate these retinal veins and inject into them without causing local damage or disruption.
For the surgeon, one disadvantage of working through robotic arms is that they no longer have the sense of touch. They are unable to sense how much force is being applied when using instruments. This lack of tactile feedback limits the range of surgical techniques that can be used.
However, the introduction of force sensors with flexible electronics attached to the instruments can remedy this. Tiny, thin, force-sensitive resistors can provide feedback to the surgeon. In a typical system, the sensor reads the force on a gripper or grasper, and actuates a corresponding pressure on the surgeon, for example via a headband.
Gripping and moving a vein or artery without being able to feel the pressure exerted is seldom attempted via robotic arms because of the danger of puncture. With a force sensor and actuator, the surgeon can differentiate between a gentle squeeze and a firm grip. He gets the feedback he needs to do delicate tasks safely.
Such haptic sensors and actuators can help the surgeon characterise and manipulate organic tissue and assess anatomical structures. They also reduce tissue damage and broken sutures.
But the design and manufacturing challenges for these sensors, as for other IoMT devices, are considerable. The whole system of strain measurement method together with electronics and connections must be tiny – surgical instruments used in MIS are typically 5mm or less in width. The system must be flexible and low powered, and fully sealed. And it must be able to endure the rigorous cleaning to which surgical instruments are subject. This typically includes autoclaving – high temperature and pressure steaming – and high pH washing.
A wide range of such sensors has now been developed for medical use and many alternative approaches to their manufacture have been explored. They include strain gauges, silicon and polymer based sensors, and fibre optic sensors. These last ones have the advantage that they are flexible.
From rigid to flexible
In the last few years, soft robotics systems have been trialled for endoscopy and bronchoscopy, catheterisation and biopsy. They have most commonly used elastomers, viscous and flexible polymers, though plastics such as PET and hydrogel have also featured. Actuation and control methods included cable driven, pneumatic, hydraulic and magnetic. Some systems can bend as much as 180 degrees, or twist. Sensors for pressure, tension and tendon length measurement are used.
Several innovative mechanisms have been explored to provide the combination of flexibility, motion and control in these robotic arms. Continuum robots are robotic devices whose bodies do not contain single rigid links or joints, but are able to bend continuously and can be considered to have an infinite number of joints. The arm of an octopus is the natural model here. Peristaltic robots are self-propelled devices that mimic the motion of earthworms and snakes. They use anisotropic friction to generate motion. Serial robots consist of several prismatic or rotational joints that are coupled together by links. This is similar to the conventional rigid robot arm structure. But soft pneumatic joints and fluidic actuators have been developed.
The wide variety of devices and operating principles being investigated suggests there is still a great deal of scope for innovation. Although many sensors and actuators are in use in traditional endoscopy, both rigid and flexible, their adaptation and implementation in soft robotics is just beginning. The market for soft tools in surgery is expected to grow by 6% annually over the next five years. Two key areas are materials and manufacture, and sensors.
Many new design and fabrication approaches have been made possible by 3D printing. Flexible photopolymers and conductive polymers form the basis of sensors. Stretchable elastomers which deform under pressure can be used to make soft actuators. Other techniques used for soft materials manufacture are Shape Deposition Manufacturing (SDM) and the Smart Composite Microstructure (SCM) process. The latter uses carbon fibre reinforced polymer rigid bodies linked by flexible polymer ligaments.
In addition to imaging, the introduction of haptic sensors will give the surgeon greater tactile control and better feedback. Sensor design for soft robots is complicated because such vehicles often have complex non-planar surfaces. Whilst some rigid sensors can be used, stretchable, deformable sensors using new materials offer more flexibility in every sense. There also remains great scope for the introduction of diagnostic sensors.
The past few years have seen an increasing quantity of published research, products in development and undergoing trials, new journals and conferences dedicated to flexible surgery and sensors. There are now potential solutions at our fingertips. Given the increasing pace of change in medicine as in other areas, it is only a matter of time before we see these technologies being implemented routinely in the operating theatre.
Medical Sensor Software by Fluffy Spider
At Fluffy Spider, we develop the complex software systems needed to collect data from the sensors and actuators on-board flexible devices and integrate that data to cloud services. We have a dedicated team of professionals, experienced in the development of robust, secure, commercial software, who will work with you from conceptualisation to commercialisation.
If you would like to learn more about our capabilities and solutions, please get in touch.