Sensorial and tactile information represent the base of all surgical procedures in medicine. The vision sense has been and continues to be developed extensively by use of micro-cameras, MRI, X-rays and many others. Nonetheless, in many cases, the vision is not enough. The touch sense is necessary to identify the stiffness of the underlying organ or tissue and press more or less to perform a cut, remove a tumor or even move a catheter inside a curved vain. This stiffness is transmitted to the finger of the surgeon as a “pressure-deformation” information. This haptic sense is present naturally in our fingertips. Nevertheless, with the recent development of non-invasive techniques, the surgeon operates robotic devices that deliver optical information via a screen but loses all haptic information since his/her fingers are not in direct contact with the organ. The present project aims at proposing a novel material, a magnetorheological elastomer (MRE) membrane as a haptic sensor. MREs are soft elastomeric materials comprising magnetic particles thus being able to deform significantly upon application of an external magnetic field. Recently, it was shown that by fabricating them in exotic or slender geometries one can exploit their resulting instabilities to shape surfaces, induce programmable swelling and deswelling, or even create swimming microrobots and externally controllable catheters. All those applications use MREs as actuators. By contrast, here, we plan to exploit the reverse operation that of sensing, i.e. induce magnetic field changes via deformation. The principle lies in using the inherent magneto-mechanical coupling to induce readable magnetic fields when the MRE deforms. The reading of the fields can then be translated back to a deformation and a force thus being able to sense soft or stiff objects. The very soft nature of MREs will allow for a very sensitive measurement of forces as low as those felt by touching a soft gel or baby-skin.
In simpler words, when the magnetic field is applied, the material will deform. But conversely, when it is deformed after being magnetized, it will act on the magnetic field that surrounds it. It is these properties and flexibility that will give it its haptic characteristics. "Let's say you're wearing a glove with this material. When you touch something, the material will deform, and the magnetic field will change around it. This change will be measurable. If you touch something that's stiff, the material will warp a lot. On the other hand, if you touch something very soft, the material won't deform, and the magnetic field won't change. We play with the alternations of magnetic fields following the deformation to measure touch," explains Kostas Danas. This type of technology, which is capable of measuring touch, is also called "haptic sensors".
The possible uses of these haptic sensors are diverse. "They have applications in all areas where there are robots. At the moment we don't have any robots that have this sense of touch, they only see with cameras," says Kostas Danas. The flexibility of the material created makes it possible to adapt to any form of robot. And adding meaning to robots can have multiple implications. For example, in the biomedical field, for minimally invasive surgeries1. This type of surgery consists of making very small incisions, and performing the surgery through this incision with the assistance of a video, either with long instruments or on robots equipped with small cameras. But 2D video limits the possibilities offered by these robots, so giving them extra meaning could greatly increase their capabilities and the amount of information they can return to the surgeon performing the operation. Thus, more surgeries would be eligible for this type of operation, yet the benefits of minimally invasive surgeries have been proven, including a reduction in recovery time after the operation and lower risks.
