L. Frasson

Closed-Loop Planar Motion Control of a Steerable Probe With a “Programmable Bevel” Inspired by Nature

Percutaneous intervention has attracted significant interest in recent years, but many of todays needles and catheters can only provide limited control of the trajectory between an entry site and soft tissue target. In order to address this fundamental shortcoming in minimally invasive surgery, we describe the first prototype of a bioinspired multipart probe that can steer along planar trajectories within a compliant medium by means of a novel “programmable bevel,” where the steering angle becomes a function of the offset between interlocked probe segments. A kinematic model of the flexible probe and programmable bevel arrangement is derived. Several parameters of the kinematic model are then calibrated experimentally with a fully functional scaled-up prototype, which is 12 mm in diameter. A closed-loop control strategy with feed-forward and feedback components is then derived and implemented in vitro using an approximate linearization strategy that was first developed for car-like robots. Experimental results demonstrate satisfactory 2-D trajectory following of the prototype (0.68 mm tracking error, with 1.45 mm standard deviation) using an electromagnetic position sensor that is embedded at the tip of the probe.

STING: a soft-tissue intervention and neurosurgical guide to access deep brain lesions through curved trajectories:

Abstract Current trends in surgical intervention favour a minimally invasive approach, in which complex procedures are performed through very small incisions. Specifically, in neurosurgery there is a need for minimally invasive keyhole access, which conflicts with the lack of manoeuvrability of conventional rigid instruments. In an attempt to address this shortcoming, the current state of progress is reported on a soft-tissue intervention and neurosurgical guide (STING) to access deep brain lesions through curved trajectories. The underlying mechanism of motion, based on the reciprocal movement of interlocked probe segments, is biologically inspired and was designed around the unique features of the ovipositor of certain parasitic wasps. Work to date has focused on probe development, low- and high-level control, and trajectory planning. These aspects are described, together with results on each aspect of the work, including biomimetic microtexturing of the probe surface. Progress is very encouraging and demonstrates that forward motion into soft tissue through a reciprocating mechanism is indeed viable and can be achieved through a suitable combination of microtexturing and microfabrication techniques.

Highly resolved strain imaging during needle insertion: Results with a novel biologically inspired device.

Percutaneous needle insertions are a common part of minimally invasive surgery. However, the insertion process is necessarily disruptive to the substrate. Negative side effects are migration of deep-seated targets and trauma to the surrounding material. Mitigation of these effects is highly desirable, but relies on a detailed understanding of the needle-tissue interactions, which are difficult to capture at a sufficiently high resolution. Here, an adapted Digital Image Correlation (DIC) technique is used to quantify mechanical behaviour at the sliding interface, with resolution of measurement points which is better than 0.5mm, representing a marked improvement over the state of the art. A method for converting the Eulerian description of DIC output to Lagrangian displacements and strains is presented and the method is validated during the simple insertion of a symmetrical needle into a gelatine tissue phantom. The needle is comprised of four axially interlocked quadrants, each with a bevel tip. Tests are performed where the segments are inserted into the phantom simultaneously, or in a cyclic sequence taking inspiration from the unique insertion strategy associated to the ovipositor of certain wasps. Data from around the needle-tissue interface includes local strain variations, material dragged along the needle surface and relaxation of the phantom, which show that the cyclic actuation of individual needle segments is potentially able to mitigate tissue strain and could be used to reduce target migration.

Biologically inspired microtexturing: Investigation into the surface topography of next-generation neurosurgical probes

Minimally Invasive (MI) surgery represents the future of many types of medical intervention (keyhole neurosurgery, natural orifice trans-luminal endoscopic surgery, etc.). However, the shortcomings of todays surgical tools fuel the need for the development of next-generation “smart instrumentation”, which will be more accurate and safer for the patient. This paper presents the preliminary results of a biologically inspired microtexturing method, based on UV-lithography, and its application to MI neurosurgery. These results suggest that the size and geometry of the texture “printed” on the outer surface of a neurosurgical probe clearly affect the insertion and extraction forces generated at the brain-probe interface. Thus, by carefully choosing an appropriate microtexture, unique insertion characteristics can be obtained, which can improve the performance of existing instruments (e.g. reducing slippage in permanent electrodes such as those used in deep brain stimulation) or enable the development of novel designs altogether.

Soft tissue traversal with zero net force: Feasibility study of a biologically inspired design based on reciprocal motion

This paper builds on previous work on the biologically inspired microtexturing of next-generation neurosurgical probes [3]. It reports on the outcome of a feasibility study, where a biomimetic robotic actuator was used to demonstrate effective soft tissue traversal (i.e. motion along the surface of a soft tissue) through the reciprocating motion of custombuilt anisotropic surface textures, without the need to apply an external force to push the tissue along the surface and while causing minimum tissue damage. The protocol applied to characterize the interaction between the samples and the different tissues was considered, including parameters such as texture size and geometry, normal load, frictional forces and reciprocating speed and acceleration. A number of microtextured samples, with features including every combination of two teeth geometries (triangular and fin-like) and three tooth sizes (500µm, 100µm and 50µm tooth height), were manufactured and mounted onto a custom-built reciprocating mechanism. First, a variety of soft tissue specimens was tested to qualify the sample/tissue interaction behavior. Subsequently, a comparative study on agarose gel and gelatin was performed to investigate the motion characteristics and resulting tissue damage on brainlike material and to explore what conditions are needed to achieve forward motion.

Tissue deformation analysis using a laser based digital image correlation technique.

A laser based technique for planar time-resolved measurements of tissue deformation in transparent biomedical materials with high spatial resolution is developed. The approach is based on monitoring the displacement of micrometer particles previously embedded into a semi-transparent sample as it is deformed by some form of external loading. The particles are illuminated in a plane inside the tissue material by a thin laser light sheet, and the pattern is continuously recorded by a digital camera. Image analysis yields the locally and temporally resolved sample deformation in the measurement plane without the need for any in situ measurement hardware. The applicability of the method for determination of tissue deformation and material strain during the insertion of a needle probe into a soft material sample is demonstrated by means of an in vitro trial on gelatin.

Experimental evaluation of a novel steerable probe with a programmable bevel tip inspired by nature

The ability to steer flexible needles and probes to access deep anatomical locations safely for medical diagnosis and treatment represents a current clinical and engineering research challenge. The behaviour of parasitic wasps has inspired the development of a novel steerable and flexible multi-part probe, which allows the control of its approach angle by adjusting the steering offset between probe segments, i.e. by means of a programmable bevel tip. This paper describes the experimental evaluation of several scaled-up proof-of-concept flexible probe prototypes to explore the effects of tip design (bevel-tip angle) and dimensions (outer diameter) on steering. For each prototype, a linear relationship between steering offset and curvature is confirmed. The effect of probe diameter and bevel-tip angle on steering performance is also analysed, with results confirming that larger bevel-tip angles and smaller probe diameters lead to larger curvature values, although improved steering comes at the price of a less stable insertion process.

Early Developments of a Novel Smart Actuator Inspired by Nature

Current research at Imperial College focuses on the development of a novel neurosurgical probe for Minimally Invasive Surgery (MIS), which can be used to target deep lesions in the brain by exploiting the unique design of certain ovipositing wasps. While conventional neurosurgical instruments are rigid and can only be used to achieve straight-line trajectories, the biomimetic design will enable curved paths connecting any entry point to any target within the brain to be followed autonomously. This paper reports on the successful outcome of an early feasibility study, where two of the key concepts behind the novel actuator design are investigated: a biologically inspired robotic actuator was developed to demonstrate effective soft tissue traversal (i.e. motion along the surface of a soft tissue) by reciprocating custom-built anisotropic surface textures, without the need to apply an external force to push the tissue along the surface. Then, custom-designed rigid probes with bio-inspired surface topographies were fabricated and tested on cadaveric porcine brain with the aim to characterize the insertion and extraction forces due to friction and tribological interaction with biological tissue.

Insertion experiments of a biologically inspired microtextured and multi-part probe based on reciprocal motion

While there have been significant advances in minimally invasive surgical instrumentation, the majority of tools still rely on a push from the back to aid insertion into the tissue, whether the process is manual or servo assisted. In this work, a novel approach to tool insertion is proposed which is based on the concept of a multi-part probe with at least three interlocking segments. By means of a sequential insertion process, where each segment is pushed further into the tissue while stabilized by the remaining stationary parts, the multi-part probe concept is shown to successfully “insinuate itself” within a synthetic soft tissue specimen without the need for an overall forward push. The presence of an anisotropic microtextured outer probe surface is also shown to affect the overall speed of insertion and can thus be used to optimize the interaction forces at the probe-tissue interface. A measured reduction in the force transferred to the back of the specimen also suggests that this approach to tool insertion may result in reduced tissue disruption, a result which could lead to less tissue damage and a reduction in target displacement.

Development and validation of a numerical model for cross-section optimization of a multi-part probe for soft tissue intervention

The popularity of minimally invasive surgical procedures is driving the development of novel, safer and more accurate surgical tools. In this context a multi-part probe for soft tissue surgery is being developed in the Mechatronics in Medicine Laboratory at Imperial College, London. This study reports an optimization procedure using finite element methods, for the identification of an interlock geometry able to limit the separation of the segments composing the multi-part probe. An optimal geometry was obtained and the corresponding three-dimensional finite element model validated experimentally. Simulation results are shown to be consistent with the physical experiments. The outcome of this study is an important step in the provision of a novel miniature steerable probe for surgery.