Tactile display for presenting stiffness distribution using magnetorheological fluid

This paper describes a tactile display for reproducing stiffness distributions based on magnetorheological (MR) fluid. This display can represent stiffness distribution by controlling the applied magnetic field locally. Computed tomography (CT) and endoscopy are currently used to diagnosis intravital conditions. However, CT cannot detect tumors smaller than 5 mm, and endoscopy can only diagnosis the tissue surface. Since tumors are stiffer than normal tissue, endoscopic palpation may be effective for detecting tumors smaller than 5 mm located beneath the tissue surface. To perform such palpation, a tactile display that can reproduce the spatial stiffness distribution of tissue is strongly required. For intravital tissue, the display must be capable of creating stiffness values ranging from about 200 to about 600 kPa with a spatial resolution of less than 5 mm. In the present study, a tactile display is proposed that exploits the ability of a MR fluid to change its stiffness in a magnetic field. In the proposed device, the MR fluid is encapsulated in an acrylic chamber covered by a thin flexible membrane. We first characterized the mechanical properties of the device and then, conducted sensory experiments with five subjects to verify that the device could display stiffness distribution. The magnetic field was produced by a cylindrical permanent magnet with a diameter of 5 mm, and the applied field strength was controlled by varying the separation between the magnet and the display. The experimental results indicated that the proposed display could successfully recreate the stiffness distribution including stiffness of tumor tissue under a local magnetic field of 200 mT. The device was then evaluated using five subjects, who were asked to touch the device with their index fingers and estimate the size of the stiff spot. Although the results varied among subjects, all were capable of perceiving spots smaller than 5mm

Design, Development, and Analysis of a Tactile Display Based on Composite Magnetorheological Elastomers

In minimally invasive surgery, surgeons carry out the operations by employing small tools and viewing equipment into the patient’s body by means of small incisions. In manual and robotic minimally invasive surgery, surgeons do not have direct touch and natural sense of touch, due to utilization of long and often flexible instruments, and palpation is a necessity to provide perfect diagnoses. As a potential candidate, magnetorheological elastomers were investigated as a stiffness display for surgical application. To this end, three different samples of magnetorheological elastomers with various volume fraction of iron particles and one non-MRE rubber sample were fabricated. Six composite MREs were made by combining two layers of different fabricated samples. The samples were characterized under compression test and perpendicular to the applied magnetic field (MF). The compression test was carried out with the strain range of (5 - 25%) at magnetic field densities of 0, 143, 162, 198, 238, 287, 365 mT. It was observed that the elastic modulus of one-layered MREs and bi-layered MREs increase with increasing the magnetic fields. Moreover, MR-effect was enhanced via bi-layer composition, e.g. mono-layered 45%vol iron particles: 211%, bi-layered 45%vol iron particles: 253%. Afterward, a solution for the medical need of the tactile display during minimally invasive surgeries was proposed. To this end, a tactile display based on the composite magnetorheological elastomers, MiTouch , was designed and prototyped. Also, the electromechanical parameters of MiTouch were identified through a transfer function optimization and a PID controller was fine-tuned to achieve a desired stiffness. Later, validation experiments were carried out to showcase the feasibility of MiTouch for pulse examinations and maintaining a desired stiffness. The results revealed that MiTouch applied a pulsed contact force of 0.6N to the phantom finger. The results were within the range of reported pulse examination forces, i.e. 0.5-2N. In addition, the system was capable of following a desired stiffness of 4N=mm and maintaining it within a range of 4:07 +/- 0:41N/mm. In the end, results confirmed the hypothesis of the feasibility of the suggested solution for surgical applications

Design, Modeling and Control of a Magnetostriction-based Force Feedback System for Robot-assisted Cardiovascular Intervention Systems

Magnetorheological elastomers (MREs), as a class of smart materials, have a property called Magnetostriction means mechanical properties, including deformation of MREs, could be changed in response to an external magnetic field. Because of the controllable deformation, MRE is a suitable candidate for rendering the loss of haptic feedback in Robot-Assisted Cardiovascular (RCI) applications. In the recently-designed such force feedback systems, i.e. TorMag, the effect of matrix shear modulus and filler volume percentage was not studied comprehensively. Tormag also exposed limitations in force range. In the current study, a previously proposed and validated constitutive model of MREs was adopted. Then, twelve MREs with three silicon rubber matrices and four filler volume fractions were fabricated and characterized to improve the limitations mentioned above in Tormag. The average relative error between analytical force range and experiment was 10.2\%, while the maximum force range was 5.29 N (stiffest matrix and 40\% filler), and the minimum range was 1.06 N (softest matrix and 10\% filler). Increasing filler percentage from 10\% to 40\% increased the force feedback range up to 288\%. The state-space analysis of Tormag revealed that this system did not fully cover the required force range and zero force rendering. As an approach, structural optimization of the system is performed using the local and global optimization process. Next, a neural network (NN)-based model as the control framework was proposed and validated to obtain the necessary force for the desired input data. Then, a nearest neighbour search (NNS) method was added to the NN model to find the required magnetic field for a force-displacement profile as input. The proposed neural network accurately predicted the force-displacement behaviour of three types of MREs ($R^2=0.97$, mean-absolute-error=1.26 N). Also, the NN+ NNS model successfully obtained the required magnetic field (mean-absolute error=3.64 mT)

Enabling wearable soft tactile displays with dielectric elastomer actuators

PhDTouch is one of the less exploited sensory channels in human machine interactions. While the introduction of the tactile feedback would improve the user experience in several fields, such as training for medical operators, teleoperation, computer aided design and 3D model exploration, no interfaces able to mimic accurately and realistically the tactile feeling produced by the contact with a real soft object are currently available. Devices able to simulate the contact with soft bodies, such as the human organs, might improve the experience. The existing commercially available tactile displays consist of complex mechanisms that limit their portability. Moreover, no devices are able to provide tactile stimuli via a soft interface that can also modulate the contact area with the finger pad, which is required to realistically mimic the contact with soft bodies, as needed for example in systems aimed at simulating interactions with virtual biological tissues or in robot-assisted minimally invasive surgery. The aim of this thesis is to develop such a wearable tactile display based on the dielectric elastomer actuators (DEAs). DEAs are a class of materials that respond to an electric field producing a deformation. In particular, in this thesis, the tactile element consists of a so-called hydrostatically coupled dielectric elastomer actuator (HC-DEAs). HC-DEAs rely on an incompressible fluid that hydrostatically couples a DEA-based active part to a passive part interfaced to the user. The display was also tested within a closed-loop configuration consisting of a hand tracking system and a custom made virtual environment. This proof of concept system allowed for a validation of the abilities of the display. Mechanical and psychophysical tests were performed in order to assess the ability of the system to provide tactile stimuli that can be distinguished by the users. Also, the miniaturisation of the HC-DEA was investigated for applications in refreshable Braille displays or arrays of tactile elements for tactile maps

Identification of Mechanical Properties of Nonlinear Materials and Development of Tactile Displays for Robotic Assisted Surgery Applications

This PhD work presents novel methods of mechanical property identification for soft nonlinear materials and methods of recreating and modeling the deformation behavior of these nonlinear materials for tactile feedback systems. For the material property identification, inverse modeling method is employed for the identification of hyperelastic and hyper-viscoelastic (HV) materials by use of the spherical indentation test. Identification experiments are performed on soft foam materials and fresh harvested bovine liver tissue. It is shown that reliability and accuracy of the identified material parameters are directly related to size of the indenter and depth of the indentation. Results show that inverse FE modeling based on MultiStart optimization algorithm and the spherical indentation, is a reliable and scalable method of identification for biological tissues based on HV constitutive models. The inverse modeling method based on the spherical indentation is adopted for realtime applications using variation and Kalman filter methods. Both the methods are evaluated on hyperelastic foams and biological tissues on experiments which are analogous to the robot assisted surgery. Results of the experiments are compared and discussed for the proposed methods. It is shown that increasing the indentation rate eliminates time dependency in material behavior, thus increases the successful recognition rate. The deviation of an identified parameter at indentation rates of V=1, 2 and 4 mm/s was found as 28%, 21.3% and 7.3%. It is found that although the Kalman filter method yields less dispersion in identified parameters compared to the variance method, it requires almost 900 times more computation power compared to the variance method, which is a limiting factor for increasing the indentation rate. Three bounding methods are proposed and implemented for the Kalman filter estimation. It was found that the Projection and Penalty bounding methods yield relatively accurate results without failure. However, the Nearest Neighbor method found with a high chance of non-convergence. The second part of the thesis is focused on the development of tactile displays for modeling the mechanical behavior of the nonlinear materials for human tactile perception. An accurate finite element (FE) model of human finger pad is constructed and validated in experiments of finger pad contact with soft and relatively rigid materials. Hyperfoam material parameters of the identified elastomers from the previous section are used for validation of the finger pad model. A magneto-rheological fluid (MRF) based tactile display is proposed and its magnetic FE model is constructed and validated in Gauss meter measurements. FE models of the human finger pad and the proposed tactile display are used in a model based control algorithm for the proposed display. FE models of the identified elastomers are used for calculation of control curves for these elastomers. An experiment is set up for evaluation of the proposed display. Experiments are performed on biological tissue and soft nonlinear foams. Comparison between curves of desired and recreated reaction force from subject's finger pad contact with the display showed above 84% accuracy. As a complementary work, new modeling and controlling approaches are proposed and tested for tactile displays based on linear actuators. Hertzian model of contact between the human finger pad and actuator cap is derived and curves of material deformation are obtained and improved based on this model. A PID controller is designed for controlling the linear actuators. Optimization based controller tuning approach is explained in detail and robust stability of the system is also investigated. Results showed maximum tracking error of 16.6% for the actuator controlled by the PID controller. Human subject tests of recreated softness perception show 100% successful recognition rate for group of materials with high difference in their softness

Novel pneumatic circuit for the computational control of soft robots

Soft robots are of significant research interest in recent decades due to their adaptability to unstructured environments and safe interaction with humans. Soft pneumatic robots, one of the most dominant subsets of soft robots, utilize the interaction between soft elastomeric materials and pressurized air to achieve desired functions. However, the systems currently used for signal computation and pneumatic regulation often make use of rigid valves, pumps, syringe drivers, microcontrollers et al. These bulky and non-integrable devices limit the performance of pneumatically-driven soft robots, carrying challenges for the robot to be miniaturized, untethered, and agile. This DPhil aims to develop pneumatic circuits that can be integrated into the soft robot bodies while performing both onboard computation and control. This thesis presents our contributions towards the aforementioned objective step by step. Firstly, we designed a 3D-printable bistable valve with tunable behaviours for controlling soft pneumatic robots. As an integrable control device, the valve stores one bit of binary information without requiring a constant energy supply and correspondingly controls a pneumatic chamber. Secondly, in order to reduce the number of valves required to control multi-chamber soft robots, we introduced a modular approach to design multi-channel bistable valves based on the previous work. Thirdly, in order to achieve continuous pressure modulation with integrable devices, we designed a soft proportional valve, utilizing the continuous deformation of Magnetorheological Elastomer (MRE) under magnetic flux. Apart from the analogue activation manner, this design also ensures a fast response time, operating at a time scale of tens of milliseconds, much shorter than the mechanical response time of most soft pneumatic actuators. Fourthly, to achieve onboard proportional control of multi-chamber soft robots, we developed an MRE valve array with an embedded cooling chamber. Physical experiments showed that our MRE valve array ensured the independence and accuracy of each valve unit within it, with a significantly lowered temperature of 73.9 $^o$C under 5 minutes of operation. Lastly, we developed an open-source software toolbox supporting the design of integrable pneumatic logic circuits to enhance their accessibility and performance. The toolbox comes with a graphical user interface (GUI) to take users' desired logic functions in the form of a truth table and a set of 2D space constraints related to the available space onboard the robot. It then schedules the pneumatic circuit which performs the desired computation within the space constraints and produces a 3D-printable CAD file that can be fabricated and used directly. The work presented in this thesis enables the community to simplify the process of integrating control devices into soft pneumatic robots, thereby paving the way for a new generation of fully untethered and autonomous soft robots

Image-Based Force Estimation and Haptic Rendering For Robot-Assisted Cardiovascular Intervention

Clinical studies have indicated that the loss of haptic perception is the prime limitation of robot-assisted cardiovascular intervention technology, hindering its global adoption. It causes compromised situational awareness for the surgeon during the intervention and may lead to health risks for the patients. This doctoral research was aimed at developing technology for addressing the limitation of the robot-assisted intervention technology in the provision of haptic feedback. The literature review showed that sensor-free force estimation (haptic cue) on endovascular devices, intuitive surgeon interface design, and haptic rendering within the surgeon interface were the major knowledge gaps. For sensor-free force estimation, first, an image-based force estimation methods based on inverse finite-element methods (iFEM) was developed and validated. Next, to address the limitation of the iFEM method in real-time performance, an inverse Cosserat rod model (iCORD) with a computationally efficient solution for endovascular devices was developed and validated. Afterward, the iCORD was adopted for analytical tip force estimation on steerable catheters. The experimental studies confirmed the accuracy and real-time performance of the iCORD for sensor-free force estimation. Afterward, a wearable drift-free rotation measurement device (MiCarp) was developed to facilitate the design of an intuitive surgeon interface by decoupling the rotation measurement from the insertion measurement. The validation studies showed that MiCarp had a superior performance for spatial rotation measurement compared to other modalities. In the end, a novel haptic feedback system based on smart magnetoelastic elastomers was developed, analytically modeled, and experimentally validated. The proposed haptics-enabled surgeon module had an unbounded workspace for interventional tasks and provided an intuitive interface. Experimental validation, at component and system levels, confirmed the usability of the proposed methods for robot-assisted intervention systems

Mechanical characterization, constitutive modeling and applications of ultra-soft magnetorheological elastomers

Mención Internacional en el título de doctorSmart materials are bringing sweeping changes in the way humans interact with engineering devices. A myriad of state-of-the-art applications are based on novel ways to actuate on structures that respond under different types of stimuli. Among them, materials that respond to magnetic fields allow to remotely modify their mechanical properties and macroscopic shape. Ultra-soft magnetorheological elastomers (MREs) are composed of a highly stretchable soft elastomeric matrix in the order of 1 kPa and magnetic particles embedded in it. This combination allows large deformations with small external actuations. The type of the magnetic particles plays a crucial role as it defines the reversibility or remanence of the material magnetization. According to the fillers used, MREs are referred to as soft-magnetic magnetorheological elastomers (sMREs) and hard-magnetic magnetorheological elastomers (hMREs). sMREs exhibit strong changes in their mechanical properties when an external magnetic field is applied, whereas hMREs allow sustained magnetic effects along time and complex shape-morphing capabilities. In this regard, end-of-pipe applications of MREs in the literature are based on two major characteristics: the modification of their mechanical properties and macrostructural shape changes. For instance, smart actuators, sensors and soft robots for bioengineering applications are remotely actuated to perform functional deformations and autonomous locomotion. In addition, hMREs have been used for industrial applications, such as damping systems and electrical machines. From the analysis of the current state of the art, we identified some impediments to advance in certain research fields that may be overcome with new solutions based on ultrasoft MREs. On the mechanobiology area, we found no available experimental methodologies to transmit complex and dynamic heterogeneous strain patterns to biological systems in a reversible manner. To remedy this shortcoming, this doctoral research proposes a new mechanobiology experimental setup based on responsive ultra-soft MRE biological substrates. Such an endeavor requires deeper insights into the magneto-viscoelastic and microstructural mechanisms of ultra-soft MREs. In addition, there is still a lack of guidance for the selection of the magnetic fillers to be used for MREs and the final properties provided to the structure. Eventually, the great advances on both sMREs and hMREs to date pose a timely question on whether the combination of both types of particles in a hybrid MRE may optimize the multifunctional response of these active structures. To overcome these roadblocks, this thesis provides an extensive and comprehensive experimental characterization of ultra-soft sMREs, hMREs and hybrid MREs. The experimental methodology uncovers magneto-mechanical rate dependences under numerous loading and manufacturing conditions. Then, a set of modeling frameworks allows to delve into such mechanisms and develop three ground-breaking applications. Therefore, the thesis has lead to three main contributions. First and motivated on mechanobiology research, a computational framework guides a sMRE substrate to transmit complex strain patterns in vitro to biological systems. Second, we demonstrate the ability of remanent magnetic fields in hMREs to arrest cracks propagations and improve fracture toughness. Finally, the combination of soft- and hard-magnetic particles is proved to enhance the magnetorheological and magnetostrictive effects, providing promising results for soft robotics.Los materiales inteligentes están generando cambios radicales en la forma que los humanos interactúan con dispositivos ingenieriles. Distintas aplicaciones punteras se basan en formas novedosas de actuar sobre materiales que responden a diferentes estímulos. Entre ellos, las estructuras que responden a campos magnéticos permiten la modificación de manera remota tanto de sus propiedades mecánicas como de su forma. Los elastómeros magnetorreológicos (MREs) ultra blandos están compuestos por una matriz elastomérica con gran ductilidad y una rigidez en torno a 1 kPa, reforzada con partículas magnéticas. Esta combinación permite inducir grandes deformaciones en el material mediante la aplicación de campos magnéticos pequeños. La naturaleza de las partículas magnéticas define la reversibilidad o remanencia de la magnetización del material compuesto. De esta manera, según el tipo de partículas que contengan, los MREs pueden presentar magnetización débil (sMRE) o magnetización fuerte (hMRE). Los sMREs experimentan grandes cambios en sus propiedades mecánicas al aplicar un campo magnético externo, mientras que los hMREs permiten efectos magneto-mecánicos sostenidos a lo largo del tiempo, así como programar cambios de forma complejos. En este sentido, las aplicaciones de los MREs se basan en dos características principales: la modificación de sus propiedades mecánicas y los cambios de forma macroestructurales. Por ejemplo, los campos magnéticos pueden emplearse para inducir deformaciones funcionales en actuadores y sensores inteligentes, o en robótica blanda para bioingeniería. Los hMREs también se han aplicado en el ámbito industrial en sistemas de amortiguación y máquinas eléctricas. A partir del análisis del estado del arte, se identifican algunas limitaciones que impiden el avance en ciertos campos de investigación y que podrían resolverse con nuevas soluciones basadas en MREs ultra blandos. En el área de la mecanobiología, no existen metodologías experimentales para transmitir patrones de deformación complejos y dinámicos a sistemas biológicos de manera reversible. En esta investigación doctoral se propone una configuración experimental novedosa basada en sustratos biológicos fabricados con MREs ultra blandos. Dicha solución requiere la identificación de los mecanismos magneto-viscoelásticos y microestructurales de estos materiales, según el tipo de partículas magnéticas, y las consiguientes propiedades macroscópicas del material. Además, investigaciones recientes en sMREs y hMREs plantean la pregunta sobre si la combinación de distintos tipos de partículas magnéticas en un MRE híbrido puede optimizar su respuesta multifuncional. Para superar estos obstáculos, la presente tesis proporciona una caracterización experimental completa de sMREs, hMREs y MREs híbridos ultra blandos. Estos resultados muestran las dependencias del comportamiento multifuncional del material con la velocidad de aplicación de cargas magneto-mecánicas. El desarrollo de un conjunto de modelos teórico-computacionales permite profundizar en dichos mecanismos y desarrollar aplicaciones innovadoras. De este modo, la tesis doctoral ha dado lugar a tres bloques de aportaciones principales. En primer lugar, este trabajo proporciona un marco computacional para guiar el diseño de sustratos basados en sMREs para transmitir patrones de deformación complejos in vitro a sistemas biológicos. En segundo lugar, se demuestra la capacidad de los campos magnéticos remanentes en los hMRE para detener la propagación de grietas y mejorar la tenacidad a la fractura. Finalmente, se establece que la combinación de partículas magnéticas de magnetización débil y fuerte mejora el efecto magnetorreológico y magnetoestrictivo, abriendo nuevas posibilidades para el diseño de robots blandos.I want to acknowledge the support from the Ministerio de Ciencia, Innovación y Universidades, Spain (FPU19/03874), and the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 947723, project: 4D-BIOMAP).Programa de Doctorado en Ingeniería Mecánica y de Organización Industrial por la Universidad Carlos III de MadridPresidente: Ramón Eulalio Zaera Polo.- Secretario: Abdón Pena Francesch.- Vocal: Laura de Lorenzi

Magnetic Hybrid-Materials

Externally tunable properties allow for new applications of suspensions of micro- and nanoparticles in sensors and actuators in technical and medical applications. By means of easy to generate and control magnetic fields, fluids inside of matrices are studied. This monnograph delivers the latest insigths into multi-scale modelling, manufacturing and application of those magnetic hybrid materials