Publications

2025

Meta-Ori: monolithic meta-origami for nonlinear inflatable soft actuators
Hugo de Souza Oliveira, Xin Li, Johannes Frey and Edoardo Milana
arXiv: 2503.23375 (2025)
The nonlinear mechanical response of soft materials and slender structures is purposefully harnessed to program functions by design in soft robotic actuators, such as sequencing, amplified response, fast energy release, etc. However, typical designs of nonlinear actuators - e.g. balloons, inverted membranes, springs - have limited design parameters space and complex fabrication processes, hindering the achievement of more elaborated functions. Mechanical metamaterials, on the other hand, have very large design parameter spaces, which allow fine-tuning of nonlinear behaviours. In this work, we present a novel approach to fabricate nonlinear inflatables based on metamaterials and origami (Meta-Ori) as monolithic parts that can be fully 3D printed via Fused Deposition Modeling (FDM) using thermoplastic polyurethane (TPU) commercial filaments. Our design consists of a metamaterial shell with cylindrical topology and nonlinear mechanical response combined with a Kresling origami inflatable acting as a pneumatic transmitter. We develop and release a design tool in the visual programming language Grasshopper to interactively design our Meta-Ori. We characterize the mechanical response of the metashell and the origami, and the nonlinear pressure-volume curve of the Meta-Ori inflatable and, lastly, we demonstrate the actuation sequencing of a bi-segment monolithic Meta-Ori soft actuator.
Electropneumatic Oscillators Using Nonlinear Inflatables
Elias De Smet, Lorenzo Migliorini, Edoardo Milana, Paolo Milani and Benjamin Gorissen
Advanced Intelligent Systems, 2400695 (2025)
Animals and robots employ central pattern generators, networks that invoke rhythmic patterns from constant inputs, to orchestrate limb movements during locomotion. Artificial central pattern generators (CPGs) can be either implemented in software or constructed in a physical domain. The former lacks embodiment, inhibiting direct interactions with the physical world. The latter is restricted by a complex translation of abstract functions to the physical domain, e.g., negative feedback, if-then behavior, etc. Here, self-oscillators, a rudimentary type of central pattern generators, that find ground in both the pneumatic and electrical domains are demonstrated. First, the hysteretic behavior of previously developed conical membranes is analyzed in deformation space. This state-space behavior is then transformed to the electrical domain by means of a stretchable strain sensor. Next, an analog comparator distinguishes between the two states and instructs a pneumatic solenoid valve to counteract the current state. As a result, a stable oscillation emerges, with a frequency that is dominated by the physical characteristic of the pneumatic circuit. As such, the proposed electropneumatic oscillator provides a promising platform for building complex CPGs that control interactive neuromorphic robots.
Mechanical metamaterial sensors: from design to applications
Hugo de Souza Oliveira, Niloofar Saeedzadeh Khaanghah, Giulia Elli, Luisa Petti, Giuseppe Cantarella, Edoardo Milana and Niko Münzenrieder
Journal of Physics D: Applied Physics 58, 133002 (2025)
The integration of mechanical metamaterials with sensor technology has opened new frontiers in the design and application of advanced sensing systems due to their ability to impart unique mechanical properties that enhance sensor functionality. Mechanical metamaterials, with properties derived from their engineered structures rather than their material composition, offer unique advantages such as negative Poisson’s ratio, high strength-to-weight ratios, and programmable behaviors. This review explores the dual approaches of incorporating sensors with mechanical metamaterials: metamaterial-supported sensors, where metamaterials provide structural support and enhanced durability to traditional sensors, and metamaterial-integrated sensors, where the metamaterial itself serves as the sensing element. Incorporating metamaterials in sensor design can offer increased sensitivity and precision, enhanced structural integrity and durability, programmability and reconfigurability, as well as lightweight and compact design solutions. Key advancements in the field are presented, highlighting how metamaterials properties can enhance sensor performance in terms of sensitivity, precision, and operational versatility. The review covers the primary materials and fabrication techniques used, including additive manufacturing, molding, and physical vapor deposition, and discusses the challenges associated with the mechanical integration of metamaterials and sensors. Practical applications in pressure, strain, temperature, and biomedical sensing are examined, demonstrating the transformative potential of mechanical metamaterials in creating high-performance, multifunctional sensor systems. The discussion concludes with an outlook on future research directions and potential advancements in the field.

2024

Proprioceptive multistable mechanical metamaterial via soft capacitive sensors
Hugo de Souza Oliveira, Niloofar Saeedzadeh Khaanghah, Martijn Oetelmans, Niko Münzenrieder and Edoardo Milana
arXiv: 2503.23389 (2024)
The technological transition from soft machines to soft robots necessarily passes through the integration of soft electronics and sensors. This allows for the establishment of feedback control systems while preserving the softness of the robot embodiment. Multistable mechanical metamaterials are excellent building blocks of soft machines, as their nonlinear response can be tuned by design to accomplish several functions. In this work, we present the integration of soft capacitive sensors in a multistable mechanical metamaterial, to enable proprioceptive sensing of state changes. The metamaterial is a periodic arrangement of 4 bistable unit cells. Each unit cell has an integrated capacitive sensor. Both the metastructure and the sensors are made of soft materials (TPU) and are 3D printed. Our preliminary results show that the capacitance variation of the sensors can be linked to state transitions of the metamaterial, by capturing the nonlinear deformation.
A bistable pneumatic actuator based on flexible metamaterials
Anagha Navale, Ritika Gupta, Hugo de Souza Oliveira, Viacheslav Slesarenko and Edoardo Milana
In ACTUATOR 2024: International Conference and Exhibition on New Actuator Systems and Applications, Wiesbaden, Germany, pp. 245-248 (2024)
In this work, we combine the programmable mechanical response of flexible mechanical metamaterials with pneumatic actuation, with the aim of constructing soft machines with mechanically programmed actuation patterns. We design and fabricate flexible metamaterials that undergo snap-through instabilities under tension, enabled by the unit cell comprising double-clamped curved beams. Specifically, we test snapping metamaterials with cylindrical topologies, each composed of a repeating array of four unit cells along the circumferential direction. These metamaterials are fabricated using FDM 3D printing with NinjaFlex filament (TPU 85A), and their nonlinear mechanical response under tension is characterized using a uniaxial testing machine. We design a linear actuator by coupling the flexible metamaterial with a pneumatic single-acting cylinder, thereby placing the nonlinear elasticity of the metamaterial in parallel with the piston. The actuators are characterized in terms of their snapping pressures using a measurement setup comprising a motorized syringe pump for controlled inflation and a pressure sensor.

2023

Nonlinear Inflatable Actuators for Distributed Control in Soft Robots
Bert Van Raemdonck, Edoardo Milana, Michael De Volder, Dominiek Reynaerts and Benjamin Gorissen
Advanced Materials 35, 35 (2023)
As soft robotic systems grow in complexity and functionality, the size and stiffness of the needed control hardware severely limits their application potential. Alternatively, functionality can be embodied within actuator characteristics, drastically reducing the amount of peripherals. Functions such as memory, computation, and energy storage then result from the intrinsic mechanical behavior of precisely designed structures. Here, actuators are introduced with tunable characteristics to generate complex actuation sequences from a single input. Intricate sequences are made possible by harnessing hysteron characteristics encoded in the buckling of a cone-shaped shell incorporated in the actuator design. A large variety of such characteristics are generated by varying the actuator geometry. This dependency is mapped and used for creating a tool to determine the actuator geometry that yields a desired characteristic. Using this tool, a system with six actuators is created that plays the final movement of Beethoven's Ninth Symphony with a single pressure supply.
Artificial Cilia–Bridging the Gap with Nature
Sam Peerlinck, Edoardo Milana, Elias De Smet, Michael De Volder, Dominiek Reynaerts and Benjamin Gorissen
Advanced Functional Materials 33, 32 (2023)
Human ingenuity has found a multitude of ways to manipulate fluids across different applications. However, the fundamentals of fluid propulsion change when moving from the macro- to the microscale. Viscous forces dominate inertial forces rendering successful methods at the macroscale ineffective for microscale fluid propulsion. Nature however has found a solution; microscopic active organelles protruding from cells that feature intricate beating patterns: cilia. Cilia succeed in propelling fluids at small dimensions; hence they have served as a source of inspiration for microfluidic applications. Mimicking biological cilia however remains challenging due to their small size and the required kinematic complexity. Recent advances have pushed artificial cilia technology forward, yet discrepancies with natural cilia still exist. This study identifies this gap by analyzing artificial cilia technology and benchmarking them to natural cilia, to pinpoint the remaining design and manufacturing challenges that lay at the basis of the disparity with nature.
Out-of-Plane Soft Lithography for Soft Pneumatic Microactuator Arrays
Edoardo Milana, Benjamin Gorissen, Eline De Borre, Frederik Ceyssens, Dominiek Reynaerts and Michael De Volder
Soft Robotics 10, 1 (2023)
Elastic pneumatic actuators are fueling new devices and applications in soft robotics. Actuator miniaturization is critical to enable soft microsystems for applications in microfluidics and micromanipulation. This work proposes a fabrication technique to make out-of-plane bending microactuators entirely by soft lithography. The only bonding step required is to seal the embedded fluidic channels, assuring the structural integrity of the microactuators. The process consists of fabricating two SU8 mold halves using different lithographic layers. Polydimethilsiloxane is poured on the bottom mold, which is subsequently aligned and assembled with the top mold. The process allows for out-of-plane actuators with a diameter of 300 μm and for fabricating arrays of up to 36 actuators that are row addressable. These active micropillars have an aspect ratio of 1:1.5 and, when pressurized at 1 bar, show a bending angle of ∼30°.
FONS: a Python framework for simulating nonlinear inflatable actuator networks
Arne Baeyens, Bert Van Raemdonck, Edoardo Milana, Dominiek Reynaerts and Benjamin Gorissen
In 2023 IEEE International Conference on Soft Robotics (RoboSoft), Singapore, Singapore, pp. 1–6 (2023)
Soft robots designed within a conventional robotic framework typically consist of individually addressable compliant actuators that are merged together into a deformable body. For inflatable soft robots, this comes at a high cost of tethering which drastically limits their autonomy and versatility. This cost can be decreased by connecting multiple actuators in a fluidic network and partially offloading control to the passive interactions within the network. This type of morphological control necessitates some of the elements in the network to have nonlinear characteristics. However a standardized simulation framework for such networks is lacking. Here, we introduce the open-source python library FONS (Fluidic object-oriented network simulator), a tool for simulating fluidic interactions in lumped fluidic networks of arbitrary size. It is compatible with both gaseous and liquid fluids and supports analytical, simulated and measured characteristics for all components. These components can be defined using a library of standard components or can be implemented as custom objects following a modular object-oriented framework. We show that FONS is capable of simulating a multitude of systems with highly non-linear components exhibiting morphological control.

2022

Towards enduring autonomous robots via embodied energy
Cameron A. Aubin, Benjamin Gorissen, Edoardo Milana, Philip R. Buskohl, Nathan Lazarus, Geoffrey A. Slipher, Christoph Keplinger, Josh Bongard, Fumiya Iida, Jennifer A. Lewis and Robert F. Shepherd
Nature 602 (2022)
Autonomous robots comprise actuation, energy, sensory and control systems built from materials and structures that are not necessarily designed and integrated for multifunctionality. Yet, animals and other organisms that robots strive to emulate contain highly sophisticated and interconnected systems at all organizational levels, which allow multiple functions to be performed simultaneously. Herein, we examine how system integration and multifunctionality in nature inspires a new paradigm for autonomous robots that we call Embodied Energy. Whereas most untethered robots use batteries to store energy and power their operation, recent advancements in energy-storage techniques enable chemical or electrical energy sources to be embodied directly within the structures and materials used to create robots, rather than requiring separate battery packs. This perspective highlights emerging examples of Embodied Energy in the context of developing autonomous robots.
Soft robotics for infrastructure protection
Edoardo Milana
Frontiers in Robotics and AI 9 (2022)
The paradigm change introduced by soft robotics is going to dramatically push forward the abilities of autonomous systems in the next future, enabling their applications in extremely challenging scenarios. The ability of soft robots to safely interact and adapt to the surroundings is key to operate in unstructured environments, where the autonomous agent has little or no knowledge of the world around it. A similar context occurs when critical infrastructures face threats or disruptions, for examples due to natural disasters or external attacks (physical or cyber). In this case, autonomous systems may be employed to respond to such emergencies and have to be able to deal with unforeseen physical conditions and uncertainties, where the mechanical interaction with the environment is not only inevitable but also desirable to successfully perform their tasks. In this perspective, I discuss applications of soft robots for the protection of infrastructures, including recent advances in pipelines inspection, rubble search and rescue, and soft aerial manipulation, and promising perspectives on operations in radioactive environments, underwater monitoring and space exploration.
Towards Half-moon-shaped Soft Pneumatic Cilia
Edoardo Milana, Sam Peerlinck, Sean Flaherty, Dominiek Reynaerts and Benjamin Gorissen
In 2022 IEEE 5th International Conference on Soft Robotics (RoboSoft), Edinburgh, United Kingdom, pp. 681–686 (2022)
Symmetry breaking mechanisms lead to locomotion with a reduced number of inputs, with soft robotic solutions emerging for both terrestrial and aquatic propulsion. For fluids, natural cilia are a source of inspiration that display an asymmetric actuation cycle where the forward and backwards stroke are following a different path. This paper reports on the design, simulation, fabrication, and characterization of a soft bending pneumatic actuator with an unconventional half-moon shaped cross-section. The actuator is fabricated through a single-step molding process out of silicone rubber and optimized using iterative structural simulations.
Monolithic Three-Dimensional Functionally Graded Hydrogels for Bioinspired Soft Robots Fabrication
Marco Piazzoni, Elisa Piccoli, Lorenzo Migliorini, Edoardo Milana, Federica Iberite, Lorenzo Vannozzi, Leonardo Ricotti, Irini Gerges, Paolo Milani, Claudia Marano, Cristina Lenardi and Tommaso Santaniello
Soft Robotics 9, 2 (2022)
Bioinspired soft robotics aims at reproducing the complex hierarchy and architecture of biological tissues within artificial systems to achieve the typical motility and adaptability of living organisms. The development of suitable fabrication approaches to produce monolithic bodies provided with embedded variable morphological and mechanical properties, typically encountered in nature, is still a technological challenge. Here we report on a novel manufacturing approach to produce three-dimensional functionally graded hydrogels (3D-FGHs) provided with a controlled porosity gradient conferring them variable stiffness. 3D-FGHs are fabricated by means of a custom-designed liquid foam templating (LFT) technique, which relies on the inclusion of air bubbles generated by a blowing agent into the monomer-based template solution during ultraviolet-induced photopolymerization. The 3D-FGHs' apparent Young's modulus ranges from 0.37 MPa (bulky hydrogel region) to 0.09 MPa (highest porosity region). A fish-shaped soft swimmer is fabricated to demonstrate the feasibility of the LFT technique to produce bioinspired robots. Mobility tests show a significant improvement in terms of swimming speed when the robot is provided with a graded body. The proposed manufacturing approach constitutes an enabling solution for the development of macroscopic functionally graded hydrogel-based structures usable in biomimetic underwater soft robotics applications.
Morphological Control of Cilia-Inspired Asymmetric Movements Using Nonlinear Soft Inflatable Actuators
Edoardo Milana, Bert Van Raemdonck, Andrea Serrano Casla, Michael De Volder, Dominiek Reynaerts and Benjamin Gorissen
Frontiers in Robotics and AI 8 (2022)
Soft robotic systems typically follow conventional control schemes, where actuators are supplied with dedicated inputs that are regulated through software. However, in recent years an alternative trend is being explored, where the control architecture can be simplified by harnessing the passive mechanical characteristics of the soft robotic system. This approach is named “morphological control”, and it can be used to decrease the number of components (tubing, valves and regulators) required by the controller. In this paper, we demonstrate morphological control of bio-inspired asymmetric motions for systems of soft bending actuators that are interconnected with passive flow restrictors. We introduce bending actuators consisting out of a cylindrical latex balloon in a flexible PVC shell. By tuning the radii of the tube and the shell, we obtain a nonlinear relation between internal pressure and volume in the actuator with a peak and valley in pressure. Because of the nonlinear characteristics of the actuators, they can be assembled in a system with a single pressure input where they bend in a discrete, preprogrammed sequence. We design and analyze two such systems inspired by the asymmetric movements of biological cilia. The first replicates the swept area of individual cilia, having a different forward and backward stroke, and the second generates a travelling wave across an array of cilia.

2021

Model-Based Control Can Improve the Performance of Artificial Cilia
Edoardo Milana, Francesco Stella, Benjamin Gorissen, Dominiek Reynaerts and Cosimo Della Santina
In 2021 IEEE International Conference on Soft Robotics (RoboSoft), New Haven, CT, USA, pp. 527-530 (2021)
Artificial cilia are a prominent example of physical intelligence. Their mechanical properties are often designed so to achieve desired motions in response to very simple actuation patterns. Yet, variability in the mechanical properties are inherent in these systems. This may critically disrupt the input-output relation, resulting in a final behavior completely different from the desired one. In this Communication we investigate the possibility of designing a robotic brain that helps the cilium to maintain its physical intelligence. We achieve that by closing a model-based control loop which tracks the position of the end effector while compensating for drag forces. We propose experiments to characterize our model, and extensive simulations validating the results in different conditions. This work is intended as a proof of concept, which will be further expanded in future work.

2020

Shaping soft robotic microactuators by wire electrical discharge grinding
Edoardo Milana, Mattia Bellotti, Benjamin Gorissen, Jun Qian, Michaël De Volder and Dominiek Reynaerts
Micromachines 11, 7 (2020)
Inflatable soft microactuators typically consist of an elastic material with an internal void that can be inflated to generate a deformation. A crucial feature of these actuators is the shape of ther inflatable void as it determines the bending motion. Due to fabrication limitations, low complex void geometries are the de facto standard, severely restricting attainable motions. This paper introduces wire electrical discharge grinding (WEDG) for shaping the inflatable void, increasing their complexity. This approach enables the creation of new deformation patterns and functionalities. The WEDG process is used to create various moulds to cast rubber microactuators. These microactuators are fabricated through a bonding-free micromoulding process, which is highly sensitive to the accuracy of the mould. The mould cavity (outside of the actuator) is defined by micromilling, whereas the mould insert (inner cavity of the actuator) is defined by WEDG. The deformation patterns are evaluated with a multi-segment linear bending model. The produced microactuators are also characterised and compared with respect to the morphology of the inner cavity. All microactuators have a cylindrical shape with a length of 8 mm and a diameter of 0.8 mm. Actuation tests at a maximum pressure of 50 kPa indicate that complex deformation patterns such as curling, differential bending or multi-points bending can be achieved.
Metachronal patterns in artificial cilia for low Reynolds number fluid propulsion
Edoardo Milana, Rongjing Zhang, Maria Rosaria Vetrano, Sam Peerlinck, Michael De Volder, Patrick R. Onck, Dominiek Reynaerts and Benjamin Gorissen
Science Advances 6, 49 (2020)
The influence of metachronal patterns on ciliary fluid flow has been studied using a soft robotic cilia array and simulations. Cilia are hair-like organelles, present in arrays that collectively beat to generate flow. Given their small size and consequent low Reynolds numbers, asymmetric motions are necessary to create a net flow. Here, we developed an array of six soft robotic cilia, which are individually addressable, to both mimic nature’s symmetry-breaking mechanisms and control asymmetries to study their influence on fluid propulsion. Our experimental tests are corroborated with fluid dynamics simulations, where we find a good agreement between both and show how the kymographs of the flow are related to the phase shift of the metachronal waves. Compared to synchronous beating, we report a 50% increase of net flow speed when cilia move in an antiplectic wave with phase shift of −pi/3 and a decrease for symplectic waves. Furthermore, we observe the formation of traveling vortices in the direction of the wave when metachrony is applied.
Fabrication of High-Aspect-Ratio Cylindrical Micro-Structures Based on Electroactive Ionogel/Gold Nanocomposite
Edoardo Milana, Tommaso Santaniello, Paolo Azzini, Lorenzo Migliorini and Paolo Milani
Applied Nano 1, 1 (2020)
We present a fabrication process to realize 3D high-aspect-ratio cylindrical micro-structures of soft ionogel/gold nanocomposites by combining replica molding and Supersonic Cluster Beam Deposition (SCBD). Cylinders’ metallic masters (0.5 mm in diameter) are used to fabricate polydimethylsiloxane (PDMS) molds, where the ionogel is casted and UV cured. The replicated ionogel cylinders (aspect ratio { extgreater} 20) are subsequently metallized through SCBD to integrate nanostructured gold electrodes (150 nm thick) into the polymer. Nanocomposite thin films are characterized in terms of electrochemical properties, exhibiting large double layer capacitance (24 μF/cm2) and suitable ionic conductivity (0.05 mS/cm) for charge transport across the network. Preliminary actuation tests show that the nanocomposite is able to respond to low intensity electric fields (applied voltage from 2.5 V to 5 V), with potential applications for the development of artificial smart micro-structures with motility behavior inspired by that of natural ciliate systems.
EELWORM: a bioinspired multimodal amphibious soft robot
Edoardo Milana, Bert Van Raemdonck, Kevin Cornelis, Enrique Dehaerne, Jef De Clerck, Yarno De Groof, Toon De Vil, Benjamin Gorissen and Dominiek Reynaerts
In 2020 IEEE International Conference on Soft Robotics (RoboSoft), New Haven, CT, USA, pp. 766-771 (2020)
Exploration robots are challenged by a continuous adaptation to the terrain induced by ever changing environments. These adaptations can be subtle (e.g. when moving from a smooth to a rough terrain), however drastic changes in environment require robots to address different locomotion modes (e.g. crawling vs swimming). While each locomotion mode can be driven by a dedicated set of actuators, nature shows that multimodal locomotion is also possible by activating the same set of actuators in different sequences (e.g. swimming snakes). In this paper, we present EELWORM, a 40 cm long soft-bodied robot consisting out of an arrangement of five inflatable bending and elongating actuator modules that can be addressed individually. EELWORM is capable of both crawling and swimming by varying the actuation sequences within the same embodiment. We show multimodal locomotion at speeds of 2 body lengths per minute (crawling) and 3 body lengths per minute (swimming).
Advances in soft robot actuation and their morphological control
Edoardo Milana, Bert Van Raemdonck, Sam Peerlinck, Dominiek Reynaerts and Benjamin Gorissen
In 20th International Conference and Exhibition of the European Society for Precision Engineering and Nanotechnology (EUSPEN), Geneva, Switzerland, pp. 1-2 (2018)
Robotic systems have been going through a staggering evolution over the past decades, with new generations of robots, increasingly dexterous and intelligent, being developed at an unprecedented rate. Where robots are traditionally thought of as rigid systems, the introduction of compliant materials in robotics has sprouted a new robotic family tree. Because of their versatility and compliancy, these soft robots are uniquely suited to accomplish delicate tasks where a safe interaction with humans or other biological systems is preeminent: minimally invasive surgery, elderly care, exoskeletons and, more recently, even the video game industry. All these applications require robots that are able to interact safely in unstructured environments. In this context, conventional rigid robots, made out of rigid links and actuators with a fixed number of degrees of freedom, typically fail because of the extreme complexity and cost of sensor and control systems. Recent advances in soft robotics showed that this limitation can be avoided. For example, grabbing a delicate object using a soft actuator does not necessitate sensor feedback. Further, by actively harnessing structural nonlinearities, functionalities can be encoded within the distributed hardware of the system, drastically lowering the computational power needed for control. This paper will highlight recent advances in soft robotic technology with applications ranging from minimally invasive surgical tools to biomimicking artificial cilia and walking robots that harness morphological control.

2019

Precise bonding-free micromoulding of miniaturized elastic inflatable actuators
Edoardo Milana, Mattia Bellotti, Benjamin Gorissen, Michael De Volder and Dominiek Reynaerts
In 2019 IEEE International Conference on Soft Robotics (RoboSoft), Seoul, South Korea, pp. 768-773 (2019)
A new precise micromoulding process has been developed in order to fabricate elastic inflatable microactua-tors, having a corrugated inner cavity without requiring any additional bonding step. Micromoulds are machined using micromilling and wire electrical discharge grinding. This manufacturing process allows the out-of-plane fabrication of complex shapes of microactuators with smallest feature down to 40 μm, It is possible to fabricate miniaturized versions of actuators that are normally made at a larger scale. Particularly, this paper reports a silicone-made miniaturized elastic inflatable actuator showing a large bending deformation. The particular pattern of the inner cavity forms a corrugated structure and enables a fully curled deformation of the actuator due to the equal distribution of multiple bending points. As a demonstrator, three microactuators are used as active fingers of a soft microgripper.
Artificial Soft Cilia with Asymmetric Beating Patterns for Biomimetic Low-Reynolds-Number Fluid Propulsion
Edoardo Milana, Benjamin Gorissen, Sam Peerlinck, Michael De Volder and Dominiek Reynaerts
Advanced Functional Materials 29, 22 (2019)
In nature, liquid propulsion in low-Reynolds-number regimes is often achieved by arrays of beating cilia with various forms of motion asymmetry. In particular, spatial asymmetry, where the cilia follow a different trajectory in their effective and recovery strokes, is an efficient way of generating flow in low Reynolds regimes. However, this type of asymmetry is difficult to mimic and control artificially. In this paper, an artificial soft cilium that comprises two pneumatic actuators that can be controlled individually is developed. These two independent degrees of freedom allow for the first time adjustment and study of spatial asymmetry in the cilium's beating pattern. Using low-Reynolds-number flow measurements, it is confirmed that spatial asymmetry allows for the generation of fluid propulsion. These two-degree-of-freedom soft cilia provide a platform to study ciliary fluid transport mechanisms and to mimic biologic viscous propulsion.
Hardware Sequencing of Inflatable Nonlinear Actuators for Autonomous Soft Robots
Benjamin Gorissen, Edoardo Milana, Arne Baeyens, Eva Broeders, Jeroen Christiaens, Klaas Collin, Dominiek Reynaerts and Michael De Volder
Advanced Materials 31, 3 (2019)
Soft robots are an interesting alternative for classic rigid robots in applications requiring interaction with organisms or delicate objects. Elastic inﬂatable actuators are one of the preferred actuation mechanisms for soft robots since they are intrinsically safe and soft. However, these pneumatic actuators each require a dedicated pressure supply and valve to drive and control their actuation sequence. Because of the relatively large size of pressure supplies and valves compared to electrical leads and electronic controllers, tethering pneumatic soft robots with multiple degrees of freedom is bulky and unpractical. Here, a new approach is described to embed hardware intelligence in soft robots where multiple actuators are attached to the same pressure supply, and their actuation sequence is programmed by the interaction between nonlinear actuators and passive ﬂow restrictions. How to model this hardware sequencing is discussed, and it is demonstrated on an 8-degree-of-freedom walking robot where each limb comprises two actuators with a sequence embedded in their hardware. The robot is able to carry pay loads of 800 g in addition to its own weight and is able to walk at travel speeds of 3 body lengths per minute, without the need for complex on-board valves or bulky tethers.

2018

Lithographic production of vertically aligned CNT strain sensors for integration in soft robotic microactuators
Benjamin Gorissen, Edoardo Milana, Dominiek Reynaerts and Michael De Volder
In 2018 IEEE International Conference on Soft Robotics (RoboSoft), Livorno, Italy, pp. 400-405 (2018)
This paper reports on a piezoresistive strain sensor that uses vertically aligned carbon nanotube (CNT) filler elements which are embedded in a rubber matrix. Compared to previously used conductive filler elements, vertical CNTs can be patterned using lithography, making it possible to scale down the sensor footprint into the micrometer range. This technological advancement is instrumental for developing intelligent soft microrobots with embedded flexible sensors. We compare vertical CNTs and carbon black as filler elements, where newly developed lithographic production techniques are applied to shape a generic strain sensor topology that is compatible with the majority of planar inflatable microactuators. This research shows a significant improvement in sensor linearity by using vertical CNTs as filler elements over carbon black. Further, a lithographically fabricated strain sensor has been successfully embedded in an elastic inflatable bending microactuator with outer dimensions of 5.5×1×0.06mm3. The full lithographic production process to create this actuator is described in this paper, together with its characterization under a static pressure input.
Design of a bi-segmented soft actuator with hardware encoded quasi-static inflation sequence
Edoardo Milana, Benjamin Gorissen, Michael De Volder and Dominiek Reynaerts
In 2018 IEEE International Conference on Soft Robotics (RoboSoft), Livorno, Italy, pp. 108-113 (2018)
A soft actuator composed of two fluidic bending segments is designed and manufactured having a mechanically programmed sequence of inflation. The sequence is determined by analyzing the equilibrium states between the connected segments at each step of the inflation. To do this, segments are described using their inner pressure vs. volume expansion relationship. Since it may be difficult to formulate this expression analytically for complex soft inflatable structures, an approach based on nonlinear FEM simulations is here introduced: by modelling the inner cavity of an actuator as filled with incompressible fluid and generating a fluid flux during the simulation, the pressure-volume curve is easily obtained, even if highly nonlinear. The resulting nonlinearities are instrumental in generating inflation sequencing of multiple segmented actuators, which is an example of hardware-based intelligence. Exploiting these behaviours will greatly enhance the possibilities and performances of soft robots.
Design and characterization of soft microactuators based on interconnected pneumatic networks
Edoardo Milana, Mattia Bellotti, Shashwat Kushwaha, Benjamin Gorissen and Michaël De Volder
In 18th International Conference and Exhibition of the European Society for Precision Engineering and Nanotechnology (EUSPEN), Venice, Italy, pp. 1-2 (2018)
This paper shows the advantages of introducing pneumatic networks in flexible bending microactuators, in terms of motion complexity and new functionalities. A bending microactuator made out of silicone rubber is designed with an embedded pneumatic network in the inner chamber to be compatible with a bonding-free fabrication process using micro-moulding. The pneumatic network is formed by wire electrical discharge grinding of a micro rod, having the negative shape of the inner chamber, which is placed in a micromilled mould. The deformation is simulated using FEM analysis and compared to a previous developed design. The microactuator is then characterized in terms of deformation vs. pressure input, showing a significant improvement of the bending performance. The microactuator achieves a fully curled configuration at 50 kPa, enabling the possibility of being applied as soft microgripper.
Wire electrical discharge grinding of micro rods for bonding-free fabrication of soft pneumatic microactuators
Mattia Bellotti, Edoardo Milana, Benjamin Gorissen, Jun Qian and Dominiek Reynaerts
In 18th International Conference and Exhibition of the European Society for Precision Engineering and Nanotechnology (EUSPEN), Venice, Italy, pp. 1-2 (2018)
This paper presents the machining process of tungsten carbide micro rods using wire electrical discharge grinding (WEDG). The micro-rodes are used in the bonding-free moulding process of soft pneumatic microactuators. First, a multi-step cutting strategy including roughing and finishing is described. Second, two sets of grooves of different depths are WEDGed into the micro rods. Based on the concept of interconnected pneumatic networks, the machining of micro rods would enable the fabrication of soft pneumatic microactuators in which bending motion is concentrated on specific sections when they are pressurised.
Artificial Pneumatic Cilia
Benjamin Gorissen, Edoardo Milana, Michael De Volder and Dominiek Reynaerts
In Atlas of Cilia Bioengineering and Biocomputing, 5 (2018)
This chapter focuses on pneumatically controlled artificial cilia, where two types of pneumatic cilia can be distinguished: those where passive hair-like structures are implanted on top of a pneumatically actuated base membrane and those where the body of the hair-like structure itself is pneumatically actuated. As the base actuated pneumatic cilia operate at a Reynolds number of 5, it can be concluded that inertial effects play an important role on metachronally induced fluid flow. Body actuated pneumatic cilia on the other hand try to mimic these advanced beating patterns by actuating the internal structure of the cilia itself rather than actuating the base structure. The performance of these base actuated plate-like cilia has been tested using experiments in water, where the effect of orientational asymmetry and metachronal asymmetry has also been studied. The influence of orientational asymmetry is quantified by using four different inclination angles of the flaps, while keeping metachronal, temporal, and orientational asymmetry constant.