Abstract Current wearable technologies strive to incorporate more medical functionalities in wearable devices for tracking health conditions and providing information for timely medical treatments. Beyond tracking of a wearer’s physical activities and basic vital signs, the advancement of wearable healthcare devices aspires to continuously monitor health parameters, such as cardiovascular indicators. To properly monitor cardiovascular health, the wearables should accurately measure blood pressure in real-time. However, current devices on the market are not validated for continuous monitoring of blood pressure at a clinical level. To develop wearable healthcare devices such applications, they must be validated by considering various parameters, such as the effects of varying skin properties. Being located between the blood vessel and the wearable device, the skin can affect the sensor readings of the device. The primary goal of this study is to investigate the effect of skin property on the radial pulse measurements. To this end, a range of artificial vein-inserted skin samples with varying properties is fabricated using Magneto-Rheological Elastomers (MRE), materials whose mechanical properties can be altered by external magnetic fields. The samples include layers to simulate the structure of skin and a silicone vein for the pulse to pass through. Note that they are not intended to represent real human skin-vein properties but created to vary a range of stiffness properties to carry out the study. Experiments are performed using a cam system capable of generating realistic human pulse waveforms to pass through the samples. During the indentation testing, the sample is compressed using a dynamic mechanical analyzer (DMA) to record experienced surface pressure, allowing the pulse patterns to be studied. Various samples are used to probe the effects of base resin hardness, iron content, and magnetic field. A pressure sensor incorporated in the cam simulator is used to benchmark the internal pulse pressure of the vein while the DMA indents the sample in order to note the pulse pressures being passed through the sample. Test results show that the properties of the skin influence the resulting pulse behaviors, particularly the ratio of the recorded pulse pressures from the sensor and the DMA.

Abstract Nonlinear cellular structures are defined as structures with multiple scale unit cells patterned through the volume of the structure. The geometrical nonlinearity allows local high flexibility in the movement and also in the sense of strength of materials. The focus of this paper is to create a framework for design for additive manufacturing (DfAM) of a modular nonlinear cellular structure with high level of flexibility. The flexibility will be exploited in skin-like structures adaptable to freeform geometries or utilize flat printed designs for voluminous and structural 3D shapes. For the modeling of the structure CAD software is used and for the fabrication of the structure additive manufacturing (AM) is applied. These technologies work by adding the material in layers, which enables fabrication of parts with complex geometries. The working principal of AM which is opposite to the traditional manufacturing requires for changes in the design process. These changes are applied in the DfAM that we are presenting with this study. The DfAM is used to develop a systematic design approach to support the fabrication of unique structure shapes by AM.

Abstract Electronic skins, or e-skins, are electronic devices capable of sensing physical interactions such as strain, temperature, or pressure. These e-skins are of interest in a variety of fields including robotics, structural health monitoring, and medicine. E-skins should measure strains over a larger range of elongation than traditional strain sensors could. This paper explores the synthesis of a flexible biaxial strain sensor for large surface strain measurement. The sensor is made by spraying an exfoliated graphite and latex mixture onto a latex substrate to form a 4 × 4 grid of electrically conductive strips. Electrodes are connected to each sensor to collect data on deformation induced voltage difference. Two setup geometries were characterized, the behavior of a single strip in each direction in a one by one configuration as well as the behavior of a four by four setup that can measure a two-dimensional strain field. The characteristics of the sensor is studied by attaching it on a tensile testing specimen. When the sensor is subjected to strain along one or both of the two measurement axes, the voltage difference can be recorded using Arduino. The voltage drop was normalized and used to construct a strain distribution plot in MATLAB to determine the highly strained location. In addition to characterizing the behavior of the sensor, the dispersion of the exfoliated graphite in the latex is also studied using optical microscopy. The sensor is made from inexpensive materials and was able to measure large strain that cannot be achieved with commercially available strain gauges.

The adaptation of a wing contour is important for most aircraft, because of the different flight states. That’s why an enormous number of mechanisms exists and reaches from conventional slats and flaps to morphing mechanisms, which are integrated in the wing. Especially integrated mechanisms reduce the number of gaps at the wing skin and produce less turbulent flow. However these concepts are located at a certain section of the wing. This leads to morphing and fixed wing sections, which are located next to each other. Commonly, the transition between these sections is not designed or a wing fence is used. If the transition is not designed, the wing has a step with an activated morphing mechanism and that produces additional vortices. A new skin design will be presented in order to smooth the contour between a fixed wing and a morphing wing. Here the transition between a droop nose and a fixed wing is considered. The skin material is a mix of ethylene propylene diene monomer rubber and glass-fiber reinforced plastic. The rubber is the baseline material, while the glass-fiber is added as stripes in chord-wise direction. In span-wise direction the glass fiber is connected with the rubber. The rubber carries the loads in span-wise direction and reduces the required actuation force. The glass fiber stiffens the skin locally in chord wise direction and keeps the basic contour of the skin. Some geometrical parameters within the skin layup can be varied to change the transition along the span or to reduce the maximum strain within the skin. The local strain maximum is a result of the material transition with different modules. One design of a leading edge was manufactured with an existing mold and it has a span of 200 mm. There are two essential aspects from a structural point of view. One is a nearly continuous deformation along the span and the second is the maximum strain in the rubber. Both aspects are investigated in an experiment and the results are compared with a simulation model. The results show a reliable concept and its numerical model, which will be assigned to a full scale demonstrator. This demonstrator will have a span of 1000 mm and will show the smooth skin transition between a droop nose and a fixed wing.

Artificial and synthetic skins are widely used in the medical field; used in applications ranging from skin grafts to suture training pads. There is a growing need for artificial skins with tunable properties. However, current artificial skins do not take into account the variability of mechanical properties between individual humans as well as the age-dependent properties of human skin. Furthermore, there has been little development in artificial skins based on these properties. Thus, the primary purpose of this research is to develop variable stiffness artificial skin samples using magnetorheological elastomers (MREs) whose properties that can be controlled using external magnetic fields. In this study, multiple MRE skin samples were fabricated with varying filler particle volume contents. Using a precision dynamic mechanical analyzer, a series of indenting experiments were performed on the samples to characterize their mechanical properties. The samples were tested using a spherical indenter that indented a total depth of 1 mm with a speed of 0.01 mm/s and unloaded at the same rate. The results show that the modulus or stiffness increases significantly as the iron percent (w/w) in the sample increases. Additionally, the stiffness of the sample increases proportional to the intensity of the applied external magnetic field. To assess the MRE samples’ variability of properties, the testing results were compared with in vivo human skin testing data. The results show the MRE samples are feasible to represent the age-dependent stiffness demonstrated in in vivo human skin testing. The MRE materials studied will be further studied as a variable-stiffness skin model in medical devices, such as radial pulse simulators.

The main goal of this study is the optimization of vibration reduction on helicopter blade by using macro fiber composite (MFC) actuator under pressure loading. Due to unsteady aerodynamic conditions, vibration occurs mainly on the rotor blade during forward flight and hover. High level of vibration effects fatigue life of components, flight envelope, pleasant for passengers and crew. In this study, the vibration reduction phenomenon on helicopter blade is investigated. 3D helicopter blade model is used to perform the aeroelastic behavior of a helicopter blade. Blade design is created by Spaceclaim and finite element analysis is conducted by ANSYS 19.0. Generated model are solved via Fluent by using two-way fluid-solid coupling analysis, then the analyzed results (all aerodynamic loads) are directly transferred to the structural model. Mechanical results (displacement etc.) are also handed over to the Fluent analysis by helping fluid-structure interaction interface. Modal and harmonic analysis are performed after FSI analysis. Shark 120 unmanned helicopter blade model is used with NACA 23012 airfoil. The baseline of the blade structure consists of D spar made of unidirectional Glass Fiber Reinforced Polymer +45°/−45° GFRP skin. MFC, which was developed by NASA’s Langley Research Center for the shaping of aerospace structures, is applied on both upper and lower surfaces of the blade to reduce the amplitude in the twist mode resonant frequency. D33 effect is important for elongation and to observe twist motion. To foresee the behavior of the MFC, thermo-elasticity analogy approach is applied to the model. Therefore, piezoelectric voltage actuation is applied as a temperature change on ANSYS. The thermal analogy is validated by using static behavior of cantilever beam with distributed induced strain actuators. Results for cantilever beam are compared to experimental results and ADINA code results existing in the literature. The effects of fiber orientation of MFC actuator and applied voltage on vibration reduction on helicopter blade are represented. The study shows that torsion mode determines the optimum placement of actuators. Fiber orientation of the MFC has few and limited influences on results. Additionally, the voltage applied on MFC has strong effects on the results and they must be selected according to applied model.

This paper summarizes the results obtained in the framework of Clean Sky 2 REG-IADP, AIRGREEN on the development of a dedicated morphing device, i.e. a Leading Edge morphing. This device, designed so to be installed on a advanced, twin prop, regional aircraft, is conceived to guarantee high lift conditions together with a smoothed and continuous skin surface, especially important due to the presence of a laminar wing. The design of a such as complex devices required a multi-disciplinary approach, able to combine the aerodynamic performances and the structural ones related to the compliant structures concept adopted for the internal structure. The paper includes an overview of all the design challenges, the adopted solutions and finally the obtained numerical assessments.

Early research on a new concept for a morphing system based on unit structures or cells containing pressurized fluid is presented in this article. The motivation stems from the desire to achieve 3D smooth variations with multiple degrees of freedom and variations in surface area. Such a cell is composed of a hybrid between elastomeric material and stiffening material, creating an orthotropic system. When connected in a network of cells, the morphing system is highly integrated in terms of the components of the skin, substructure and actuation means. Numerical predictions of small simple prismatic cells show a force and axial strain capability of above 200 N and 14% respectively for typical elastomeric and stiffening materials at 8 bar pressure. PolyJet™ additively-manufactured models of wings show how such actuators can be integrated into aircraft structures, including when 3D geometry is highly challenging. These additively-manufactured models were operated at low pressures in the order of 0.5 bar, and a number of open questions on the design, manufacture and operation of such structures are discussed along with intended future work towards higher grade materials and working pressures.

Helicopters suffer from a number of problems raised from the high vibratory loads, noise generation, load capacity limitations, forward speed limitation etc. Especially unsteady aerodynamic conditions due to the different aerodynamic environment between advised and retreating side of the rotor cause most of these problems. Researchers study on passive and active methods to eliminate negative effects of aerodynamic loads. Nowadays, active methods such as Higher Harmonic Control (HHC), Individual Blade Control (IBC), Active Control of Structural Response (ACSR), Active Twist Blade (ATB), and Active Trailing-edge Flap (ATF) gain importance to vibration and noise reduction. In this paper, strain-induced blade twist control is studied integrated by Macro Fiber Composite (MFC) actuator. 3D model is presented to analyze the twisting of a morph and bimorph helicopter rotor blade comprising MFC actuator which is generally applied vibration suppression, shape control and health monitoring. The helicopter rotor blade is modeling with NACA23012 airfoil type and consists of D-spar made of unidirectional fiberglass, ±45° Glass Fiber Reinforced Polymer (GFRP) and foam core. Two-way fluid-structure interaction (FSI) method is used to simulate loop between fluid flow and physical structure to enable the behavior of the complex system. To develop piezoelectric effects, thermal strain analogy based on the similarities between thermal and piezo strains. The optimization results are obtained to show the influence of different design parameters such as web length, spar circular fitting, MFC chord length on active twist control. Also, skin thickness, spar thickness, web thickness are used to optimization parameters to illustrate effects on torsion angle by applying response surface methodology. Selection of correct design parameters can then be determined based on this system results.

Liquid metal (LM) alloys such as eutectic gallium indium (EGaIn) and gallium-indium-tin (Galinstan) have been used in the fabrication of soft and stretchable electronics during the past several years. The liquid-phase and high electrical conductivity of these materials make them one of the best candidates for fabrication of deformable electronics and multifunctional material systems. While liquid metals are highly reliable for fabrication of simple circuits and stretchable microfluidic devices, their application for producing complex circuits faces fabrication challenges due to their high surface tension and surface oxidization. In this study, we propose a scalable, cost-effective, and versatile technique to print complex circuits using silver nanoparticles and transform them into stretchable electronics by incorporating eutectic gallium indium alloys to the circuit. As a result, the deposited liquid metal considerably increases the electrical conductivity and stretchability of the fabricated electronics. The reliability and performance of these stretchable conductors are demonstrated by studying their electromechanical behavior and integrating them into skin-like electronics, termed electronic tattoos.

In this communication, we first introduce the concept of programmable boundary conditions, and then use it to design a nonreciprocal acoustic device: an effective, broadband, acoustic diode. Previous works showed that, using sufficiently small transducers, an active acoustic metasurface can be realized: a smart active acoustic skin with tunable acoustic properties. Using distributed control, these properties can be adapted or reconfigured in real-time. Or, it can even depend on the acoustic field itself, allowing for a programming of the (meta)surface properties: a programmable boundary condition. For instance, a partial derivative equation depending on the acoustic quantities can be imposed, in a discretized form, at the surface of such a programmable boundary. This type of non-standard boundary conditions have been shown to provide the necessary basis for nonreciprocal propagation for a plane wave interacting with a boundary with non grazing incidence, ie. for wavevectors that possess a component normal to the boundary. This restriction may appear problematic when the wavevector is then parallel to the boundary, e.g. when dealing with plane waves in a 1D waveguide, as in an acoustic diode. An acoustic diode, or acoustic isolator, is a nonreciprocal device that let acoustic power pass only in one direction, hence breaking the reciprocity of normal acoustic propagation. We propose a new model of acoustic diode, based on active components: a continuous, distributed source inside the domain. However, based on the modeling of parietal sources in ducts, in the low frequency range, we show that the boundary control approach and the distributed domain sources are equivalent. The only difference is that, in the case of the programmable boundary condition, the near-field of the boundary also contains a component normal to the boundary. Hence our acoustic diode can be realized in practice using programmable boundary conditions. Moreover, the acoustic diode is effective on a broad frequency range, since it can work both on the fundamental mode (plane waves) and on higher-order mode of the waveguide.

Morphing is a technology with high potential to reduce emissions in aviation by adapting the shape of the wings to varying external operating conditions. This paper is presenting results from the EU FP7 funded CHANGE project, where different concepts to adapt a UAV wing airfoil to different demands were investigated. The paper is concentrating on the design and experimental testing of a droop nose, which transforms the leading edge part of the 60 cm chord airfoil from a NACA 6510 shape for loiter and low speed to a NACA 2510 shape for a high speed mission. This paper is presenting the use of an especially soft skin, which reduces the needed force for morphing. That way the requirements for the servos driving the droop nose could be reduced significantly. This paper is showing the implications of such a soft design on the accuracy of the shape generated. For such a skin design, the driving mechanism of the system is designed as a compliant mechanism, which was generated by topology optimization, taking into account aerodynamic loads. For easy manufacturing reasons, thermoplastic polylactic acid (PLA) with zero warp property was used for the manufacturing of this compliant mechanism. Finally deformation measurements of the morphing skin were carried out in a series of lab tests. The match between measured and numerically derived section is quite good, especially in the root region of the wing. Finally an example of an alternative concept to the soft approach is presented. It is the metal based compliant mechanism with a rather stiff GFRP skin. A discussion on the use of different materials and the way forward towards 3D skin optimization is wrapping up the paper.

Origami-folding principles can be used with laminated composites to produce lightweight structures that are capable of drastic changes in shape. This paper presents a smart composite that can actively change its crease pattern to fold itself into different rigid shapes and provide a large range of motion. The composite uses a smart material with variable modulus sandwiched between two fiber-reinforced elastomeric skins, one of which is prestressed. Change in modulus of the sandwiched core layer allows prestress in the elastomeric skin to actuate the fold. Unfolding the structure to a flat shape can be accomplished through either embedded or external actuation. Passive composite panels were fabricated for model development and validation. An analytical model was developed based on classical laminate plate theory to study the influence of core modulus, core thickness, and elastomeric skin prestress on the equilibrium curvature of the composite structure. Selected smart materials that provide a change in modulus when stimulated are discussed as candidates for the core layer of the self-folding composite.

Morphing is a technology with high potential to reduce emissions in aviation, since it enables wings to adapt their shape to operate at a higher efficiency over the full range of flight conditions. This paper is presenting a concept to adapt camber by drooping the nose. The scope is the setup and bench top testing of a full scale wing tip leading edge wind tunnel model with a morphing droop nose. The complete model features a span of 1.3 m and a strong taper from the root to the tip. For completeness, the design approach is covered as well. The design comprises a GFRP skin to be drooped by two compliant mechanisms, which are driven by linear motors. The compliant morphing devices are “designed-through-optimization”, with the optimization algorithms including Simplex optimization for composite compliant skin design, continuum-based and load path representation topology optimization methods for compliant internal substructure design. The compliant mechanism is manufactured by nickel-titanium alloy to allow high strains in the order of several percent, which is shown to be critical in the design of such compliant mechanisms. In order to validate the models, strains within the mechanisms are measured while drooping the nose in the bench top test. This is done after installing the mechanisms into the leading edge skin. It can be shown, that the simulation for the inboard mechanism is close to the experimental results. The comparison of strain levels in the skin and in the mechanism during droop reveals that the stiffness distribution between these two components is quite different. As a result this ratio can be taken into account in future design processes in order to distribute strains more evenly. Moreover the 3D shapes of the morphed and clean skin are measured and their comparison with the target shapes is presented as well. Finally, the bench top tests are a proof of concept for the overall concept and design which resultes in a “go” for the following low speed subsonic wind tunnel tests.

The chord-wise bending airfoil wing could be achieved by employing a kind of active morphing skin which was embedded with pneumatic muscle fibers. This camber morphing structure involves the discipline of structural mechanics which is analysed in this paper. Carbon fiber composite plate is utilized for the upper surface skin of the chord-wise bending airfoil structure, so the approach described in this paper starts from deformation analysis of the upper surface skin based on the classical laminate theory. Energy method was also used for solving the shape function of the upper surface skin which was under the condition of pure bending. While the active morphing skin was actuated at a series of discrete actuation pressures, the fixed geometrical shapes of the chord-wise bending airfoil structure could be obtained. Meanwhile the finite element method (FEM) was used for analyzing this chord-wise bending airfoil structure and the deformed shapes of the upper surface skins would be obtained. Deformed shapes of the upper surface skins between numerical analysis result and FEM analysis result were compared in this paper. This structural analysis work provides useful design and camber morphing characteristics for chord-wise bending airfoil structure. Developers of future shape-adaptive air vehicles have been provided with structural design tools.

A concept for a novel folding wing is presented, which, using the Brazier effect, can snap from a stable, extended position to a folded configuration. A wing typical of size used in an unmanned aircraft vehicle (UAV) is examined, including manufacturing aspects as well as an analytical and a finite element model (FEM) of the structure. The wing is simply made of a glass fiber reinforced plastic (GFRP) skin stiffened by ribs at regular intervals. At the mid-span location, a cut-out is made in the leading and trailing edge in order to allow the pressure and suction sides of the wing to collapse inward when folding occurs (due to Brazier effect). The analytical model draws upon work from Brazier to predict the maximum bending moment the folding section can withstand before buckling. A FEM, using a quasi-static analysis and requiring a contact definition to allow the wing surfaces to meet, reproduces with accuracy the folding pattern seen on the prototype. A bending test of the demonstrator confirmed the validity of the models in terms of bending stiffness, bending snap through and folding radius of curvature.

Persons with transfemoral and transtibial protheses experience changes in the volume of their residual limb during the course of the day. These changes in volume unavoidably lead to changes in quality of fit of the prosthesis, skin irritations, and soft tissue injuries. The associated pain and discomfort can become debilitating by reducing one’s ability to perform daily activities. While significant advancements have been made in prostheses, the undesirable pain and discomfort that occurs due to the volume change is still a major challenge that needs to be solved. The goal of this program is to develop smart prosthetic sockets that can accommodate for volume fluctuations in the residual limb. In this research, fluidic flexible matrix composite wafers (f2mc) are integrated into the prosthetic socket for volume regulation. The f2mc’s are flexible tubular elements embedded in a flexible matrix. These tubular elements are connected to a reservoir, and contain an internal fluid such as air or water. Fluid flow between the tubes and reservoir is controlled by valves. The f2mc’s can achieve more than 300% increase in volume and potentially several orders of magnitude of change in stiffness. Experimental results for a prosthetic socket demonstrate that the flexible matrix composite wafers can achieve changes in volume when pressurized.

This work introduces a new span morphing concept under development at Swansea University. Known as the Adaptive Aspect Ratio wing, this concept couples a compliant skin material to a mechanism based internal structure to create a morphing wing capable of significant changes in span and aspect ratio. The four key technologies of the concept, namely the elastomeric matric composite skin, the telescoping spar, the sliding ribs and the strap drive, are first introduced and discussed. The compliant skin is established to be the dominant component in the overall design of this concept, requiring careful balancing between in-plane actuation force requirements and out-of-plane stiffness under aerodynamic loading. An initial skin design optimization exercise is then carried out using analytical models of the skin’s behaviour, providing significant insight into the interplay between the various parameters of the skin design.

Shape memory alloy (SMA) actuators have recently been developed in the form of torsional tubes that can undergo large twisting deformations. Wing twisting has been investigated as a means to reduce induced drag in cruise conditions in small aircraft, but the actuation hardware required to generate wing twist at larger scales is prohibitively cumbersome. Replacing conventional actuators with SMA torque tubes provides a way to minimize weight of the twisting system but wing structural design then becomes more challenging. This analysis-driven design study examines an SMA torque tube as applied to the twisting wing design problem. A composite skin is considered to maximize wing performance under combined twist and aerodynamic loads. The SMA has been analyzed using a 3-D thermo-mechanical constitutive model while a preliminary study was performed to determine a composite lamina with appropriate unidirectional properties. An optimization was then completed to find an ideal composite layup. This optimization also included the design of a passive torque tube used to properly balance the twist generated by the SMA against that required in the wing. Localized buckling in the twisted wing was also considered and avoided. The product of this optimization was a composite wing that twisted while considering constraints of stress on the SMA. To validate the controllable use of SMA actuators, testing was completed on a scaled wing model fitted with a rapid prototype shell.

A new approach to self-healing systems is presented that aims to overcome the inherent drawbacks of conventional liquid resin based healing systems within composites. Finite embedded systems offer limited healing potential for small volume delaminations and as such cannot effectively heal large damage volumes often associated with shear damaged sandwich panel structures or debonding between skin and core. An expanding polymer based approach aims to overcome such limitations. The mechanical and physical properties of a prepared polyepoxide foam are investigated and how the inclusion of a carbon fibre reinforcement within the foam affects processability and performance. The healing efficiency of different polymer foams to heal damaged structures is also investigated. A secondary investigation is also presented that aimed to overcome the drawbacks associated with the requirement for stoichiometric mixing of two part healing agents, or for healing agent to come into direct contact with a catalyst embedded within the matrix material. Different approaches were taken to develop a self-healing system that once deployed required no additional mixing or stimuli for healing to occur.