An innovative dynamic hybrid metamaterial structure created for an ultra-light, highly precise, and self-correcting live mirror

Paper 13431-8 SPIE Smart Material + NDE — Vancouver, Canada — Ed. 2025 An innovative dynamic hybrid metamaterial structure created for an ultra-light, highly precise and self-correcting Live Mirror Gil Moretto*a, Kritsadi Thetpraphi b, Mariano Laguna-Morenoc, Dipanjana Sahaa, Peter Königc, Johannes Hörberd, Peter Roginc, Peter W. de Oliveirac, Vincent Bruyèree, Maud Langloisa, Vincenzo Cotroneof, Alejandro M. Paolag, José A. Diesteg and Francesca Rebastia aCenter for Astrophysics of Lyon (CRAL/CNRS), Saint-Genis Laval 69770, France; b Division of Physics, School of Science, Walailak University, Tasala District 80160, Thailand; cINM, Leibniz Institute for New Materials, Campus D2 2, 66123 Saarbrucken, Germany; dKRONOS Mechatronics GmbH, Fürther Str. 246c, 90429 Nürnberg, Germany; eSIMTEC, 5 rue Felix Poulat, 38000 Grenoble, France; fBrera Astronomical Observatory, INAF, via E. Bianchi 46, 23807 Merate Lc, Italy; gAITIIP Technology Centre, 50720 Cartuja Baja, Zaragoza, Spain. ABSTRACT The Live Mirror EIC consortium (Horizon EIC #101099220), funded by the Horizon European Innovation Council (EIC), is spearheading the development of an innovative technology for a new lightweight, hybrid meta-material, and selfcorrecting mirror. This cutting-edge mirror relies on three critical elements: (1) precise processes for shaping fire-polished sheet glass; (2) dynamic hybrid metamaterial structure (HM-MS) that incorporates 3D-printed flexible electrodes and electroactive polymer actuators, designed to support the mirror and rectify any deformations resulting from thermal variations, gravitational effects, and wind loads and (3) a metrology & calibration multi-sensing in real-time closed-loop control. These advancements in smart structures are anticipated to produce cost-effective, lightweight, integrated optoelectronic systems and will illustrate novel remote sensing capabilities from both terrestrial and space-based platforms. In this manuscript, we present a thorough update regarding the ongoing research and development initiatives related to the Live Mirror HM-MS technology. Keywords: Metamaterial structure, flexible ink electrodes, electroactive polymer force actuators and sensors, additive manufacturing, fire-polished, self-correcting surfaces, electronic polishing, Live Mirror. 1. INTRODUCTION Astronomical imaging of ever-fainter objects and imaging the Earth from space require much higher angular resolution and dynamic range than current optical telescopes can deliver. Mirrors, the key elements of these systems, are technologically challenging to improve because they must maintain an exceedingly precise shape while resisting * [email protected]; phone 33 695859217; https://eic-live-mirror.eu/ Figure 1 – The Live-Mirror Project logo. Paper 13431-8 SPIE Smart Material + NDE — Vancouver, Canada — Ed. 2025 deformation from gravity and variable wind loads in the open environments in which they operate. For over a century, the primary technology for shaping mirrors has been that of “abrasive polishing,” with stiffness to resist environmental deformation provided by steel and glass, which increases the mass significantly. This is impractical for systems larger than a few meters – a 10m class segmented mirror exceeds this limit and depends on cumbersome active mirror surface shape control. The astronomical community is currently building and commissioning two large ground telescopes, while the James Webb Space Telescope was completed and commissioned in summer 2022. The future world’s largest astronomical telescope, the European ESO-led Extremely Large Telescope (ELT) (https://elt.eso.org/telescope/) , is under construction with operations planned for 2027, and the largest Webb space telescope (https://science.nasa.gov/mission/webb/) is already unveiling impressive images from the deep universe. Both employ conventional segmented mirrors and have fixed-mirror apertures measuring 39 metres (ELT) and 6.5 metres (Webb) in diameter. These are costly undertakings (ELT: €2.3 billion, JWST: US$10 billion), driven by their extensive astronomical requirements. The fabrication of the optical mirrors constitutes a significant portion of the cost, exceeding half a million of either currency per square metre of reflecting surface, and this cost does not even include the research, innovation, and infrastructure development efforts necessary to adapt the mirror fabrication technology. Indeed, the technology employed for grinding and shaping the final mirror surface is adapted from Keck-type (https://keckobservatory.org/) technology from the 1990s, with developments primarily funded through European Framework Programmes (EU FP6/FP7). Our consortium, EIC Live-Mirror, aims to utilize our interdisciplinary scientific expertise to effectively challenge and transform disruptively the current optical mirror fabrication concept that has remained essentially unchanged for over 35 years. Our proposed lightweight, diffraction-limited, meta-material-based optical Live-Mirror system technology aims to achieve significant breakthroughs in several key areas: a sevenfold reduction in the areal mass density of mirrors, a tenfold decrease in surface roughness and scattered light, and a fifteenfold reduction in production costs and time. Our strategy is to evaluate and meet the requirements of ground and space astronomy and astrophysics (A&A) applications. The specifications for optical mirrors tailored to A&A applications are particularly challenging, requiring extremely lightweight construction, precise shaping, and high surface quality to minimise light scattering. Once we validate this technology for A&A applications, our new Live-Mirror technology will also apply to a wide range of less demanding applications. 2. SCIENCE TOWARDS A TECHNOLOGICAL BREAKTHROUGH The Live-Mirror project aims to establish a new paradigm in optical technologies. We are pioneering a novel approach that involves shaping thin, extremely smooth, fire-polished, lightweight glass to a predetermined curvature. By harnessing the controllable energy of electroactive polymers (EAPs), we can dynamically regulate the stiffness of the glass, making it resistant to environmental deformation. This unique process results in a "Live" optical mirror. Biomimetics research has significantly inspired our work. The similarities between human eyes — a bio-optical system composed of ciliary muscles, the crystalline lens, and the cornea — and our dynamic optoelectronic systems are striking. Our systems feature a thin, optical, fire-polished glass sheet that is actively controlled by multi-degree-of-freedom force actuators and sensors, incorporating characteristics that mimic natural systems such as reliability, adaptability, autonomy, stiffness, areal density, and miniaturisation. This reliance on high degrees of freedom led to a conceptual breakthrough called "Electronic Polishing." Figure 2The Live Mirror & Biomimetics. Paper 13431-8 SPIE Smart Material + NDE — Vancouver, Canada — Ed. 2025 One of the primary objectives of this research is to develop highly accurate optical reflective surfaces by utilising “float” glass through the slumping process, ensuring there is no contact with the fire-polished surface. This approach effectively prevents any degradation of the final optical surface, resulting in a mirror that exhibits surface roughness significantly lower than that of traditionally abrasively shaped mirrors. Figure 3 compares two pieces of glass, highlighting how the surface microroughness of ground and polished mirror is worse than the smoothness achieved with fire-polished glass. Interferometric measurements demonstrate that the microroughness of conventionally polished glass is, on average, three times worse than that of fire-polished glass, such as that found in a standard window pane (refer to Fig. 3B). As a result, the non-specular, scattered light emitted from a conventionally polished surface (illustrated in Fig. 3A) is nearly an order of magnitude better than that originating from unabraded (fire-polished, float) glass. To create the required focusing optical surface depends on generating a large-scale aspheric shape and subsequently correcting the residual cm-scale shape errors independently. This is a two-step process inside a pressurised, tailored-made kiln; a glass slumping facility (GSF) developed by partners AITIIP/Zaragoza and INAF/Brera (Figure 4). Figure 4 : The Live Mirror deterministic glass slumping facility being developed by partners AITIIP/Saragosa and INAF/Brera. An initial aspherical optic can be achieved without grinding or melting the reflective surface, which can be done inexpensively. The scientific advancements leading to technological breakthroughs in these processes will produce a several-metre-scale aspherical (parabolic), thin (ranging from 1 to 3 mm) fire-polished lightweight surface with exceptional control over the level of diffuse scattered light, such as improved high dynamic range of optical observation, which will surpass conventionally ground mirrors by yielding ten times less scattered light. 3. THE LIVE MIRROR HYBRID META-MATERIAL STRUCTURE A thin glass sheet sags under the influence of gravity, exhibiting a 2D shape determined by the Kirchhoff-Love (K-L) biharmonic equation (Arnold et al. 2003)[1]. The K-L equation is the foundation for rectifying minor surface errors in a shaped glass plate. In a closed-loop application, a glass surface, paired with independent knowledge of its shape, can be “deformed” by applying a 2D force distribution to null shape errors iteratively. This process results in a hybrid structure with artificial stiffness, a fixed dynamic shape when subjected to external forces. As the stiffness of the active surface relies on more than just the material plate thickness and Young's modulus (unlike a simple static plate), it becomes feasible to fabricate a hybrid material with a lower areal density than that of the corresponding static material plate. Currently, we are developing this dynamic hybrid meta-materials structure (HM-MS) through optical glass surface (OS) and a reaction surface (RS) supported by lattices of EAP based force-actuators and -sensors via additive manufacturing as shown in Figure 5. This will create a novel hybrid meta-material with superior stiffness-to-density-ratio properties. The upper-optical surface (OS) is the fire-polished thin thermal slumped surface close to the final aspheric shape. We are also Figure 3: Glass-surfaces quality. Paper 13431-8 SPIE Smart Material + NDE — Vancouver, Canada — Ed. 2025 developing a deterministically distributed lattice of EAP-based force-actuators interconnected by flexible electronics via additive manufacturing. The EAP force actuators volume (thickness, shape), material and distribution (spatial resolution/pitch size) will be determined/optimised by the dynamic range of the actuators and multi-sensing close-loop calibration metrology system to correct the “shape-errors” with an imposed actuator force distribution. The process of correcting shape errors involves "touching" the backside of the optical surface (OS) through a one-pass, predictable, and precise technique referred to as "electronic polishing" (Patent-Pending). This process is responsive to (i) environmental changes, such as those resulting from temperature, gravity and wind loads, and (ii) aims to achieve shape correction for diffraction-limited performance. The configuration of the distributed lattice will incorporate various pitch sizes, layers, and distributions, which will depend on the dynamic interactions of the actuators and sensors utilised for corrections and sensing. 4. DEVELOPING TAILORED EAP AND FLEXIBLE INKS FOR THE HM-MS Electroactive polymers (EAPs) are smart materials capable of sensing changes in their surrounding environment, processing this information, and responding in real-time. They can maintain their induced response while subjected to an applied DC electric field. Their properties can be customised by incorporating “intelligence at the molecular level through a process called doping. This allows them to achieve high strain rates, fast responses, reliability, and high mechanical compliance, and they can also be configured into complex shapes. In our Live-Mirror application, we conducted preliminary trade-offs[2][3][4] between the achievable piezoelectric strain and the applied electric field. We chose a basic terpolymer, P(VDF-TrFE-CFE)†, to be doped with diisononyl phthalate (DINP) plasticiser. As illustrated in Figure 6(A), a multilayer configuration of both pure terpolymer and DINP-doped terpolymer, tested in single-layer and six-layer stacks, shows an improved strain response (ST) when stimulated by a low input electric field (E). † P(VDF-TrFE-CFE) stands for polyvinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene. Figure 5: The Live Mirror additive manufacturing facility is being developed by consortium partner Kronos Mechatronics GmbH. On the right side of the figure is the first proof of concepts for the HM-MS structures, which aim to optimise and validate tailored additive manufacturing for the final half-meter demonstrators. Paper 13431-8 SPIE Smart Material + NDE — Vancouver, Canada — Ed. 2025 To enhance the correlation between strain response (ST) and electric field (E), our consortium is investigating various trade-offs in the selection of electroactive polymers. We focus on developing terpolymers or copolymers that incorporate a particular monomer, CTFE or CFE. This selection aims to transition the standard ferroelectric behaviour of EAPs to that of a relaxor-ferroelectric. Presently, we are conducting laboratory work guided by simulations that compare the terpolymer with the copolymer P(VDF-TrFE), as depicted in Figure 6 (B). Recently, we initiated bench testing on a newly available commercial terpolymer from Piezotech® Arkema, which has a high TrFE content. This research aligns with the trade-off mentioned above (ST ´ E) and is being presented as part of the Live-Mirror consortium in this conference 13431 paper #13431-47[5]. The Live-Mirror consortium is actively pursuing the development of a new 3D printable conductive flexible circuitry ink[6][7][8] for connecting and powering hundreds of actuators in a HM-MS multilayer configuration. The 3D printing of electroactive polymer actuators and flexible interconnection ink requires careful optimisation of various parameters, including the material-to-solvent ratio and overall HM-MS electrical performance. This involves inks ensuring high conductivity while meeting specific mechanical and chemical conditions, such as viscosity, printing speed, and solvent compatibility with fluorinated polymers. Additionally, the ink must be adaptable for multilayer structures. Figure 6: (A) The baseline comparison of P(VDF-TrFE-CFE) among pure, doped, and both uniand multi-layer configurations intended for application as live mirror force actuators[3]. (B) The numerical simulations comparing the terpolymer and copolymer variants. . Figure 7: Ink AgNP viscosity change with the increasing of the shear rate. The ink was formulated with XTPL’s CL85® and EAP(P(VDF-TrFE-CTFE)/MEK solution. The ink shows a slight thinning behaviour with increasing shear rate, making it flow easier. The viscosity derivative was determined as 80 mPa·s (in orange) . . Paper 13431-8 SPIE Smart Material + NDE — Vancouver, Canada — Ed. 2025 5. DEVELOPING A TAILORED ADDITIVE MANUFACTURING FACILITY In the Live-Mirror project, we aim to tackle some of the most challenging issues by simulating, modeling, and validating them on a laboratory demonstrator, a ½ meter-class HM-MS “workhorse.” This demonstrator will feature hundreds of force actuators and sensors interconnected through flexible electronics, all 3D-printed using a custom-developed additive multi-axis manufacturing facility (AMF). This facility is being specifically designed to meet the requirements of our LiveMirror HM-MS systems by our partner, Kronos Mechatronics GmbH, based in Nürnberg, Germany. Figure 8 illustrates several current modular and in-line monitoring designs and developments for the AMF versions. These features will assess the printing process while focusing on testing and validating optimal conductivity, adhesiveness, viscosity, curing, and reactivity of the electroactive polymers (EAPs) and inks used in the process. Figure 8: The Live-Mirror development by partner Kronos Mechatronics (formerly Neotech) involves a modular multi-axis additive manufacturing facility (AMF) specifically designed to meet the performance and validation requirements of HM-MS applications. Paper 13431-8 SPIE Smart Material + NDE — Vancouver, Canada — Ed. 2025 6. RESULTS FOR LIVE-MIRROR SMART HM-MS Several proof of concepts for the HM-MS are currently underway. The primary focus is on processing and optimising the 3D printing procedures to meet the high-level specifications for validating Live-Mirror technology in a laboratory bench. Figure 9: The proof of concept for the HM-MS is currently underway, focusing on assessing and optimizing the 3D printing process to meet the high-level specifications of Live-Mirror, as shown in the top part of the figure. The bottom part displays the preliminary results regarding the optical surface displacement observed when an electric field of 20 V/μm is applied to all nine actuators. Paper 13431-8 SPIE Smart Material + NDE — Vancouver, Canada — Ed. 2025 Figure 10: Several HM-MS proof-of-concept samples were prepared using the configuration shown at the top of the figure. These samples consist of P(VDF-TrFE-CTFE/CFE) with an 8% DINP content for the electroactive polymer (EAP) and 8% carbon black for the ink composites. The lower part of the figure presents (left-side) the results and (right-side) the simulations [16] for one of these samples obtained by applying voltages of 1 kV and 2 kV. Samples from Figures 9 and 10 present preliminary outcome results using P(VDF-TrFE-CTFE/CFE) with 8% Carbon Black[3][4][10] as the materials for the electroactive polymer (EAP) and ink. The EAP force actuators are constructed from three distinct layers, each with a thickness of 160 µm. These actuators have a diameter (Da) of 8 mm and a pitch size— defined as the distance between the centroids of adjacent actuators—of 13 mm (Da + 5 mm). When an electric field is applied to multiple adjacent actuators, it creates crosstalk (X-talk), which significantly impacts the overall deformation of the optical surface. Controlling and characterising this crosstalk is crucial for the optical performance of the Live-Mirror Paper 13431-8 SPIE Smart Material + NDE — Vancouver, Canada — Ed. 2025 HM-MS, as it requires precise multi-spatial and temporal frequency-controlled surface metrology and calibration[9][10]. This oversight ensures that the information necessary to drive the force actuators is accurate, allowing for the correction of “shape errors.” To achieve this, the deformations of the optical surface must guarantee diffraction-limited form figures, compensating for the mirror's reactions to gravity, wind, and temperature variations. Each factor operates at specific temporal frequencies, necessitating multi-frequency, multi-feedback, and predictive control algorithms. To tackle these challenges, we will develop and validate a new optical metrology and calibration system that will allow simultaneous sensing of the Live-Mirror HM-MS surface shape and its focal plane using a maximum likelihood registration method. This real-time, closed-loop control and performance depend on the configuration of the distributed actuator lattice. This lattice includes configurations, pitch sizes, EAP + ink + glue layer thicknesses, diameters, weights, and their optimised materials. A customised and refined doping process is essential for these optimised and smart materials, including EAPs and flexible inks. This process enhances dielectric permittivity, reduces Young's modulus (including for the flexible glue), and improves conductivity. These enhancements aim to achieve optimal HM-MS performance, facilitating strong strain with a low-driven electric field. 7. TRANSLATING THE SMART MATERIAL RESEARCH INTO INNOVATIONS In the Horizon Europe EIC Live-Mirror consortium context, we intend to advance the fabrication of meta-materials that incorporate printed electronics capabilities by capitalising on highly disruptive technologies, specifically flexible inks and electroactive polymers (EAP) as force actuators for ground-based applications[12]. Figure 11 – The long-term direct application of Live-Mirror technology involves imaging exoplanets using specially designed highdynamic optical telescopes. However, adapting these technologies for complex operations in the space environment presents additional challenges that warrant thorough assessment. In the past decade, significant efforts have been dedicated to ensuring that EAP achieves optimal performance characterised by reliability and robustness in space conditions. This initiative primarily focuses on facilitating distributed actuation in robotics and automation through soft actuator technology.