Mechanical metamaterials have recently driven significant advancements, and this field has currently been extended to break the reciprocity principle in static mechanics and wave propagation. Here, we demonstrate a type of three-dimensional mechanical metamaterials that possess nonreciprocal static elastic behaviors and tunable dynamic wave properties. The metamaterial is designed with suitably tailored microstructure asymmetry, which exhibits vastly different deformation configurations upon loading from different sides. Such contrast in deformation induces distinct force-displacement responses, which gives nonlinear elastic moduli that are dependent on both the magnitude and direction of applied loads. We fabricate such metamaterials with 3D printing technique at the microscale. The non-reciprocal mechanical behavior is validated by analytical means, simulations, and experiments. Besides, tunable band structure characteristics are obtained when the metamaterial is loaded in opposite directions or by different magnitudes. The band structure deforms in asymmetrical ways, which indicates flexible control on transmit-prohibit switching of elastic waves propagation (in certain frequency ranges), and this is realized by only switching the external mechanical loading direction. These peculiar behaviors show great prospects in enabling unidirectional elasticity and wave transmission within a solid material, paving avenues to new one-way functional devices.

The miniaturization of mechanical mechanisms is crucial to enable the development of compact, high-performance micromachines. However, the downscaling actuation of conventional gears and micromotors has remained limited by the inherent challenges of implementing mechanical/electrical powering. Here, we present the design, fabrication, and characterization of an optomechanical, gear-driven micromachine realized through two-photon polymerization 3D printing. The actuation is achieved using optical tweezers. The device integrates a microgear transmission system with an optically actuated part, enabling light-controlled micromachines. When illuminated by a highly focused laser source, the first gear generates rotational torque within the gear assembly, converting optical energy into directional mechanical work that can be transmitted to the coupled gear. We demonstrate the fabrication of micromachines using two-photon polymerization (2PP) laser writing, enabling the fabrication of spur gear trains and bevel gears that can produce out-of-plane rotations, which is not achievable with traditional micromachining fabrication techniques. The micromachines are composed of a single gear or a train of two or three gears without any unwanted adhesion between the components, leading to functioning systems. Experimentally, the fabricated micromachines were actuated using optical tweezers, demonstrating continuous gear rotation, effective motion transmission in gear trains, out-of-plane rotations, and the ability to amplify velocity or torque. Optical-tweezer actuation broadens the potential applications of these micromachines, particularly in biomedical and lab-on-a-chip systems, where precise, minimally invasive control at the microscale is essential.

Optical tweezers use tightly focused laser beams to trap and manipulate microscopic particles by balancing scattering forces with a gradient force at the beam's focal point. This enables precise and contact-free particle control, making optical tweezers a powerful tool to activate microrobots (optobots) that give multiple degrees of freedom. However, realizing out-of-plane rotation, which is demanded for medical applications such as cellmanipulation and drilling, remains a challenge. Here, we present an optobot design leveraging chirality to achieve full-cycle out-of-plane rotation. This optobot has an elongated structure with dual optical handles and a chiral helix aligned on the long axis of the robot. The optical handles are used to be trapped by the optical tweezers beams to keep the long axis of the optobot and the helix perpendicular to the laser light propagation, while its chiral helical part generates out-of-plane rotation around its long axis under laser excitation due to broken axial parity. Finite element analysis was conducted to simulate the interaction of the chiral helix with the laser beam. The scattered beam distribution was visualized to see how it is altered by the chiral structure. The optical torque was also calculated to show that it is unidirectional following chirality. For experimental demonstration, the optobot was fabricated via two-photon lithography. The optical manipulation experiments using an optical tweezer system showed that the optobot demonstrates versatile actuation capabilities, achieving on-demand repetitive rotation, controlled-speed translation, and combined rotational-translational motion.

Hierarchical auxetic metamaterials are a class of materials which are characterized by a multi-tiered architecture and have the capability of exhibiting enhanced mechanical properties in comparison to their single-geometry counterparts. In this work, three distinct new classes of hierarchical auxetic metamaterials designed by incorporating cubic crystal lattice geometries, namely, Body-Centred Cubic (BCC), Face-Centred Cubic (FCC) and Tetrahedral Cubic (TC) are proposed into 3D rotating cube structures. Through the introduction of hierarchy, these relatively dense mechanical metamaterials are rendered lightweight, through a volume fraction reduction of over 90% in the majority of cases from their full-block (FB) counterparts, while retaining their original auxetic capabilities. These systems are also demonstrated to possess the ability to exhibit a wide range of stiffnesses and Poisson's ratios, including giant negative values, as well as superior stiffness/density ratios making them ideal for implementation in lightweight applications. Furthermore, a two-photon lithography 3D-printer is used to fabricate these new lattice structures at the microscale and test them in-situ. The results obtained provide clear and comprehensive evidence of the improvement imparted through the introduction of hierarchy and the advantages of using this method to design lightweight 3D rotating unit auxetic structures.

Tailoring material properties at the microscale is essential for advancing technologies, particularly in the field of 4D printing. The ability to manipulate thermal expansion is particularly critical for opto-mechanical systems, where precise deformation control is required. This paper introduces a novel approach that combines 4D printing with topology optimization to design and fabricate a multimaterial structure capable of mitigating undesired thermal expansion upon heat stimulation. This approach is applied to the development of a microfabricated Fabry-Perot filter as a robust alternative to directly printed cavity-based devices. Employing both approaches enables the determination of material distribution within the internal geometry of the structure, resulting to a temperature-insensitive response while maintaining optical performance. Using two- photon polymerization, the designed structure is 3D-printed with a combination of active and passive materials to achieve a controlled geometry. The final structure demonstrates a minimal change in dimensions under a temperature increase, confirming its ability to counteract thermal expansion effectively. This work showcases the potential of 4D printing and intelligent design strategies for developing devices at the microscale with precise thermal control.

Optical microrobots (OPTOBOTs) have garnered significant interest, particularly in the medical field, due to their potential for precise cell manipulation in various biological studies and microsurgical applications. Previously described OPTOBOTs demonstrate multiple degrees of freedom, yet improvements are needed, especially in achieving reliable out-of-plane rotation. Here, we propose an OPTOBOT design based on chirality that enables full-cycle out-of-plane rotations using optical tweezers. The OPTOBOT has an arrow-like structure with two handles aligned on the same axis, maintaining its horizontal orientation and facilitating controlled movement. Additionally, the OPTOBOT's tail is a chiral helix, which induces repetitive out-of-plane rotations around its longer axis when targeted by a laser beam that is due to broken axial parity. Finite element analysis is employed to design the OPTOBOT and assess its capacity to generate mono-directional high optical torque. Experimental results confirm various actuation modes, supporting future integration of OPTOBOTs in complex micromanipulation tasks.

The manipulation of microscale components with complex shape like semiconductors, 3D printed microparts, and optical lenses, remains challenging due to strong surface forces and limitations of existing methods. A 3D‐printed soft pneumatic microgripper capable of rectilinear deformation is presented in this paper. It addresses these challenges through a concave design with two operational modes (snap and continuous) and an integrated adhesion‐reducing mask. Fabricated with IP‐PDMS two‐photon polymerisation 3D printing, the microgripper achieves a 40 minimum operation diameter and demonstrates a substrate‐free release force as low as 11.1 nN with an adhesion switching ratio of 373 in the normal direction. Combined with rigid alignment, the device enables universal pick‐and‐place over a range of micro‐objects and mid‐air transition of ultralight components (≈1.14 ) in confined spaces. With over 30000 actuation cycles without performance degradation, this scalable design can execute complex manipulation tasks through a multi‐gripper system as demonstrated.

2-Photon Polymerization (2PP) 3D printing has allowed several interesting fabrication processes for microrobots. However, the use of 2PP for micromanipulators to create the largest workspace possible at small scale, and generate controllable, repeatable and multi-DoF movement capable of applying substantial forces is still an open question. Toward this goal, we investigate the behavior of 3D 100 m-scale tethered microrobots made with bi-material 2PP printing. Volumes of pNIPAM are used as actuators in an IP-S flexible mechanism, which takes the function of a structural skeleton of the microrobots. The force and displacement capabilities of pNIPAM actuators are studied experimentally and through Finite Element Modelling (FEM), revealing promising capacities in displacement and forces (measured contraction up to 30 N). The behavior of pNIPAM-actuated IP-S flexible mechanisms is investigated. The angular movement of pNIPAM-actuated IP-S rotational beam joint with varying dimensions was studied, and a max joint rotation of 23° was observed. Furthermore, a repeatable 80 μ m long motion was demonstrated on a 300 μ m long RR robot mechanism during 5 actuation cycles. Last, actuated 3D mechanisms are demonstrated with three 100 m grippers with two jaws oriented 45° around x, 45° around x and y, and three vertical jaws.

This paper proposes to investigate a bi-material approach that introduces another material in the 3D printed soft microrobot working as a structural skeleton of the active material. For the latter, pNIPAM-based hydrogels are good candidates thanks to their large deformation under stimuli. However, this same property is a trade-off with mechanical strength, and by extension the structural stability of the printed microrobots. By combining two materials, the shortcomings of one can be palliated by the second material. Here, we propose to make 3D 100 mu m-scale tethered microrobots with pNIPAM beams as an actuator into IP-S flexible mechanisms, using bi-material 2-Photon Polymerization (2PP) printing as a fabrication process. The capacity of the pNIPAM beams as actuators is studied experimentally and via Finite Element Modelling, revealing promising capacities in displacement and forces during simulations (contraction up to 25%, force up to 166 mu N). The behavior of pNIPAM-actuated IP-S flexible mechanisms is explored in a 2PP-printed microrobot, demonstrating a state-of-the-art motion range (joint rotation up to 14 degrees) easily controllable by temperature change. Furthermore, actuated 3D mechanisms are demonstrated with three 100 mu m 3D grippers.

Materials and structures with tunable mechanical properties are essential for numerous applications. However, constructing such structures poses a great challenge since it is normally very complicated to change the properties of a material after its fabrication, particularly in pure force fields. Herein, we propose a multi-step and elastically stable 3D mechanical metamaterial having simultaneously tunable effective Young's modulus and auxeticity controlled by the applied compressive strain. Metamaterial samples are fabricated by 3D printing at the centimetric scale, with selective laser sintering, and at the micrometric scale, with two-photon lithography. Experimental results indicate an elementary auxeticity for small compressive strains but superior auxeticity for large strains. Significantly, the effective Young's modulus follows a parallel trend, becoming larger with increasing compressive strain. A theoretical model explains the variations of the elastic constants of the proposed metamaterials as a function of geometry parameters and provides a basic explanation for the appearance of the multi-step behavior. Furthermore, simulation results demonstrate that the proposed metamaterial has the potential for designing metamaterials exhibiting tunable phononic band gaps. The design of re-usable elastically-stable multi-step metamaterials, with tunable mechanical performances supporting large compression, is made possible thanks to their delocalized deformation mode.

Over the years, research and development into micro-force sensing techniques has gained a lot of traction, especially for microrobotic applications, such as micromanipulation and biomedical material characterization studies. Moreover, in recent years, new microfabrication techniques have been developed, such as two-photon polymerization (2PP), which enables fast prototyping, high resolution features, and the utilization of a wide range of materials. In this work, these two fields are combined to realize the first fully 3D printed vision-based micro-force sensor. The sensor exhibits tunable stiffness properties, which are simulated and compared with calibration values for a variety of 2PP printing settings. Furthermore, a novel bimaterial printing approach was utilized to fabricate sensors with a highly compliant sensing structure and rigid body. Lastly, the sensors are used to measure the mechanical properties of fish eggs as a cell analog to showcase the possible applications of the system.

In this letter, we propose a mobile microgripper actuated using laser optothermal and magnetic actuation in liquid environments. This hybrid actuation scheme allows the full decoupling between the in-plane positioning of the mobile microgripper and the actuation of its gripping mechanism, which reduces the control complexity of such microgrippers. The developed mobile microgripper with the dimensions of 1500 $\times 700\times$ 250 μ $\mathrm{m^{3}}$ can realize an open and close gripping motion of 35 μm, a magnetic positioning accuracy of 6 μm for translation over an area of 1 × 1 mm and 2 $^{\circ }$ for rotation, and a laser steering accuracy of 25 μm. Finally, the mobile microgripper is used to control the position of a microbead inside a liquid environment. Our work provides a proof of concept of laser optothermal-magnetic hybrid actuation, which has the potential to enhance the deployment of microtools in biomedical applications including cell manipulation and lab-on-chip devices.

The ability to control Poisson’s ratio of functional materials has been one of the main objectives of researchers attempting to develop structures efficient from the perspective of protective, biomedical and soundproofing devices. This task becomes even more challenging at small scales, such as the microscale, where the possibility to control mechanical properties of functional materials is very significant, like in the case of flexible electronics. In this work, we propose novel microscopic 2D and 3D functionally-graded mechanical metamaterials capable of exhibiting a broad range of Poisson’s ratio depending on their composition. More specifically, we show that upon adjusting the number of structural elements corresponding to one type of the substructure at the expense of another, it is possible to change the resultant Poisson’s ratio of the entire system from highly positive to highly negative values as well as to achieve arbitrary intermediate values. Finally, in addition to static properties, we also analyze the dynamic properties of these structures. Namely, we show how the variation in the composition of the considered mechanical metamaterials affects the velocity of a wave propagating through the system. This, in turn, could be essential in the case of applications utilizing localized wave attenuation or sensors.

Non-Newtonian liquids are characterized by stress and velocity-dependent dynamical response. In elasticity, and in particular, in the field of phononics, reciprocity in the equations acts against obtaining a directional response for passive media. Active stimuli-responsive materials have been conceived to overcome it. Significantly, Milton and Willis have shown theoretically in 2007 that quasi-rigid bodies containing masses at resonance can display a very rich dynamical behavior, hence opening a route toward the design of non-reciprocal and non-Newtonian metamaterials. In this paper, we design a solid structure that displays unidirectional shock resistance, thus going beyond Newton’s second law in analogy to non-Newtonian fluids. We design the mechanical metamaterial with finite element analysis and fabricate it using three-dimensional printing at the centimetric scale (with fused deposition modeling) and at the micrometric scale (with two-photon lithography). The non-Newtonian elastic response is measured via dynamical velocity-dependent experiments. Reversing the direction of the impact, we further highlight the intrinsic non-reciprocal response.

The ability to significantly change the mechanical and wave propagation properties of a structure without rebuilding it is currently one of the main challenges in the field of mechanical metamaterials. This stems from the enormous appeal that such tunable behavior may offer from the perspective of applications ranging from biomedical to protective devices, particularly in the case of micro-scale systems. In this work, a novel micro-scale mechanical metamaterial is proposed that can undergo a transition from one type of configuration to another, with one configuration having a very negative Poisson's ratio, corresponding to strong auxeticity, and the other having a highly positive Poisson's ratio. The formation of phononic band gaps can also be controlled concurrently which can be very useful for the design of vibration dampers and sensors. Finally, it is experimentally shown that the reconfiguration process can be induced and controlled remotely through application of a magnetic field by using appropriately distributed magnetic inclusions.

Energy absorption and dissipation features of mechanical metamaterials have widespread applications in everyday life, ranging from absorbing shock impacts to mechanical vibrations. This article proposes novel bioinspired friction‐based mechanical metamaterials with a zero Poisson's ratio behavior inspired from parrot's beaks and manufactured additively. The mechanical performances of the corresponding metamaterials are studied at both macro and micro scales by experiments and finite element analysis (FEA). An excellent agreement is observed between the FEA and both microscopic and macroscopic scale experiments, showing the accuracy of the developed digital tool. Performances are compared to traditional triangular lattice metamaterials. Both experimental tests and FEA results demonstrate the following advantages: 1) absorbing and dissipating energy per unit of mass (SEA) at large compressive strains without global buckling; 2) bistable deformation patterns including friction‐based and interlocking mechanisms; 3) reversible deformation patterns after unloading; 4) shape recovery behavior after a heating–cooling process; and 5) the higher elastic modulus of micro metamaterials compared with their macro counterparts. This is the first demonstration of a bioinspired friction‐based design of 3D‐printed mechanical metamaterials that feature absorbing/dissipating energy, stability, and reversibility properties to cater to a wide range of sustainable meta‐cylinders in micro and macro scales.

In this paper, we proposed an integration method of a mm-scale high voltage (HV) driver and electrostatic actuators on a Parylene-C flexible substrate. With our unique three-layer metal structure (Cr/Au/Cr), we have demonstrated the contamination-less integration of an HV driver made of deep trench separated series silicon P-N junctions, bonded on the gold electrodes of the actuator. This technique enables CMOS LSI chips to directly be integrated with flexible electronics. Thus, novel applications of flexible electronics in various fields will be developed with the enhanced ability of signal processing, communications, and power delivery by CMOS LSI.

In this article, we propose laser actuated microjoints which can be remotely actuated in both air and water. Their actuation relies on the optothermal response of a spiral bimaterial. The microjoints are fabricated using two-photon polymerization technology that offers the ability to tune the thermal and mechanical properties of the material by controlling the laser printing power. Modeling is first conducted to verify the parameters of the spiral that affect the rotational displacement and generated torque of the microjoint. Then, microjoints having a diameter of less than 200 $\mu$m are characterized. The microjoints can realize a maximum deflection of approximately 8.5$^{\circ }$, a force in the $\mu$N-order using a 265-$\mu$m long arm, an actuation repeatability of more than 100 times, and a time response of approximately 34 ms. Finally, the microjoints are implemented in a microgripper and an xy serial microarm. Successful micromanipulation of 40 $\mu$m microbeads using the microgripper, and the simultaneous actuation of multiple microjoints of the xy serial microarm with two degrees of freedom are shown. This kind of rotational, compact, selective, and remotely actuated microjoints would allow the deployment of individually controlled mobile microrobots with several degrees of freedom for complex applications such as cell manipulation and microassembly.

Three-dimensional direct laser writing technology enables one to print polymer microstructures whose size varies from a few hundred nanometers to a few millimeters. It has been shown that, by tuning the laser power during writing, one can adjust continuously the optical and elastic properties with the same base material. This process is referred to as gray-tone lithography. In this paper, we characterize by Brillouin light scattering the complex elastic constant C11 of different reticulated isotropic polymers, at longitudinal phonon frequencies of the order of 16 GHz. We estimate the real part of the C11 constant to vary from 7 to 11 GPa as a function of laser power, whereas its imaginary part varies between 0.25 and 0.6 GPa. The linear elastic properties are further measured at a fixed laser power as a function of temperature, from 20∘C to 80∘C. Overall, we show that our 3D printed samples have a good elastic quality with high Q factors only ten times smaller than fused silica at hypersonic frequencies.

Well-designed stretching-dominated lattices can form elastic metamaterials with high specific stiffness and strength. Their strongly anisotropic and unstable nonlinear mechanical properties, however, limit their application to energy absorption. In contrast, bending-dominated lattices are well known for high energy absorption capacity and stable nonlinear response, but poor elastic response. Here, we propose a new class of light-weight elastic isotropic bending-dominated truss lattice that combines both advantages. Numerical simulations reveal that the proposed lattices not only exhibit elastic isotropy, but also nearly isotropic inelastic large deformation response. In particular, for a relative density smaller than 1% the metamaterial almost attains the upper bound of Poisson’s ratio for an isotropic material, i.e ν=0.5. Compared to BCC truss lattices, uniaxial compression tests show that the relative modulus is twice as large, and that the relative collapse strength and specific energy absorption are about 1.6 times as large. The designed metamaterial is thus a noteworthy alternative for load bearing, energy absorption, and transformation acoustics.