We present a sound absorbing meta-atom which exhibits two subwavelength (λ/20 and λ/14) absorption peaks in the low frequency range of 350 to 500 Hz. The meta-atom combines two locally resonant systems by using a slit embedded neck Helmholtz resonator with a folded quarter-wavelength resonator crown. Finite element simulations demonstrate how the frequency of the peak absorption coefficients can be tuned by adjusting key geometric parameters. We develop an analytical model to predict the absorption coefficient of the structure under plane wave excitation by using the transfer matrix method. A four-cell array of the proposed unit cell was fabricated using masked stereolithography and its absorption was measured using the two-microphone method in a square cross section impedance tube. Experimental results demonstrate two distinct absorption peaks at 340 Hz (α=0.91) and 484 Hz (α=0.98) with a structure height of 50 mm. Characterised by a constant 2D cross section, this unit cell may be better suited to scalable manufacturing techniques when compared with other laboratory based low frequency absorbers.

The adult Buthus occitanus scorpion has an interesting hair-like mechanoreceptor (called trichobothrium), which is flat instead of circular and allows great sensitivity for airflow measurements. The aim of this work is to develop a bioinspired artificial hair-like sensor (AHS) with a similar flat hair shaft using 3D printing. The sensor was successfully manufactured using digital light processing (DLP), a technique that allows multiple sensors to be 3Dprinted sensor in one batch, or arrays of them in a singular structure. The sensor is then sputter-coated with platinum to add piezoresistive capabilities and allow the transduction of airflow velocities into voltage levels. The sensor has been characterized to show a sensing range between 6.8 and 22.3 m/s and an average measurement error <1% under the tested conditions. The upper range warrants further study as it was limited by the experimental setup. The sensor can be used for several applications covering robotics, biomedical engineering, and meteorology, and the use of DLP allows for great customization capability.

Acoustic metamaterials (AMMs) offer significant promise for ultrasound probe backing layers due to their capacity to enhance acoustic energy dissipation through tailored sub wavelength structures. However, practical implementation remains challenging due to difficulties in reliably reproducing the micrometer-scale features required for MHz-frequency operation and the lack of quality assurance processes linking design intent to fabricated performance. This work presents the evaluation of a 3-D-printed acoustic isolator-type metamaterial (AI-MM) backing designed for MHz operation using a custom aluminum oxide resin. Directional transmission intensity measurements revealed frequency-dependent asymmetry in forward and backward wave propagation (in both experiments and simulations), consistent with passive acoustic isolator behavior. X-ray micro-computed tomography (micro-CT) imaging of AI-MM samples revealed dimensional deviations, apex rounding, and local density variation. Attenuation spectra showed that AI-MM backings consistently outperformed homogeneous controls in both simulation and experiment, with frequency-dependent trends indicating enhanced scattering and viscous losses. A local attenuation peak near 2.6 MHz was within the operational range estimated from the measured geometry (2.22-2.94 MHz), underscoring the importance of linking performance to real-world fabrication. These findings support the potential of AI-MMs as tunable passive components in ultrasound systems and highlight the need for integrated design, fabrication, and validation workflows.

Insects and arachnids have mechanoreceptors and similar sensilla which allow a wide variety of sensing mechanisms, from air flow to olfactory stimuli. These structures, often hair-like, vary in shape and size based on their function. The trichoid sensilla is a hair-like sensing mechanism that reacts to low-frequency near-field sound. This project successfully 3-D-printed structures inspired by both insects and arachnids. Of particular inspiration is the flat hair structure of the adult Buthus occitanus scorpion, which allows for a larger area of air particles to excite the sensor. The multi-material sensor design mimics trichoid sensilla, with the rigid hair transferring excitation energy to the softer basal area responsible for transduction. Using a Laser Doppler Vibrometer (LDV) it has been possible to show a change in frequency response due to a change in hair structure. Essentially showing a different frequency sensitivity based on hair length, thickness, and shape. The 3-D-printing technique used allows the production of an array of sensors in shorter times. It proves therefore useful for printing sensing arrays that would allow separate frequency band acquisition. Early development to convert mechanical displacement into electric signals has begun. [This work is funded by the Defence Science and Technology Laboratory (DSTL), United Kingdom.]

Critical-sized bone defects cannot heal spontaneously and receive poor clinical prognosis due to limitations in modern treatment strategies. Next-generation therapies are applying biomaterials incorporating BMP-2 to effectively promote and support bone regeneration, but adverse effects are linked to uncontrolled BMP-2 egress from the biomaterial. Implementing extracellular matrix proteins to biomaterials is a favourable approach to alleviate these drawbacks, and self-assembling peptide hydrogels are rapidly emerging as modulable and versatile biomaterials. Here, we describe the creation of a tenascin-c-functionalised peptide hydrogel designed to regenerate critical-sized bone defects. A recombinant fragment of tenascin-c spanning from the 3rd to 5th fibronectin-like domains is integrated into the fibre network. We demonstrate that this nascent construct effectively retains BMP-2 to differentiate mesenchymal stem cells into mature osteoblasts and achieves complete unionisation of murine critical-sized bone defects under low BMP-2 dose. All in all, we demonstrate tenascin-c as a suitable candidate to functionalise biomaterials intended for bone engineering applications and the promising potential of self-assembling peptide hydrogels in treating critical-sized bone defects.

Enhancing osteogenesis in mesenchymal stromal (stem) cells is essential for advancing cellular therapies that target and alleviate skeletal pathologies. By employing a bespoke bioreactor capable of delivering nano-amplitude vibration (30 nm, 1kHz) to human adipose-derived mesenchymal stromal cells, we observed osteo-specific differentiation. We related this to the mechanotransductive mechanism by showing that inhibition of intracellular tension results in the loss of cytoskeletal organisation and myosin activation driven by nanovibration. Further, we dissect the mechanism of osteogenesis using a panel of Wnt agonists and antagonists and highlight the role of non-canonical Wnt. Then, using Bcl3 -/- cells and by stimulating with BCL3 peptide, we show that non-canonical Wnt, osteogenesis-related inflammation and osteogenesis itself are all regulated by BCL3. This is important as nanoscale direct cell-stimulation is gaining interest, and there is an emerging consensus that such signals can be osteogenic. While prior research has only broadly hinted at how nanovibrational signals convert to an osteogenic phenotype, this new work pinpoints critical mechanistic insights, thereby advancing our understanding of this promising avenue in musculoskeletal cell therapy. ### Competing Interest Statement The authors have declared no competing interest. European Union, https://ror.org/019w4f821, 874889

Nature has always inspired humans to create innovative tools. Arachnids show exceptional and functional sensory receptors at a small scale. Air flow mechanoreceptors, commonly called trichobothria, are used in different shapes and sizes by several arachnid species. The goal of this work is to develop flat hair-like sensors inspired by the adult Buthus occitanus scorpion. A sensor that responds to airflow has been developed and realized using multi-material additive manufacturing (also known as 3D printing). The sensor has been sputter coated with platinum to add piezoresistive sensing capabilities. Preliminary results with an unbalanced voltage divider show a promising response in the mV region.

Hydrogel-based tissue phantoms are widely used in ultrasound imaging research due to their ability to replicate the acoustic and mechanical properties of biological tissues. However, conventional hydrogels suffer from rapid dehydration and poor long-term stability, limiting their effectiveness in reliable or repeatable imaging studies-particularly for delicate structures such as the retina. To address this limitation, we present a scalable method for extending hydration stability in hydrogel-based retinal phantoms through the development of a hydro-locking polymer network. Two sulphuric acid (H2SO4) treatment approaches were investigated: (1) pre-polymerization incorporation into the hydrogel precursor, and (2) post-polymerization immersion. Acid concentrations ranging from 10% to 40% (w/w) were evaluated, with thermal processing at 65 degrees C applied to stabilize chemical bonding. Supplementary storage measures, including refrigeration and cling film wrapping, were implemented to further preserve phantom integrity. Results demonstrate that H2SO4 promotes ionic and hydrogen bonding between water and methacrylated monomers, significantly reducing water loss. The most effective treatment extends hydration stability from minutes to several weeks without compromising acoustic performance. These findings support the development of durable, reusable hydrogel-based retinal phantoms suitable for ophthalmic ultrasound system calibration and testing, with ongoing imaging validation studies in progress.

The manufacture of acoustic metamaterials (AMMs) is a significant challenge within the field, which developments in additive manufacture have the potential to address. This research presents the optimization of a new Stereolithography (SL) 3D printable resin, with magnetic properties incorporated to be utilized in adjustable AMMs. The core aim of this study is the synthesis of a magnetic resin for improved adjustable-bandwidth performance in membrane-coupled AMMs. The material features considered relevant here for resin optimization are curing rate, Young’s modulus, and magnetization of saturation. Studies were conducted to analyze the effect of various resin components, comprising single and interpenetrated polymer networks, surfactants, photoblocker concentrations, and magnetic fillers, on the resin properties. Magnetic hysteresis plots were recorded to demonstrate the effect of using different particle sizes of magnetite and carbonyl iron. The goal of this is to optimize the magnetic composite selection to maximize magnetization while reducing the need for magnetic poling post-manufacture, further contributing to the ease of manufacture of the resin formula. The final formula had a density of 1205.30±0.56 kg/m3, peak tensile Young’s modulus of 6.50 MPa and ultimate tensile strength of 0.744 MPa - printed with 25 μm layer thickness. The magnetization of saturation for the optimized resin formula was 3.326 - 4.647 emu/g at 5 %wt magnetite content, dependent on the poling regime.

Previous work has shown underwater gas bubbles to be an amplification mechanism for weak-sources outputting at the bubble's Minnaert resonance. Here further work to capture bubbles in manufactured shells and cages is reported. 3-D printed resin cages with direct air–water interfaces, and rubberized shells of specified volume and resting tension and a downward facing pipe-based system, were created. The effect these have on the resonance of the amplification system and amplitude of the output is analyzed. These devices are miniscule compared to other low-frequency devices of a similar center frequency, for example, a 500-Hz cage has a 7-mm radius sphere. Another key feature explored is utilizing inter-bubble coupling at different distances, configurations, and bubble volume deltas as a means of frequency output control and the effect that the different bubble-holding systems have on this coupling. Controlling the exact frequency response of this system is important as one of the major applications for these devices will be as a finely tuned underwater sensor for weak signals in noisy environments.

Achroia grisella (Fabricius, 1794) (Lepidoptera: Pyralidae) is a pyralid moth with two ears in its abdomen that it uses for detecting mates and predators. Despite no connection between the two ears having been found and no other elements having been observed through X-ray scans of the moth, it seems to be capable of directional hearing with just one ear when one of them is damaged. It is therefore suspected that the morphology of the eardrum can provide directional cues for sound localization. Here, we use finite element modelling software COMSOL to model a simplified version of the eardrum, an elliptical plate with two sections of different thicknesses and a mass load at the centre of the thin section, to try to determine if the morphology of the ear is responsible for the moth’s monoaural directional hearing. Results indicate that the resonance mode and directionality response of the elliptical plate with two thicknesses and a mass load match that of the moth closely and provide an enhanced response to sounds coming from the front of the moth. Damping is also considered in the resonant mode, and it is observed to improve the resemblance of the simulation to real moth ear measurements.

Photopolymerization-based additive manufacturing requires selectively exposing a feedstock resin to ultraviolet (UV) light, which in digital light processing is achieved either using a digital micromirror device or a digital mask. The minimum tolerances and resolution for a multilayer process are separate for resolution through the Z-axis, looking through the thickness of a printed part, and resolution in the XY-axes, in the plane of the printed layer. The former depends wholly on the rate of attenuation of the incident UV light through the material relative to the mechanical motion of the build layer, while the latter is determined by a two-dimensional pattern of irradiance on the resin formed by the digital micromirror device or the digital mask. The size or the spacing of elements or pixels of this digital mask is frequently given by manufacturers as the "resolution" of the device, however, in practice the achievable resolution is first determined by the beam distribution from each pixel. The beam distribution is, as standard, modeled as a two-parameter Gaussian distribution but the key parameters of peak intensity and standard deviation of the beam are hidden to the user and difficult to measure directly. The ability of models based on the Gaussian distribution to correctly predict the polymerization of printed features in the microscale is also typically poor. In this study, we demonstrate an alternative model of beam distribution based on a heavy-tailed Lorentzian model, which is able to more accurately predict small build areas for both positive and negative features. We show a simple calibration method to derive the key space parameters of the beam distribution from measurements of a single-layer printed model. We propose that the standard Gaussian model is insufficient to accurately predict a print outcome as it neglects higher-order terms, such as beam skew and kurtosis, and in particular failing to account for the relatively heavy tails of the beam distribution. Our results demonstrate how the amendments to the beam distribution can avoid errors in microchannel formation, and better estimates of the true XY-axes resolution of the printer. The results can be used as the basis for voxel-based models of print solidification that allow software prediction of the photopolymerization process.

Many animals employ a second frequency filter beyond the initial filtering of the eardrum (or tympanal membrane). In the field cricket ear, both the filtering mechanism and the transmission path from the posterior tympanal membrane (PTM) have remained unclear. A mismatch between PTM vibrations and sensilla tuning has prompted speculations of a second filter. PTM coupling to the tracheal branches is suggested to support a transmission pathway. Here, we present three independent lines of evidence converging on the same conclusion: the existence of a series of linked membranes with distinct resonant frequencies serving both filtering and transmission functions. Micro-computed tomography (µ-CT) highlighted the ‘dividing membrane (DivM)’, separating the tracheal branches and connected to the PTM via the dorsal membrane of the posterior tracheal branch (DM-PTB). Thickness analysis showed the DivM to share significant thinness similarity with the PTM. Laser Doppler vibrometry indicated the first of two PTM vibrational peaks, at 6 and 14 kHz, originates not from the PTM but from the coupled DM-PTB. This result was corroborated by µ-CT-based finite element analysis. These findings clarify further the biophysical source of neuroethological pathways in what is an important model of behavioural neuroscience. Tuned microscale coupled membranes may also hold biomimetic relevance.

Controlling the absorption and diffusion of sound in the audible range is an exciting field of research. Achieving miniaturized acoustic systems able to operate at audio frequencies is one of the main challenges of acoustic engineering for many practical applications. One viable approach to tackle this challenge is by using meta materials such that deep subwavelength control can be achieved. This work investigates the fabrication and experimental characterization of membrane-coupled Helmholtz resonators as directional attenuators via 3D-printing, and introduces a 3D-printed piezoelectric component to transform acoustic attenuation to electrical outputs.

Osteoporosis disrupts the fine-tuned balance between bone formation and resorption, leading to reductions in bone quantity and quality and ultimately increasing fracture risk. Prevention and treatment of osteoporotic fractures is essential for reductions in mortality, morbidity, and the economic burden, particularly considering the aging global population. Extreme bone loss that mimics time-accelerated osteoporosis develops in the paralyzed limbs following complete spinal cord injury (SCI). In vitro nanoscale vibration (1 kHz, 30 or 90 nm amplitude) has been shown to drive differentiation of mesenchymal stem cells toward osteoblast-like phenotypes, enhancing osteogenesis and inhibiting osteoclastogenesis simultaneously. Here, we develop and characterize a wearable device designed to deliver and monitor continuous nanoamplitude vibration to the hindlimb long bones of rats with complete SCI. We investigate whether a clinically feasible dose of nanovibration (two 2 h/day, 5 days/week for 6 weeks) is effective at reversing the established SCI-induced osteoporosis. Laser interferometry and finite element analysis confirmed transmission of nanovibration into the bone, and microcomputed tomography and serum bone formation and resorption markers assessed effectiveness. The intervention did not reverse SCI-induced osteoporosis. However, serum analysis indicated an elevated concentration of the bone formation marker procollagen type 1 N-terminal propeptide (P1NP) in rats receiving 40 nm amplitude nanovibration, suggesting increased synthesis of type 1 collagen, the major organic component of bone. Therefore, enhanced doses of nanovibrational stimulus may yet prove beneficial in attenuating/reversing osteoporosis, particularly in less severe forms of osteoporosis.

An industrially relevant challenge for acoustic meta-material (AMM) research is to incorporate frequency tuneability, with the aim of creating adjustable acoustic sensors and sound attenuators. The resonant frequencies of AMMs are largely fixed after manufacture, with adjustable mechanisms often requiring complex materials that are difficult to implement in AMM construction. This research presents an entirely 3D printable adjustable AMM device, with a bespoke magnetic resin developed for the adjustable mechanism. This device demonstrates significant potential for resonance tuneability at ultra-subwavelength dimensions for low-frequency audio bandwidths. The resin characterisation is documented, with different material weightings, magnetic nanoparticle sizes, and base polymers trialled to optimize the formula. The photo-responsive resin is developed specifically for stereolithography (SLA) 3D printers, and the superparamagnetic properties achieved ensure no postprocessing poling stage is needed, supporting ease of manufacture. Enabling simple and efficient manufacture of tuneable AMMs is the first step to introducing AMMs into widespread use for industry and academia alike.

Acoustic metamaterials (AMMs) exhibit unique acoustic properties not found in conventional materials. Despite extensive research, applying AMMs in practice remains difficult, especially when translating mathematical models into functional devices. The challenge stems from the required manufacturing resolution for high frequency operation, where unit cell sizes must be scaled down to micrometer level. However, AMMs hold significant potential for applications where material size can be minimized without compromising performance, such as in the backing layers of ultrasound probes, which are essential components in medical ultrasound devices. A 3D-printable resin containing 60%wt aluminum oxide was developed to create acoustic diode-type metamaterial backing layers. These backing layers were experimentally (through-transmission tests) and theoretically (finite element analysis) evaluated and their reflection and transmission coefficients were compared to those of a homogeneous backing layer. This study marks a significant step towards advancing the fabrication and testing of AMMs for megahertz frequency applications such as medical ultrasound imaging.

Conventional osteogenic platforms utilize active growth factors to repair bone defects that are extensive in size, but they can adversely affect patient health. Here, an unconventional osteogenic platform is reported that functions by promoting capture of inactive osteogenic growth factor molecules to the site of cell growth for subsequent integrin-mediated activation, using a recombinant fragment of latent transforming growth factor beta-binding protein-1 (rLTBP1). It is shown that rLTBP1 binds to the growth-factor- and integrin-binding domains of fibronectin on poly(ethyl acrylate) surfaces, which immobilizes rLTBP1 and promotes the binding of latency associated peptide (LAP), within which inactive transforming growth factor beta 1 (TGF-β1) is bound. rLTBP1 facilitates the interaction of LAP with integrin β1 and the subsequent mechanically driven release of TGF-β1 to stimulate canonical TGF-β1 signaling, activating osteogenic marker expression in vitro and complete regeneration of a critical-sized bone defect in vivo.