Passive Structures Research Articles

4D fabrication of shape-changing systems for tissue engineering: state of the art and perspectives

AbstractIn recent years, four-dimensional (4D) fabrication has emerged as a powerful technology capable of revolutionizing the field of tissue engineering. This technology represents a shift in perspective from traditional tissue engineering approaches, which generally rely on static—or passive—structures (e.g., scaffolds, constructs) unable of adapting to changes in biological environments. In contrast, 4D fabrication offers the unprecedented possibility of fabricating complex designs with spatiotemporal control over structure and function in response to environment stimuli, thus mimicking biological processes. In this review, an overview of the state of the art of 4D fabrication technology for the obtainment of cellularized constructs is presented, with a focus on shape-changing soft materials. First, the approaches to obtain cellularized constructs are introduced, also describing conventional and non-conventional fabrication techniques with their relative advantages and limitations. Next, the main families of shape-changing soft materials, namely shape-memory polymers and shape-memory hydrogels are discussed and their use in 4D fabrication in the field of tissue engineering is described. Ultimately, current challenges and proposed solutions are outlined, and valuable insights into future research directions of 4D fabrication for tissue engineering are provided to disclose its full potential.

Optimizing thermal efficiency: Advancements in flat plate heat exchanger performance through baffle integration

Compact heat exchangers have been gaining popularity in many industrial applications. Various types of passive turbulising structures such as corrugations, protrusions and ribs are introduced in the flow path to increase the effective heat transfer area and the level of turbulence in the flow path. This study investigates the impact of introduction of baffles on the performance of PHEs in terms of flow characteristics, pressure drop, and heat transfer. Two distinct types of baffle structures, namely wedge and aerofoil configurations, were introduced at varying numbers - 1, 3, and 5. An extensive experimentation is conducted for a FPHE in a thermal power plant of 500 × 2 MW and various flow and thermal parameters are measured. Computational Fluid Dynamics is utilized in this study to find the optimum baffle configuration. A detailed validation study is executed to obtain the correct computational algorithm, that is the right mesh count, optimum turbulence model, and precise numerical algorithm by comparing the numerical results with the available experimental results. Wedge- type baffles create increased turbulence and pressure drop, while aerofoil-type baffles minimize stagnation and exhibit lower pressure drop. Both baffle configurations lead to a substantial increase in heat transfer, with the 5-wedge-baffle setup showing the highest up to a 55% enhancement of Nusselt number. The Performance Evaluation Criterion (PEC) of wedge and aerofoil type is about 1.24 to 1.3 and 1.22 to 1.24 to that of conventional one respectively.

Surgical treatment of isolated tricuspid valve disease: indications, new evidence, and risk stratification

The tricuspid valve, long neglected as a passive structure and often termed the "forgotten valve", has recently gained attention from the international medical and cardiological community due to the association of tricuspid regurgitation with an unfavorable prognosis. Isolated tricuspid regurgitation represents a relatively unknown and variable condition, closely linked to the shape and function of the right ventricle and the state of the pulmonary circulation. Currently, guidelines are not clear regarding the optimal treatment strategy, the process of patient selection, and the surgical or transcatheter procedural timing, nor for predicting patient outcomes. Surgical procedures specifically aimed at correcting isolated tricuspid regurgitation, without other concomitant indications for open-heart surgery, have been considered complex and risky, with a high rate of postoperative complications and a poorly understood impact on patient survival and life expectancy. In this review, we will attempt to examine tricuspid valve pathology by analyzing preoperative assessment, essential for risk stratification, various surgical techniques, and outcomes.

Ultrathin aerogel-structured micro/nanofiber metafabric via dual air-gelation synthesis for self-sustainable heating

Incorporating passive heating structures into personal thermal management technologies could effectively mitigate the escalating energy crisis. However, current passive heating materials struggle to balance thickness and insulating capability, resulting in compromised comfort, space efficiency, and limited thermoregulatory performance. Here, a dual air-gelation strategy, is developed to directly synthesize ultrathin and self-sustainable heating metafabric with 3D dual-network structure during electrospinning. Controlling the interactions among polymer, solvent, and water enables the microphase separation of charged jets, while adjusting the distribution of carbon black nanoparticles within charged fluids to form fibrous networks composed of interlaced aerogel micro/nanofibers with heat storage capabilities. With a low thickness of 0.18 mm, the integrated metafabric exhibits exceptional thermal insulation performance (15.8 mW m−1K−1), superhydrophobicity, enhanced mechanical properties, and high breathability while maintaining self-sustainable radiative heating ability (long-lasting warming of 8.8 °C). This strategy provides rich possibilities to develop advanced fibrous materials for smart textiles and thermal management.

Capacitive Sensors Based on Recycled Carbon Fibre (rCF) Composites.

Recycled carbon fibre (rCF) composites are increasingly being explored for applications such as strain sensing, manufacturing of automobile parts, assistive technologies, and structural health monitoring due to their properties and economic and environmental benefits. The high conductivity of carbon and its wide application for sensing makes rCF very attractive for integrating sensing into passive structures. In this paper, capacitive sensors have been fabricated using rCF composites of varying compositions. First, we investigated the suitability of recycled carbon fibre polymer composites for different sensing applications. As a proof of concept, we fabricated five touch/proximity sensors and three soil moisture sensors, using recycled carbon fibre composites and their performances compared. The soil moisture sensors were realised using rCF as electrodes. This makes them corrosion-resistant and more environmental-friendly, compared to conventional soil moisture sensors realised using metallic electrodes. The results of the touch/proximity sensing show an average change in capacitance (ΔC/C~34) for 20 mm and (ΔC/C~5) for 100 mm, distances of a hand from the active sensing region. The results of the soil moisture sensors show a stable and repeatable response, with a high sensitivity of ~116 pF/mL of water in the linear region. These results demonstrate their respective potential for touch/proximity sensing, as well as smart and sustainable agriculture.

Numerical examination on seismic response behavior of a piping system considering the plastic deformation of supports

A previous study proposed a passive safety structure to mitigate severe damage to crucial components under the beyond design basis event (BDBE). To examine the applicability of this concept to the piping system, the effect of the elastic-plastic behavior of the piping support structure on the seismic response of the piping system was investigated through elastic-plastic time history response analyses on a piping system model with multiple support structures. The numerical analyses considered the elastic-plastic behavior of supports and/or the inelastic characteristics of pipe material. The analysis results confirmed that the response of the piping system can be suppressed by adequately introducing the inelastic behavior of support structures. The results show the possibility of realizing of passive safety structure in piping systems and addressing some issues, such as changing vibration modes due to the elastic-plastic deformation of the support or the order of generation of inelastic behavior of pipe material and supports.

Exploration HTL-Free all inorganic mixed halide perovskite solar cells: effects of 4-ADPA passivation

The incredible PV performance of thin-film perovskite solar cells has garnered the attention of researchers. Mixed halide perovskite outweighs pure halide perovskite in its ability to optimize PV performance while performing material composition engineering. All inorganic mixed halide (AIMH) perovskite CsPbI2Br has shown stable performance against thermal variations. This study mainly highlights the performance of HTL (Hole transport layer) free, passivated solar cell structure with utilization of the SCAPS-1D simulator. The inclusion of passivation layer 4-ADPA(4-aminodiphenylamine) between active layer CsPbI2Br and the end electrode mitigates the occurrence of charge carrier recombination. The thickness of passivation layer 4-ADPA is optimized for the range 100 nm–1000 nm, and 100 nm is decided as the optimum width based on the evaluated PV performance of SnO2/CsPbI2Br/4-ADPA/anode. 4-ADPA layer with an optimum thickness of 100 nm, is embedded with a CsPbI2Br layer, and the performance of solar cell has been investigated under the collective impact of BDD (bulk defect density)/thickness of CsPbI2Br for the range (1012 cm−3 to 1018 cm−3)/(50 nm to 500 nm) respectively. Further, this study investigated the capacitance–voltage (C-V), Mott—Schottky (1/C2), and Nyquist plot (C-F) performance of solar cells under the influence of only BDD for two cell configurations (corresponding to maximum and minimum delivered PCE i.e., thickness/BDD is 200 nm/1012 cm−3 and 500 nm/1018 cm−3 respectively). The highest 13.27% of PCE is extracted from HTL-free, 4-ADPA passivated all inorganic PSC, at 200 nm/1012 cm−3 of thickness/BDD respectively. This technique encourages researchers to explore more cost-effective, HTL-free passivated solar cell structures.

Hard- and Soft-Coded Strain Stiffening in Metamaterials via Out-of-Plane Buckling Using Highly Entangled Active Hydrogel Elements.

Metamaterials show elaborate mechanical behavior such as strain stiffening, which stems from their unit cell design. However, the stiffening response is typically programmed in the design step and cannot be adapted postmanufacturing. Here, we show hydrogel metamaterials with highly programmable strain-stiffening responses by exploiting the out-of-plane buckling of integrated pH-switchable hydrogel actuators. The stiffening upon reaching a certain extension stems from the initially buckled active hydrogel beams. At low strain, the beams do not contribute to the mechanical response under tension until they straighten with a resulting step-function increase in stiffness. In the hydrogel actuator design, the acrylic acid concentration hard-codes the configuration of the metamaterial and range of possible stiffening onsets, while the pH soft-codes the exact stiffening onset point after fabrication. The utilization of out-of-plane buckling to realize subsequent stiffening without the need to deform the passive structure extends the application of hydrogel actuators in mechanical metamaterials. Our concept of out-of-plane buckled active elements that stiffen only under tension enables strain-stiffening metamaterials with high programmability before and after fabrication.

The Effect of Different Planning Conditions versus Explicit Grammar Instruction on Young EFL Learners’ Oral Production of a Picture Promoted Task and Grammar Gain

This study examined the effects of three task conditions on young EFL learners’ oral performance of a picture prompted task and their grammatical knowledge gain. To this end, 34 EFL learners were randomly assigned into three experimental groups, namely online planning, pre-planning, and explicit instruction, and one control group. Pictures were employed as prompts to implement the tasks in all groups. A Grammatical Judgment Test was used to compare students’ knowledge of simple present passive structure before and after the treatment. Learners’ oral task performance was measured in terms of Complexity, Accuracy, and Fluency (CAF). The findings demonstrated significant differences among groups as for CAF measures. However, no significant differences were spotted in GJT posttest scores. The paper calls for the incorporation of diverse prompt-based planning conditions in task-oriented teaching practices in order to target learners' higher levels of oral competence.

Associations amongst dynamic knee stiffness during gait, quadriceps stiffness, and the incidence of knee osteoarthritis over 24 months: a cohort study with a mediation analysis

BackgroundDecreased strength and increased stiffness of the quadriceps have been associated with a higher risk of developing knee osteoarthritis (OA) in elders. Dynamic joint stiffness (DJS) represents collective resistance from active and passive knee structures for dynamic knee motions. Elevated sagittal knee DJS has been associated with worsening of cartilage loss in knee OA patients. Altered quadriceps properties may affect DJS, which could be a mediator for associations between quadriceps properties and knee OA. Hence, this study aimed to examine whether DJS and quadriceps properties would be associated with the development of clinical knee OA over 24 months, and to explore the mediation role of DJS in associations between quadriceps properties and knee OA.MethodsThis was a prospective cohort study with 162 healthy community-dwelling elders. Gait analysis was conducted to compute DJS during the loading response phase. Quadriceps strength and stiffness were evaluated using a Cybex dynamometer and shear-wave ultrasound elastography, respectively. Knee OA was defined based on clinical criteria 24 months later. Logistic regression with generalized estimating equations was used to examine the association between quadriceps properties and DJS and incident knee OA. Mediation analysis was performed to explore the mediation role of DJS in associations between quadriceps properties and the incidence of knee OA.ResultsA total of 125 participants (65.6 ± 4.0 years, 58.4% females) completed the 24-month follow-up, with 36 out of 250 knees identified as clinical knee OA. Higher DJS (OR = 1.86, 95%CI: 1.33–2.62), lower quadriceps strength (1.85, 1.05–3.23), and greater quadriceps stiffness (1.56, 1.10–2.21) were significantly associated with a higher risk of clinical knee OA. Mediation analysis showed that the DJS was not a significant mediator for the associations between quadriceps properties and knee OA.ConclusionsHigher sagittal knee dynamic joint stiffness, lower quadriceps strength, and greater quadriceps stiffness are potential risk factors for developing clinical knee OA in asymptomatic elders. Associations between quadriceps properties and knee OA may not be mediated by dynamic joint stiffness. Interventions for reducing increased passive properties of the quadriceps and knee joint stiffness may be beneficial for maintaining healthy knees in the aging population.

Transmissive reconfigurable metasurface enabling independent control of active and passive modules through weak coupling

Metasurfaces have demonstrated rich electromagnetic control capabilities and degrees of freedom in past years. As is well known, for passive metasurfaces, their functionalities cannot be further expanded accordingly once prototypes are established. Therefore, reconfigurable metasurfaces, utilizing active devices to replace geometric changes in passive structures, have received widespread attention, especially with the development of wireless communication recently. In reconfigurable metasurfaces, artificial meta-atoms are composed of active devices and passive structures combined together. However, these two modules are usually utilized as a whole due to the tight coupling of the active devices and the passive structures, which results in passive structures not receiving sufficient attention and being utilized as independent degrees of freedom. In this article, we propose the concept of weakly coupled reconfigurable metasurfaces in transmissive systems, enabling independent control of active and passive modules through weak coupling. As the proof of concept, a simple weakly coupled system is proposed, which can realize the transmission wavefront engineering through the geometric changes of meta-structures in passive mode, while achieving switching between transmission and reflection states in active mode, respectively. Our exploration lies in making use of the physical structure, which is easily neglected in traditional reconfigurable metasurface design, emphasizing the collaborative work of active and passive modules, exploring more available variables within the same aperture, and providing a potential solution for balancing functionality and resource consumption in practical applications.

The Passive Voice in Urdu: A Syntactic Investigation and Transformational Analysis

This paper explores passive structures in the Urdu language. It aims to understand their syntactic configuration and how to convert active sentences into passive ones. Limited research exists on Urdu's passivization compared to English. The research questions focus on the structure of passive sentences in Urdu and the necessary modifications for passivization. A qualitative research methodology is used, analyzing written texts and daily conversations of native Urdu speakers. The literature review compares Urdu's passive structures with English, including the standard "be" passive and less common "get passive" and "have passive." Findings show that Urdu passives use the auxiliary “Gaya” and the agent can be explicitly mentioned using "from-PP" rather than "by-PP." The paper also covers passivization in imperative sentences, using “ja:” and “cha:hi:e” morphemes. Moreover, voice neutrality is examined, showing that passive and active sentences in Urdu often convey the same event with similar agent-patient relationships. The study’s conclusive remarks on passive construction in Urdu have the potential to ignite future research in this domain. The paper contributes to the knowledge of passive constructions in Urdu which can significantly impact translation accuracy and interpretation between Urdu and English. This research also adds to the big picture of how languages work.

Effect of rapid cooling, frozen storage, and thawing on the passive viscoelastic properties and structure of the rat aorta

Biological tissues decay over time after harvesting, which alters their biomechanical properties. This poses logistical challenges for studies investigating passive arterial biomechanics as tissues need to be characterized shortly after excision. Freezing and cryopreservation methods can help alleviate the need for biomechanical testing of fresh tissue in human ex vivo studies. However, these methods tend to eliminate or reduce arterial cell functionality and affect passive biomechanics. Furthermore, their impact on dynamic arterial biomechanics remains unknown despite arterial viscoelastic properties being an integral component contributing to arterial stiffness under in vivo loading conditions. The present study aims to investigate the impact of rapid cooling and subsequent storage at −80 °C on the passive viscoelastic properties of arterial tissue and aid in ascertaining whether this is a suitable method to delay tissue analysis for studies investigating passive arterial biomechanics. Control and frozen abdominal rat aorta segments were quasi-statically and dynamically tested using a biaxial testing set-up. The results were modeled using a constituent-based quasi-linear viscoelastic modeling framework, yielding directional stiffness parameters, individual constituent biomechanical contributions, and a quantification of viscoelastic stiffening under dynamic pressurization conditions. Frozen samples displayed significantly decreased wall thickness, viscoelastic dissipation, viscoelastic stiffening, and significantly decreased circumferential deformation with changes in luminal pressure. Furthermore, frozen samples displayed significantly increased circumferential stiffness, pulse wave velocity, and collagen load bearing. Consequently, these changes should be considered when utilizing this tissue preservation method to delay biomechanical characterization of rat aortic tissue.

Adaptive nonlinear damping control of active secondary suspension for hunting stability of high-speed trains

This study employs an active secondary suspension with adaptive nonlinear damping to enhance the hunting stability of high-speed trains. Adjusting passive suspension parameters to optimize ride comfort and hunting stability simultaneously in varied extreme operational conditions poses a significant challenge for high-speed trains. This research integrates high-order displacement-dependent nonlinear damping into the secondary lateral suspension, drawing on insights from field test data. We analyze three typical hunting motion bifurcations: carbody hunting and both subcritical and supercritical bifurcations of bogie hunting. Theoretical analysis demonstrates that (a) the adaptive nonlinear damping narrows the unstable speed range and reduces the amplitude of the limit cycle in carbody hunting, (b) while it does not increase the critical speed for supercritical bogie hunting bifurcation, it substantially reduces the amplitude of the limit cycle, and (c) it increases the nonlinear critical speed for subcritical bogie hunting bifurcation, with the potential to alter the bifurcation from subcritical to supercritical, which holds considerable practical significance. Implementing such nonlinear damping directly through passive structure is challenging. Therefore, in view of this underactuated control problem, the constraint-following control is applied in the active suspension system to inherit the benefits of adaptive nonlinear damping. The simulation results show that the proposed active suspension system can effectively suppress the abnormal vibration caused by hunting motions.

A compliant constant-force mechanism with sub-Newton force and millimeter stroke output

Force control technology plays a pivotal role in manipulating vulnerable objects. This paper presents a new compliant mechanism with constant-force output characteristics, which can prevent force damage due to displacement overshoot. Due to its unique passive structure, it is exempted from intricate sensors and complicated controllers. The quasi-static constant-force mechanism is obtained by the parallel combination of positive- and negative-stiffness mechanisms. Such a straightforward method facilitates a rapid development and implementation process. The procedures of architectural arrangement selection, theoretical model establishment, parameter sensitivity evaluation, and multi-objective structural optimization are carried out to achieve specified constant-force characteristics. Moreover, the prototype fabrication and performance testing of a constant-force mechanism have been performed. The results demonstrate that the mechanism delivers a constant output force of 339 mN in the motion range of 1.84 mm. The constant-force feature is solely governed by the actuating displacement and is independent of driving speed and acceleration. The promising application of the constant-force mechanism has been demonstrated by manipulating small force-sensitive objects such as biological eggs.

A Comparative Analysis of Passive Voice in Farsi and Pashto Languages

A Comparative Analysis of Passive Voice in Farsi and Pashto Languages

Model Predictive Control With Stability Guarantee for Second-Order DC/DC Converters

This paper proposes a continuous-control-set model-predictive control (CCS-MPC) designed explicitly for second-order dc/dc converters such as the boost, buck, buck-boost, and non-inverting buck-boost converters. These converters share a bilinear dynamic structure that allows designing a generalized control. In contradistinction to the conventional finite-control-set approach, the proposed control uses a pulse-width modulation that allows a continuous control set with constant switching frequency. The modulation index's saturation is considered an inequality constraint in the optimization model. The proposed control guarantees stability in the sense of Lyapunov. This property is proven by using the passive structure of these converters. Experimental results on the four converters mentioned above show the superior performance of the proposed control in terms of robustness, offset, computation time in the central processing unit and, of course, stability.

Experimental investigation on transpiration cooling of ammonium carbonate in porous composite structure at high-temperature

This paper presents and experimental investigation of a novel transpiration cooling concept using ammonium carbonate as the solid coolant. Ammonium carbonate is a promising solid transpiration coolant with stable physicochemical properties at room temperature, high heat absorption by decomposition, and no solid residual after decomposition. In order to enhance the load-bearing properties of the thermal protection structure, alumina and zirconia ceramic foams with high porosity and large pore size were used as the coolant carrier, and the foam layer could still provide thermal insulation capacity after the coolant was exhausted. The transpiration cooling experiment were conducted with a stainless-steel wire mesh porous panel as the heated surface of the cooling structure within the high-temperature radiant apparatus, and the cooling performance was compared with that of hydrogel and a passive thermal protection structure. The results indicated that the thermal protection effect using the transpiration cooling technique has a significant advantage over the passive thermal protection method. The maximum cooling efficiencies of the ammonium carbonate cooling structure were up to 6.5 % and 33.7 % for the surface and bottom, respectively. In the transpiration cooling system without coolant injection force, the addition of a thin foam layer can extend the cooling time of ammonium carbonate from 981 s to 1800 s. However, this approach is detrimental to the cooling performance of the hydrogel. Influenced by the huge heat absorption of ammonium carbonate, the difference in heat transfer between alumina and zirconia foams within the cooling structure can be negligible.

Spring-Damped Underactuated Swashplateless Rotor on a Bicopter Unmanned Aerial Vehicle

The stabilisation capabilities of unmanned aerial vehicles (UAVs) with bicopter underactuated swashplateless rotors are highly sensitive to motor-induced vibration. Due to the requirement of the active control of underactuated swashplateless rotors, conventional designs are limited in reducing vibration through control optimisation. A solution with customized passive spring-damping structures on a unique underactuated swashplateless rotor of a tiltrotor bicopter platform is presented. The implementation of this structure effectively reduces the self-coherent vibration in flights. As a result, a higher level of control authority has been achieved without setting excessive low-pass filtering for vibration. Experimentally obtained inertial measurement unit (IMU) data, rotor speed, rotor tilt angle, and the cyclic stator response are presented for comparison with Simulink model predictions.

A locally resonant metamaterial beam with tunable electromagnetic stiffness based on the electromechanical analogy network

Locally resonant acoustic metamaterials offer distinctive advantages in controlling low-frequency elastic waves. However, passive structures often face limitations due to narrow bandgaps and fixed working band once they are fabricated. This paper introduces a novel metamaterial beam with tunable bandgaps. This achievement is made possible by employing one electromagnet and three permanent magnets to create an electromagnetic spring. The initial stiffness, provided by the spiral beam, collaborates with the electromagnetic components to establish a local resonant unite cell featuring tunable composite stiffness. Subsequently, an analogy network is formulated for the metamaterial beam based on electromechanical analogy theory. This network not only elucidates the generation mechanism and regulatory principles of bandgaps but also serves as a paradigm for the proactive design of the metamaterial beam with external control sources, particularly under finite period conditions. Finally, theoretical analysis and experimental results collectively demonstrate the flexibility of the proposed metamaterial beam in effectively suppressing low-frequency elastic waves across a wide frequency range.