Conventional Sheet Research Articles

A Comprehensive Review on the Incremental Sheet Forming of Polycarbonate.

Incremental sheet forming has emerged as an excellent alternative to other material forming procedures, incrementally deforming flat metal sheets into complex three-dimensional profiles. The main characteristics of this process are its versatility and cost-effectiveness; additionally, it allows for greater formability compared to conventional sheet forming processes. Recently, its application has been extended to polymers and composites. The following review aims to present the current state of the art in the incremental sheet forming of polycarbonate, an outstanding engineering plastic, beginning with initial studies on the feasibility of this process for polymers. Attention is given to the advantages, drawbacks, and main applications of incrementally formed polycarbonate sheets, as well as the influence of process parameters and toolpath strategies on features such as formability, forming forces, deformation and failure mechanisms, geometric accuracy, surface quality, etc. Additionally, new hybrid forming methods for process optimisation are presented. Finally, a discussion is provided on the technical challenges and future research directions for incremental sheet forming of polycarbonate and, more generally, thermoplastics. Thus, this review aims to offer an extensive overview of the incremental forming of polycarbonate sheets, useful to both academic and industrial researchers working on this topic.

Self-supported electrochemical sensor based on uniform palladium nanoparticles functionalized porous graphene film for monitoring H2O2 released from living cells.

Graphene film has been considered a promising material for the construction of self-supported electrodes due to its favorable flexibility and high conductivity. However, the film fabricated from pristine graphene or conventional graphene sheet reduced graphene oxide processes limited electrocatalytic performance. Decorating active metal species or incorporating heteroatoms intothegraphene framework have been proved to be effective methods to enhance the electrocatalytic efficiency of graphene film-based self-supported electrodes. Herein, we present a freestanding electrode composed of uniform Pd nanoparticles decorating N,S co-doped porous graphene film (Pd/NSPGF) and explore its practical application in differentiating various human colon cell types by in situ tracking the amount of H2O2 secreted from live cells. Our findings reveal that, on the one hand, the NSPGF has abundant surface and inner pores, which promote active site exposure, and mass diffusion during electrochemical reactions; on the other hand, the substitutional doping ofthe graphene framework with heteroatoms (e.g., N or S) can tailor its electronic and chemical properties, and facilitate the uniform loading of high-density Pd nanoparticles. Moreover, the intrinsic activity of Pd/NSPGF is regulated by the interaction of Pd nanoparticles withtheNSPGF support. Taking the advantages of morphology and composition, the self-supported Pd/NSPGF electrode displays remarkable electrochemical performance with a wide linear range up to 2.0mM, low detection limit of 0.1μM (S/N = 3), high sensitivity of 665 µA cm-2mM-1, and good selectivity. When applied in real-time trackingofthe H2O2 released from normal human colon epithelial cells and human colorectal cancer cells, the Pd/NSPGF-based electrochemical sensing system can distinguish the cell types by testing the number of extracellular H2O2 molecules released per cell, which holds considerable potential for early detection and monitoring of disease-related clinical specimens.

Towards fiber-reinforced front-sheets for lightweight PV modules in VIPV

Novel approaches in the field of photovoltaics, such as building or vehicle integration require investigations of lightweight PV module concepts [1]. This research proposes and evaluates a lightweight PV module concept using glass fiber-reinforced polymers (GFRP) based on epoxy composites within the module stack. The usage of GFRP as front material as proposed in this work, reduces weight by 44–74 % compared to conventional glass-back sheet modules. We show results that with the proposed module stack hail impact resistant module can be built with small and large size avoiding any cell cracks after impact. Additionally, modules with GFRP front layer withstand thermal cycling tests with low power losses ranging from −0.9 % to −1.1 % at Pmpp depending on the individual GFRP layer. We also show results of cut susceptibility tests with an increased performance compared to modules with a polymeric front layer. Development topics remain as shown by damp-heat and UV irradiation testing, were an optimization of the module stack still needs to be performed.

Thermo-Mechanical Numerical Simulation of Friction Stir Rotation-Assisted Single Point Incremental Forming of Commercially Pure Titanium Sheets.

Single point incremental forming (SPIF) is becoming more and more widely used in the metal industry due to its high production flexibility and the possibility of obtaining larger material deformations than during conventional sheet metal forming processes. This paper presents the results of the numerical modeling of friction stir rotation-assisted SPIF of commercially pure 0.4 mm-thick titanium sheets. The aim of this research was to build a reliable finite element-based thermo-mechanical model of the warm forming process of titanium sheets. Finite element-based simulations were conducted in Abaqus/Explicit software (version 2019). The formability of sheet metal when forming conical cones with a slope angle of 45° was analyzed. The numerical model assumes complex thermal interactions between the forming tool, the sheet metal and the surroundings. The heat generation capability was used to heat generation caused by frictional sliding. Mesh sensitivity analysis showed that a 1 mm mesh provides the best agreement with the experimental results of total forming force (prediction error 3%). It was observed that the higher the size of finite elements (2 mm and 4 mm), the greater the fluctuation of the total forming force. The maximum temperature recorded in the contact zone using the FLIR T400 infrared camera was 157 °C, while the FE-based model predicted this value with an error of 1.3%. The thinning detected by measuring the drawpiece with the ARGUS non-contact strain measuring system and predicted by the FEM model showed a uniform thickness in the drawpiece wall zone. The FE-based model overestimated the minimum and maximum wall thicknesses by 3.7 and 5.9%, respectively.

Self-assembled multicolor fluorescent nanoparticles derived from dopamine analogues: A versatile platform for biomedical applications

Fluorescent polydopamine (PDA) nanoparticles (FPNPs) have gained recognition as invaluable resources for a wide range of biomedical applications. Typically, these FPNPs are synthesized solely from dopamine (DA). In this study, our focus lies in the design of PDA-based multicolor fluorescent nanoparticles (m-FNPs) utilizing DA and its analogues, including levodopa (LVD), norepinephrine (NPP), 6-hydroxydopamine (HDA), and epinephrine (EPP) monomers, thereby paving the way for the next generation of FPNPs. The m-FNPs, respectively labeled as L-FNPs, N-FNPs, H-FNPs, E-FNPs, and D-FNPs, exhibit distinct band gaps influenced by the monomer’s structure, allowing versatile multicolor fluorescence bioimaging across a broad fluorescence emission spectrum ranging from 410 to 680 nm. Significantly, the catechol groups present on the surfaces of m-FNPs enable chelation with various theranostic metal ions and subsequent release under endo/lysosome pH conditions in cancer cells, thereby eliciting a cancer-specific “OFF-ON” fluorescence signal in vivo. Furthermore, integrating m-FNPs into poly(dimethylsiloxane) (PDMS) produces a tissue-adhesive fluorescence sheet that permits fluorescence monitoring of internal tissue surfaces within living organisms for 15 days. These m-FNPs-incorporated PDMS (m-FNPs-PDMS) sheets, enriched with catechol, carboxyl, and amine groups on their surfaces, exhibit significantly enhanced adhesion to biological tissues compared to conventional PDMS sheets containing PDA. Our findings suggest that m-FNPs pave the way for advancing PDA-based fluorescent nanoparticles with diverse applications in the biomedical field, supported by comprehensive in vitro and in vivo evaluations.

2D Transition Metal Dichalcogenides‐Based Composites for Electromagnetic Interference Shielding and Microwave Absorption Applications

Due to the advancements in the electronics industry, the demand for new electromagnetic interference (EMI) shielding and microwave absorption (MA) materials has increased significantly in recent decades. Researchers are investigating a variety of new materials to replace conventional metal sheets in response to these growing demands. Consequently, there is a growing interest in lightweight EMI shielding materials that can meet the demand for lightweight and highly integrated electronic equipment. 2D structural transition metal dichalcogenides (TMDCs) are good candidates for EMI shielding and MA due to their unique properties. This article examines the latest developments in TMDCs and their composite nanomaterials, focusing on their EMI shielding effectiveness and MA performance. The investigation includes a thorough examination of these materials’ shielding effectiveness, maximum reflection loss value, and effective absorption bandwidth. Moreover, this review analyzes the challenges and opportunities that arise in the development of TMDCs.

Systematic and quantitative comparison of axially overlapped multi-focus and Gaussian light sheets

Overlapped multi-focus light sheet (MF-LS) along beam propagation microscope has generated significant interest due to its reportedly superior axial resolving power when compared to conventional Gaussian light sheet (GLS) systems. By axially overlapped multiple light sheets in the illumination path, it would weld the combined field of view (FOV) and be wider than stationary counterparts. However, the effect of diffractive energy entering the focal space of desired beam, impacting the optical sectioning capabilities of the system has not been studied. It introduces the question that does a MF-LS performance better than standard GLS? Here we quantitatively and systematically analyzed the properties between optimized MF-LS and traditional GLS on z-axis resolution, signal to background ratio (SBR), and FOV. We first optimized the MF-LS by analyzing the diffractive energy to acquire the best sectioning capability. Then, we demonstrated a comparable optical sectioning performance between the two types of LS when matching with specific optical parameters including the illumination numerical aperture (NA) and foci numbers. A novel equation has been proposed to describe the equivalence relation. Analysis in the research suggests “putative FOV-extension” characteristics of MF-LS due to energy effects and interference between beams. Finally, we performed simulation and imaging experiments approaches on both fluorescence microspheres and biological samples to validate the viewpoint of this research.

Performance evaluation of fabricated conventional horizontal perforated sheet aerator models for aquafarm

Performance evaluation of fabricated conventional horizontal perforated sheet aerator models for aquafarm

Positive temperature coefficient material based on silicone rubber/ paraffin/ graphite/ carbon nanotubes for wearable thermal management devices

PTC materials provide substantial advantages such as adaptive temperature control performance and excellent safety. However, their use in wearable devices has been limited due to the essential requirements for flexibility and stability. This study prepared a new type of flexible PTC material, with a PTC intensity of 6.3 and a resistivity of only 400 Ω·cm in the low-temperature region. The flexibility of the material is evidenced by its 2 mm bending diameter and its ability to maintain structural integrity even after 100 bends. Furthermore, experimental evidence proves that PTC material has excellent thermal stability and can withstand extreme temperatures ranging from −20 °C to 120 °C. In the thermal control experiment, the PTC material maintains a temperature of approximately 25 °C to accommodate the requirements of the human body in a cold environment. When compared to the conventional PI heating sheet, the newly prepared PTC material exhibits advantages such as rapid response, adaptive temperature control performance, and safety. Consequently, it presents a broad application prospect in the field of wearable heating devices.

Unraveling the Role of Enkephalin Signaling in Respiratory Control

Opioid-induced respiratory depression (OIRD) is the primary cause of death during opioid overdose characterized by a dramatic decrease in respiratory frequency and regularity. Opioid drugs cause neural depression in the ventral respiratory column (VRC) of the medulla via activation of mu-opioid receptors (MORs) that partially contribute to breathing cessation. Nonetheless, little is known about the role of endogenous opioid signaling in respiratory modulation. This study investigates the role of endogenous MORs ligands, enkephalins encoded by the Penk gene, in breathing control. To test whether enkephalin-expressing neurons of the preBӧtzinger Complex (preBӧtC) — the inspiratory rhythm-generating region of the VRC — are necessary for breathing rhythmogenesis, we permanently silenced the region’s Penk+ population via AAV1 injections encoding tetanus toxin light chain. Using whole-body plethysmography, we recorded the animals' respiratory performance within 14 days post-injection. During the second week of testing, we observed unexpected mortality in some experimental but not control animals; post-hoc analysis of breathing traces revealed the development of severe tachypnea in succumbed mice. These data suggest that Penk+ preBӧtC cells are integral for normal respiration. Still, these observations did not assess the specific contribution of enkephalin, as opposed to other types of signaling, to breathing stability. To directly dissect the effects of enkephalin on respiratory rhythm, we utilized optogenetics and pharmacological approaches in brainstem slice preparation from neonatal mice. Brief optogenetic activation of Penk+ neurons in the preBӧtC evoked population bursts while prolonged stimulation increased the rhythm frequency. However, the application of MOR antagonist Naloxone did not affect the network’s responses to light stimulations indicating the lack of MOR/enkephalin involvement in mediating these excitatory effects. High fidelity and temporal coordination of evoked bursts implicated glutamatergic transmission as the primary mechanism of excitation. Likewise, we were able to optogenetically drive respiratory rhythm in urethane-anesthetized spontaneously breathing adult mice by stimulating the preBӧtC. Further, there was a small but significant difference in breathing responses in the presence of Naloxone compared to control conditions. We hypothesized that longitudinal projections to the VRC that might be disrupted during slice preparation contributed enkephalin to the respiratory circuits in the whole-body preparation. We utilized recombinase-dependent reporter mice crosses and retrograde AAV tracers to identify brain-wide Penk inputs to the preBӧtC. Using conventional and light sheet fluorescence microscopy, we located regions in the pons, midbrain, and forebrain that send direct enkephalinergic projections to the VRC. Functional characterizations of these projections will potentially uncover state-dependent mechanisms of respiratory modulation via endogenous opioids. Untimely, this study will help elucidate the role of enkephalin signaling in breathing control and may establish therapeutic targets for preventing OIRD. R01HL166317, R00HL145004. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Unexpected texture transition induced by grain growth during static annealing of a dilute Mg-1Al alloy

Conventional rolled Mg-Al alloy sheets typically exhibit strong basal textures that remain and may even strengthen after recrystallization annealing due to the preferential growth of basal-oriented grains, resulting in poor formability at room temperature. Therefore, the knowledge of recrystallization and grain growth is critical for modifying textures of Mg-Al alloy sheets. The static recrystallization and texture evolution in a cold-rolled dilute Mg-1Al (wt.%) alloy during various annealed temperatures ranging from 300 °C to 450 °C, have been investigated using the quasi in-situ electron backscatter diffraction (EBSD) method. The as-rolled Mg-1Al alloy shows a dominant basal texture, which weakens and broadens in the rolling direction (RD) during the subsequent annealing, accompanied by the formation of 〈101¯0〉 texture component. Particularly, the 〈101¯0〉 texture component is more pronounced after annealing at high temperatures. The quasi in-situ EBSD results show that recrystallized grains are mainly induced by shear bands, which exhibit a wide spectrum of orientations with c-axis tilt angles ranging 20°-45° from the normal direction (ND). Orientations of shear band-induced recrystallized grains are retained during the entire recrystallization process, resulting in a reduction in the texture intensity. Moreover, recrystallized grains belonging to the 〈101¯0〉 texture component grow preferentially compared to those with other orientations, which is attributed to low energy grain boundaries, especially grain boundaries with ∼30° misorientation angles. Furthermore, the high temperature annealing facilitates the rapid growth of grain boundaries having a 30° misorientation angle, leading to the occurrence of distinct 〈101¯0〉 texture after annealing at 450 °C for 1 h. The results provide insights for texture modification of rare earth-free low-alloyed Mg alloys by controlling annealing parameters.

Effect of brazing and artificial aging on the corrosion and strengthening behavior of a quad-layer Al alloy composite

PurposeThis study aims to investigate the influence mechanism of brazing and aging on the strengthening and corrosion behavior of novel multilayer sheets (AA4045/AA7072/AA3003M/AA4045).Design/methodology/approachPolarization curve tests, immersion experiments and transmission electron microscopy analysis were used to study the corrosion behavior and tensile properties of the sheets before and after brazing and aging.FindingsThe strength of the sheet is weakened after brazing due to brittle eutectic phases, and recovered after aging due to enhanced precipitation strengthening in the AA7072 interlayer. The core of nonbrazed sheets cannot be protected due to the significant galvanic coupling effect between the intermetallic particles and the substrate. Brazing and aging treatments promote the redissolved of second phased and limit corrosion along the eutectic region in the clad, allowing the core to be protected.Originality/valueAA7xxx alloy was added to conventional brazed sheets to form a novel Al alloy composite sheet with AA4xxx/AA7xxx/AA3xxx structure. The strengthening and corrosion mechanism of the sheet was proposed. The added interlayer can sacrificially protect the core from corrosion and improves strength after aging treatment.

A new hybrid stretch forming and double-layer two-point incremental sheet forming process

This research presents a new hybrid stretch forming and double-layer two-point incremental sheet forming (hybrid stretch forming and DL-TPIF). The novelty of the proposed hybrid process is first to employ double-layer blank sheets in the conventional two-point incremental sheet forming (TPIF) process, which allows the material in the flange region to flow into the deformation region for achieving more uniform thickness distribution and less thinning. The use of an upper dummy sheet prevents the forming tool from direct contact the target blank sheet to improve the surface quality. Secondly, the combination of the stretch forming and the DL-TPIF processes enables seamless integration of the two forming processes to take advantages of both processes. The proposed new hybrid process was evaluated by two case studies through experimental testing and finite element (FE) simulation. Three pre-stretching conditions were examined to evaluate the effect of the prior stretching operation. The results showed that the maximum thickness reduction of the dome shape produced by the new stretch forming and DL-TPIF process was less than one third of that obtained from the conventional TPIF process. The geometrical accuracy and surface quality were also improved by using this new hybrid forming process.

Development and characterization of cranberry pomace extract incorporated and vitamin E fortified edible films as an edible separation sheet for fruit leather.

The separation sheets for fruit leather are traditionally made of plastic film or wax paper, which not only leads to environmental issues but also is inconvenience to consumers. This study evaluated edible fruit leather separation sheets using food polymers, including hydroxypropyl methyl cellulose (HPMC) and incorporation of cranberry pomace water extract (CPE) for providing natural fruit pigment, flavor, and phenolics. HPMCCPE film was then further improved by incorporating hydrophobic compound (oleic acid, OA) and vitamin E (VE) via cellulose nanocrystal (CNC) Pickering emulsion (CNCP) for enhancing film hydrophobicity and nutritional benefit, respectively. The CNCP-HPMCCPE film exhibited reduced water vapor permeability (∼0.033gmm/m2dPa) compared to HPMCCPE film (∼0.59gmm/m2dPa) and had the least change in mass and moisture content when wrapping fruit leather for up to 2 weeks of ambient storage. The fruit leather wrapped by CNCP-HPMCCPE film showed lower weight change than those by films without CNCP due to low mass transfer between film and fruit leather. CNCP resulted in controlled release of VE into a food simulating solvent (ethanol). The developed colorful and edible fruit leather separation sheet satisfied the increased market demands on sustainable food packaging. PRACTICAL APPLICATION: Colorful and flavorful edible films made of edible polymers, fruit pomace water extract, and emulsified hydrophobic compounds with vitamin E were created. The films have the satisfactory performance to replace the conventional fruit leather separation sheet made of plastic or wax paper. The edible films can be eaten with packaged fruit leather for not only reducing packaging waste but also providing convenience and nutritional benefit to consumers. These functional edible films may also be utilized to package other food products for promoting packaging sustainability and nutritional benefit.

Multifunctional composite material based on piezoelectric nanofibers and Cu-CFRP electrodes for sensing applications

To monitor possible failures of a composite, several Structural Health Monitoring (SHM) systems have been developed. However, these methods typically involve embedding commercial sensors within the laminate, potentially compromising the material's strength.In this study, a self-sensing composite laminate was fabricated by interleaving poly(vinylidenefluoride-trifluoroethylene) (P(VDF-TrFE)) piezoelectric nanofibers between Glass Fiber Reinforced Plastic (GFRP) prepreg plies. Instead of conventional metallic sheets, hybrid Copper-Carbon Fiber Reinforced Plastic (Cu-CFRP) was used as electrodes to collect piezoelectric signals. This innovative approach offers two main advantages: enhanced interlaminar fracture toughness due to nanometric piezoelectric fibers and an intrinsic connection between copper wires and carbon, eliminating the need for additional electrical cables within the laminate. The effect of stacking sequence parameters on the self-sensing laminate's electromechanical response was investigated using a Design of Experiment (DoE) based on the Box-Benken method. Additionally, a lumped electric circuit model was employed to gain analytical insights into the piezoelectric behavior of the laminates.

Explicating tensile response of AA6061-T6 sheet post single point incremental forming: Two camera-DIC strain measurement and texture analysis

Incremental sheet forming (ISF) produces 3D components imposing localized deformation by spherical-ended tool moving on a predefined path without requiring component-specific die/punch. The formability achieved in ISF is much higher than the conventional sheet forming processes; however, the deformation imposed during ISF alters the mechanical properties of the component. The present work investigates the tensile behavior and strain localization during the uniaxial tensile test of the AA6061-T6 component after single point incremental forming (SPIF). The influence of incremental depth (Δz) and tool diameter (dt) on the tensile response and strain localization is illustrated by a digital image correlation setup with two cameras. The tensile strength of SPIF components increases with Δz and mildly decreases with dt. The changes in scallop heights/undulations formed due to tool movement at higher Δz and dt influence the localized strain accumulation. Regions of localized strain are more when Δz is high, but strain accumulation is more even with an increase in dt. Texture analysis showed increased dislocation density due to the strain hardening after SPIF. The presence of brass {011}<211>, copper {112}<111>, and S {123}<634> texture indicates the plane-strain deformation in the wall region of the component formed by SPIF.

Enhancing Metal Forging Tools and Moulds: Advanced Repairs and Optimisation Using Directed Energy Deposition Hybrid Manufacturing

This article investigates the efficacy of directed energy deposition (DED) processes in repairing forging tools and moulds, comparing mechanical properties between specimens fabricated from conventional sheet metal and those manufactured by DED techniques. A comparative analysis reveals significant mechanical differences between subtractive and DED-manufactured specimens, emphasising the nuanced balance between tensile strength and ductility in DED-produced components influenced by layering. Notable insights from scatter plot analyses highlight distinct material behaviours, particularly layer-dependent tendencies in DED-manufactured specimens. Regression-based predictive models aid in understanding material behaviours, aiding in informed material selection for manufacturing processes. Additionally, this article underlines the advantages of DED-based repair processes, highlighting precision, material efficiency, reduced lead times, and cost-effectiveness. The article studies die and mould repair, tool restoration, and critical considerations like material compatibility and quality assurance. The study concludes by emphasising the role of hybrid manufacturing in extending product lifecycles, in conformity with specific mechanical requirements, and fabricating complex geometries, despite potential higher costs in materials and technologies. Overall, this research demonstrates the efficacy of DED processes in enhancing component reliability and lifespan in metalworking industries.

Fibronectin Connecting Cell Sheet Based on Click Chemistry for Wound Repair.

As a living repair material, cell sheet exhibits significant potential in wound repair. Nonetheless, wound healing is a complicated and protracted process that necessitates specific repair functions at each stage, including hemostasis and antibacterial activity. In this work, on the basis of harvesting the cell sheet via a photothermal response strategy, a fibronectin attached cell sheet (FACS) is prepared to enhance its wound repair capability. For this purpose, the azide group (N3 ) is initially tagged onto the cell surface through metabolic glycoengineering of unnatural sugars, and then the conjugate (DBCO-fibronectin) comprises of the dibenzocyclooctyne (DBCO) and fibronectin with multiple wound repair functions is linked to N3 using click chemistry. Biological evaluations following this demonstrates that the FACS preparation exhibits excellent biocompatibility, and the fibronectin modification enhances the capacity for cell proliferation and migration. Moreover, in vivo wound healing experiment confirms the reparative efficacy of FACS. It not only has a wound closure rate 1.46 times that of a conventional cell sheet but also reduces inflammatory cell infiltration, promotes hair follicle and blood vessel regeneration, and encourages collagen deposition. This strategy holds enormous clinical potential and paves the way for advanced functional modifications of cell sheets.

Coated electrodes for liquid thermoelectric conversion devices to enhance Fe2+/Fe3+ redox kinetics

A graphite-dispersion coated electrode enhances the output power of a LTE by 37.5% as compared with that of a LTE composed of a conventional graphite sheet.

Shape and size tunability of sheets of interlocked ring copolymers.

Mechanically bonded membranes of interlocked ring polymers are a significant generalization of conventional elastic sheets, where connectivity is provided by covalent bonding, and represent a promising class of topological meta-materials. In this context, two open questions regard the large-scale reverberations of the heterogeneous composition of the rings and the inequivalent modes of interlocking neighboring rings. We address these questions with Langevin dynamics simulations of chainmails with honeycomb-lattice connectivity, where the rings are block copolymers with two segments of different rigidity. We considered various combinations of the relative lengths of the two segments and the patterns of the over- and under-passes linking neighboring rings. We find that varying ring composition and linking patterns have independent and complementary effects. While the former sets the overall size of the chainmail, the latter defines the shape, enabling the selection of starkly different conformation types. Notably, one of the considered linking patterns favors saddle-shaped membranes, providing a first example of spontaneous negative Gaussian curvature in mechanically bonded sheets. The results help establish the extent to which mechanically bonded membranes can differ from conventional elastic ones, particularly for the achievable shape and size tunability.