Degradation Of Magnesium Alloy Research Articles

Degradation Behavior of Medical MgZZC-1 in Various Simulated Body Fluids.

Magnesium-based biodegradable metal bone implants exhibit superior mechanical properties compared to biodegradable polymers for orthopedic and cardiovascular stents. In this study, MgZZC-x (x = 1, 1.2) alloys were screened by in vitro biocompatibility tests in three simulated body fluids under nontoxic conditions. The MgZZC-1 alloys with better biocompatibility were selected to predict the days required for complete degradation. The evolution of degradation products was analyzed, and the mechanism of formation of the product film was inferred. A degradation kinetic model was established to investigate the effect of MEM components on the degradation of the alloys. The results demonstrate that the proteins in MEM can greatly retard the degradation progress by attaching to the surface of MgZZC-1 alloys, which are predicted to degrade completely within 341 days. The carbonate and phosphate buffers were adjusted to pH in MEM solution, delaying the degradation of magnesium alloys. This process in MEM more accurately reflects the actual degradation in the body and is superior to that in Hanks and SBF solutions. This study will promote the application of biodegradable materials in clinical medicine.

Fixation technique of biodegradable magnesium alloy suture anchor in the rotator cuff repair of the shoulder in a goat model: a technical note

BackgroundShoulder disorders, particularly rotator cuff tears, are prevalent musculoskeletal conditions related to aging. Although the widely used suture anchor technique provides strong mechanical support to the tendon, it is associated with a risk of postoperative tendon retearing. The conventionally used titanium alloys can affect the interpretation of magnetic resonance imaging. Degradable magnesium alloys possess excellent biocompatibility, similar mechanical property to the bone, and stimulating bone formation ability from Mg2+. The purpose of this experiment was to develop innovative magnesium-based suture anchors to enhance rotator cuff repair by improving fixation materials, and to evaluate their feasibility in a goat model.MethodsWe developed fluoridized ZK60 suture anchors as the implantation material for two goats, who underwent rotator cuff repair surgery on both shoulders. Computed tomography (CT) and histological analysis were performed at 12 weeks postoperatively, and the results were compared between the magnesium and titanium alloy groups. Additionally, a hematological examination was conducted, which included assessments of red blood cells, white blood cells, platelets, coagulation function, liver function, kidney function, and magnesium ion concentration.ResultsThe 12-week postoperative CT images showed intact MgF2 ZK60 suture anchors, effectively reconnecting the infraspinatus tendon to the humeral head. The anchors became less visible on CT scans, indicating absorption by surrounding tissues. New bone formation in the MgF2 group surpassed that in the Ti group, demonstrating superior osseointegration. The similarity between cortical bone and magnesium reduced stress-shielding and promoted bone regeneration. Histological analysis revealed successful tendon healing with MgF2 anchors, while the Ti group showed discontinuous interfaces and reduced collagen secretion. Hematological examination showed stable liver, renal function, and magnesium ion levels.ConclusionsThe findings indicate that MgF2-coated suture anchors are feasible for rotator cuff repair and potentially other orthopedic applications. We hope that magnesium alloy anchors can become the solution for rotator cuff tendon repair surgery.

Aneurysm healing following treatment with biodegradable embolization materials: assessment in a rat sidewall aneurysm model

BackgroundBiodegradable materials that dissolve after aneurysm healing are promising techniques in the field of neurointerventional surgery. We investigated the effects of various bioabsorable materials in combination with degradable magnesium alloy...

Surface Analysis of Magnesium AZ31 Samples Immersed in Various Aqueous Solutions

Rapid degradation in body fluids is known to be the main shortcoming of the AZ31 magnesium alloy that is aimed to be controlled in this study by chemical conversion of its surface in various phosphate and chloride solutions. Deposited layers on the surface of bare alloy plates were subjected to compositional and morphological analyses to assess their performance as barriers to degradation. Also changes in the mass of the samples and pH of the solutions were monitored during 21 day immersion periods. Formations of prismatic, platelike, needlelike crystals of various compositions including calcium phosphates, magnesium phosphates, magnesium chlorides were observed by scanning electron microscopy and their atomic compositions were determined by EDX and quantitative XRD analyses. The results indicate that a layer of ceramic of various thicknesses can stably form on the base alloy by simple adsorption of the particles suspended in the solution or by nucleation and growth of the products of reactions between dissolved ions and the metal ions released from the surface. These deposition layers that are solely induced by the electrochemical potential of the species in the solution offer facile surface modification methods and novel phases to control the degradation of magnesium alloys in aggressive environments such as body fluids or marine environments.

Effects of friction stir processing and nano-hydroxyapatite on the microstructure, hardness, degradation rate and in-vitro bioactivity of WE43 alloy for biomedical applications

Nowadays, magnesium alloys are emerging in biomedical implants for their similar properties to natural bones. However, the rapid degradation of magnesium alloys in biological media hinders successful implantation. Refinement of microstructure, as well as reinforcement particles can significantly improve the degradation rate. In this work, multi-pass friction stir processing (FSP) was proposed to synthesize WE43/nano-hydroxyapatite (nHA) surface composite, the microstructure, reinforced particle distribution, micro-hardness, corrosion behavior and in-vitro bioactivity were studied. The subsequent FSP passes of WE43 alloy and WE43/nHA composite refined the grain size which was reduced by 94.29% and 95.92% (2.63 and 1.88 µm, respectively) compared to base metal after three passes. This resulted in increasing the microhardness by 120% (90.86 HV0.1) and 135% (105.59 HV0.1) for the WE43 and WE43-nHA, respectively. It is found that increasing FSP passes improved the uniform distribution of nHA particles within the composite matrix which led to improved corrosion resistance and less degradation rate. The corrosion rate of the FSPed WE43/nHA composite after three passes was reduced by 38.2% (4.13 mm/year) and the degradation rate was reduced by 69.7% (2.87 mm/y). This is attributed to secondary phase (Mg24Y5 and Mg41Nd5) particle fragmentation and redistribution, as well as a homogeneous distribution of nHA. Additionally, the growing Ca-P and Mg(OH)2 layer formed on the surface represented a protective layer that reduced the degradation rate. The wettability test revealed a relatively hydrophilic surface with water contact angle of 49.1 ± 2.2° compared to 71.2 ± 2.1° for base metal. Also, biomineralization test showed that apatite layer grew after immersion 7d in simulated body fluid with atomic ratio of Ca/P 1.60 approaching the stoichiometric ratio (1.67) indicating superior bioactivity of FSPed WE43/nHA composite after three passes. These results raise that the grain refinement by FSP and introduction of nHA particles significantly improved the degradation rate and in-vitro bioactivity of WE43 alloy for biomedical applications.

Additive manufacturing of degradable magnesium alloys and their application in orthopedic implants

ABSTRACT Recently, with the continuous research on magnesium alloy materials, the production and processing technology of magnesium alloys are continuously improved, bringing in their improved mechanical properties and corrosion resistance. Therefore, magnesium alloys have once again become one of the research hotspots of orthopedic degradable materials. Meanwhile, the biocompatibility and biosafety properties of magnesium alloys were considered. The magnesium ions generated after the magnesium alloy is degraded in the body, will participate in the human body's metabolism, regulate bone metabolism activities, and contribute to the synthesis of new bone. Some research studied that magnesium ions can regulate the metabolic function of the human body through multiple signaling pathways, and in vitro cells, and experiments have proved this point of view. Therefore, magnesium alloys are expected to become a promising material in orthopedic treatment, due to their good biocompatibility, degradability, and mechanical properties. In this review, we provide an overview of the degradable magnesium alloys and some key surface treatments and additive manufacturing of these materials to promote their application in orthopedic implants.

Degradation behavior of ZE21C magnesium alloy suture anchors and their effect on ligament-bone junction repair.

Current materials comprising suture anchors used to reconstruct ligament-bone junctions still have limitation in biocompatibility, degradability or mechanical properties. Magnesium alloys are potential bone implant materials, and Mg2+ has been shown to promote ligament-bone healing. Here, we used Mg-2 wt.% Zn-0.5wt.% Y-1wt.% Nd-0.5wt.% Zr (ZE21C) alloy and Ti6Al4V (TC4) alloy to prepare suture anchors to reconstruct the patellar ligament-tibia in SD rats. We studied the degradation behavior of the ZE21C suture anchor via in vitro and in vivo experiments and assessed its reparative effect on the ligament-bone junction. In vitro, the ZE21C suture anchor degraded gradually, and calcium and phosphorus products accumulated on its surface during degradation. In vivo, the ZE21C suture anchor could maintain its mechanical integrity within 12 weeks of implantation in rats. The tail of the ZE21C suture anchor in high stress concentration degraded rapidly during the early implantation stage (0-4weeks), while bone healing accelerated the degradation of the anchor head in the late implantation stage (4-12weeks). Radiological, histological, and biomechanical assays indicated that the ZE21C suture anchor promoted bone healing above the suture anchor and fibrocartilaginous interface regeneration in the ligament-bone junction, leading to better biomechanical strength than the TC4 group. Hence, this study provides a basis for further research on the clinical application of degradable magnesium alloy suture anchors.

Preparation and characterization of hydroxyapatite coating by magnetron sputtering on Mg–Zn–Ag alloys for orthopaedic trauma implants

The aim of the present paper was to evaluate the effect of hydroxyapatite coatings on the two types of Mg–Zn–Ag alloys as a possible solution to control magnesium alloy degradation. The coatings were prepared by the radio frequency magnetron sputtering method at a deposition temperature of 300 °C. To perform this evaluation, the coated alloys were immersed in a simulated body fluid solution at body temperature (37 ± 0.5 °C) to determine the corrosion resistance through electrochemical and immersion tests. Moreover, the investigation also consisted of the evaluation of microchemical, mechanical, and morphological properties. The deposition temperature of 300 °C was enough to obtain a crystalline hydroxyapatite structure with a Ca/P ratio close to the stochiometric one. The adhesion of coatings was not influenced by the nature of Mg–Zn–Ag alloys, so similar values for both coated alloys were found. The results showed that the coating was homogonous deposited on the Mg–Zn–Ag alloys and the corrosion resistance of uncoated magnesium alloys was improved.

Copper-loaded chitosan coating for improved in-vitro corrosion resistance and endothelialization of magnesium alloy stents

For degradable magnesium alloy stents, excellent corrosion resistance and rapid endothelialization are two essential properties. In this paper, we plan to use a copper-loaded chitosan coating to accelerate the endothelialization process of the stent, which not only improves the biocompatibility of the magnesium alloy stent, but also its corrosion resistance by improving the service environment in vivo. Copper is an impurity in magnesium alloys, so how copper-containing coatings affect the corrosion resistance of magnesium alloy stents is the focus of this paper. The corrosion properties of the material were characterized by electrochemical experiments, immersion experiments, and in vitro cellular experiments characterized the effect of the copper-loaded coating on endothelial cells. The results showed that the Icorr of the copper-loaded chitosan coated sample and the Icorr of the unloaded copper chitosan coated sample were both in the order of 10−8, and the decrease of the corrosion resistance of the magnesium alloy by the copper-containing chitosan composite coating was not significant, and it still had a protective effect on the magnesium alloy substrate. During the immersion process, magnesium and copper ions were slowly released, followed by the hydrogen released. In addition, the weight loss rate of different coated samples presented the tendency of slower at inital, faster at last, and the weight loss rate of 0.041% for the copper-loaded chitosan coated sample was not significantly different from that of 0.035% for the unloaded copper coating group, after 28 days of immersion. The in vitro degradation products of the copper-loaded composite coating on the surface of magnesium alloy were also analyzed by XRD and EPMA, which included Mg(OH)2, Mg3(PO4)2 and Ca3(PO4)2; in addition the degradation process was analyzed. The cellular experiments results showed that the copper-loaded magnesium alloy had a proliferation rate of 243.0% for endothelial cells, which indicated that the copper-loaded coating could endow magnesium alloy stent a good biofunctional properties.

Investigation of structural properties of forsterite coating on AZ91 magnesium alloy by sol-gel method

This study examined the synthesis and characterization of a nanostructured forsterite coating on a biodegradable AZ91 alloy to enhance its bioactivity. The coating was applied using sol-gel technique, resulting in a homogeneous and crack-free surface as confirmed by scanning electron microscopy. X-ray diffraction was used to identify the phases present in the coating, which confirmed the presence of forsterite particles with an average size of 35 nm. To evaluate the effectiveness of the coating, potentiodynamic polarization electrochemical tests were conducted in simulated body fluid (SBF), which showed that the corrosion current density decreased from 11 μA cm−2 for uncoated samples to 4.3 μA cm−2 for coated samples. Moreover, the surface of the coated magnesium alloy showed fewer cracks and cavities after being immersed in SBF for 672 h. Given that the early degradation of magnesium alloy poses challenges for prostheses and bone stabilizers, this coating could promote bone healing and positive interaction with bone tissue while preventing toxicity from metal ions.

Shortening the manufacturing process of degradable magnesium alloy minitube for vascular stents by introducing cyclic extrusion compression

Due to its excellent biocompatibility and biodegradability, Mg and its alloys are considered to be promising materials for manufacturing of vascular sent. However, the manufacture of high-precision and high-performance Mg alloys minitubes is still a worldwide problem with a long manufacturing processing caused by the poor workability of Mg alloys. To solve this problem, the cyclic extrusion compression (CEC) was used to pretreat the billet by improving the workability of Mg alloys, finally shortening the manufacturing process. After CEC treatment, the size of grains and second phase particles of Mg alloys were dramatically refined to 3.2 µm and 0.3 µm, respectively. Only after three passes of cold drawing, the wall thickness of minitube was reduced from 0.200 mm to 0.135 mm and a length was more than 1000 mm. The error of wall thickness was measured to be less than 0.01 mm, implying a high dimensional accuracy. The yield strength (YS), ultimate tensile strength (UTS) and elongation of finished minitube were 220±10 MPa, 290±10 MPa and 22.0 ± 0.5%, respectively. In addition, annealing can improve mechanical property and corrosion resistance of minitubes by improving the homogeneity of the microstructure and enhancing the density of basal texture.

Effect of PLGA+MAO composite coating on the degradation of magnesium alloy in vivo and in vitro

Effect of PLGA+MAO composite coating on the degradation of magnesium alloy in vivo and in vitro

The biological properties of 3D-printed degradable magnesium alloy WE43 porous scaffolds via the oxidative heat strategy.

As a biodegradable material, magnesium alloy has a modulus similar to that of bone, and given the biological activity of its degradation products, it has the potential to be a bone grafting material. Oxidation heat treatment is a very effective passivation method that may reduce the rate of magnesium alloy degradation. Oxidation heat treatment increases the rare earth oxide content of the scaffold as well as the corrosion resistance of the scaffold. The overall cytotoxicity of the as-printed scaffolds (APSs) and oxidation heat-treated scaffolds (OHSs) showed that OHSs accelerated cell proliferation. In the apoptosis experiment, the OHS group had a cell survival rate between that of the control group and of the as-printed group. In the osteogenic induction experiment, the alkaline phosphatase activity and the quantity of mineralized nodules were greater in the APS and OHS groups than in the control group. Marker proteins for bone growth were expressed at higher levels in the APS and OHS groups than in the control group. Therefore, oxidation heat-treated 3D printing scaffolds with good biocompatibility and osteogenic properties have great potential to be made into advanced biomaterials that can be used to fix bone defects.

Influence of Spin Coating and Dip Coating with Gelatin/Hydroxyapatite for Bioresorbable Mg Alloy Orthopedic Implants: In Vitro and In Vivo Studies.

Suitable biomechanical properties, good biocompatibility, and osteoconductivity of a degradable magnesium (Mg) alloy make it a potential material for orthopedic implants. The main limitation of Mg is its high corrosion rate in the human body. Surface modification is necessary to improve the Mg corrosion resistance. In this work, a polymeric layer of gelatin/nanohydroxyapatite (Gel/nHA) was coated on a ZK60 Mg alloy by dip coating and spin coating to test the corrosion resistance and biocompatibility in vitro and in vivo. The results from the in vitro test revealed that the coated groups reduced the corrosion rate with the corrosion current density by 59 and 81%, from 31.22 to 12.83 μA/cm2 and 5.83 μA/cm2 in the spin coating and dip coating groups, respectively. The dip coating group showed better corrosion resistance than the spin coating group with the lowest released hydrogen content (17.5 mL) and lowest pH value (8.23) and reducing the current density by 45%. In vitro, the relative growth rate was over 75% in all groups tested with MG63, demonstrating that the Mg substrate and coating materials were within the safety range. The dip coating and spin coating groups enhanced the cell proliferation with significantly higher OD values (3.3, 3.0, and 2.5, respectively) and had better antihemolysis and antiplatelet adhesion abilities than the uncoated group. The two coating methods showed no difference in the cellular response, cell migration, hemolysis, and platelet adhesion test. In in vivo tests in rats, the dip coating group also showed a higher corrosion resistance with a lower corrosion rate and mass loss than the spin coating group. In addition, the blood biochemistry and histopathology results indicated that all materials used in this study were biocompatible with living subjects. The present research confirmed that the two methods have no noticeable difference in cell and organ response but the corrosion resistance of dip coating was higher than that of spin coating either in vitro or in vivo.

Assessment of Mg(OH)2/TiO2 coating in the Mg‐Ca‐Zn alloy for improved corrosion resistance and antibacterial performance

The high activity of metallic magnesium and alloys limits its potential in biomedical applications; in recent years, extensive efforts have been devoted to modulating this reactivity. In this work, we present Mg(OH) 2 and TiO 2 barrier coatings to reduce the degradation of magnesium alloy (Mg-Ca-Zn) surfaces. These coatings were deposited by the anodization method and the spin-coating technique, respectively. The anodized layer was coated with TiO 2 generated from the hydrolysis of 3% weight of TTIP (Ti[OCH(CH 3 ) 2 ] 4 , Titanium(IV) isopropoxide) in 2-Propanol deposited by the spin-coating method. Studying the degradation in Ringer's solution by electrochemical impedance spectroscopy and OCP revealed a 98% reduction in pittings in uncoated samples after 14 days of immersion. The pH measurements revealed that the TiO 2 coating reduced the alkalization of the physiological environment, keeping the pH at 6.0 values. In vitro studies of two types of bacteria ( E. coli and S. aure us) exhibited zones of inhibition in the agar and activity bactericidal (kill time test). The mechanisms behind the improved degradation resistance and enhanced antibacterial activity are presented and discussed here. Surface modification with Mg(OH) 2 /TiO 2 coatings is a promising strategy to control the biodegradation of magnesium implants for bone regeneration.

Enhanced abrasive-mixed-µ-EDM performance towards improved surface characteristics of biodegradable Mg AZ31B alloy

The non-degradable metallic implants, such as bone screws, often act as the source of dysfunction and harmful corrosion products in the aqueous environment inside the human body. Many of these implants are fixed either temporarily or permanently into the human body, and therefore, both need to match tight tolerances with a remarkably finished surface to eradicate burrs or striations. In this regard, the new generation of degradable magnesium (Mg) alloy implants with excellent osseointegration and low elasticity (like that of human bone), minimizing stress shielding, have been identified as potential candidates to challenge surgical procedures reintervention. However, the biological response of an implant toward the cells in vivo can be predominantly regulated by modifying the surface chemistry, morphology, and corrosion characteristics. Powder or abrasive-mixed-micro-electric discharge machining (A-M-µ-EDM) is gaining attention for executing precision machining and achieving a simultaneous surface modification on micro-manufactured surfaces, suitable for clinical applications. Therefore, the present research aimed at improving the surface characteristics of Mg AZ31B alloy via an augmented performance of A-M-µ-EDM by adopting copper and brass-micro-electrodes (C-µ-E and B-µ-E) in association with distinct abrasive particle concentrations (APCs: 0, 1.5, 3, 4.5, and 6g/l) of bioactive zinc abrasives. To enhance the A-M-µ-EDM capabilities, the experiments were designed with a one-variable-at-a-time (OVAT) strategy, and the trial runs were conducted using different combinations of µ-electrodes and APCs. The superior performance of A-M-µ-EDM was noticed with the fusion of C-µ-E and 3g/l APC in terms of minimum machining time (MT) and dimensional deviation (DD). The additional outcomes of this work reported favorable improvements in surface morphology, chemistry, topography, wettability, microhardness, and corrosion resistance on the A-M-µ-EDMed sample of interest.

Degradable Pure Magnesium Used as a Barrier Film for Oral Bone Regeneration.

The barrier membrane plays an extremely critical role in guided bone regeneration (GBR), which determines the success or failure of GBR technology. In order to obtain barrier membranes with high mechanical strength and degradability, some researchers have focused on degradable magnesium alloys. However, the degradation rate of pure Mg-based materials in body fluids is rather fast, thus posing an urgent problem to be solved in oral clinics. In this study, a novel micro-arc oxidation (MAO) surface-treated pure Mg membrane was prepared. Electrochemical tests, immersion experiments and in vivo experiments were carried out to investigate its potential use as a barrier membrane. The experimental results showed that the corrosion resistance of a pure Mg membrane treated by MAO is better than that of the uncoated pure Mg. The results of cell experiments showed no obvious cytotoxicity, which suggests the enhanced differentiation of osteoblasts. At the same time, the MAO-Mg membrane showed better biological activity than the pure Ti membrane in the early stage of implantation, exhibiting relatively good bone regeneration ability. Consequently, the MAO membrane has been proven to possess good application prospects for guided bone regeneration.

Outlook of magnesium alloy for rhinoplasty material

Rhinoplasty is one of the famous cosmetic surgeries all over the world, as it not only benefits congenital nasal bone dysplasia patients, but also satisfy a part of people who want to look more beautiful and perfect. The materials commonly used in current rhinoplasty are prone to failure of rhinoplasty because of autologous absorption, loosening and slipping of nasal scaffold due to poor tissue compatibility, and difficulties in secondary repair[1-2]. Biodegradable magnesium alloy material is one of the most ideal biomedical materials with great application value. Mechanically, the biodegradable magnesium alloy material has good mechanical properties, similar to the mechanical properties of human bone; the density is similar to that of human bone. Biologically, degradable magnesium alloy material has the characteristics of excellent biocompatibility, biological activity, degradation products can be easily absorbed or excreted by human tissues; degradable magnesium alloy can induce cell differentiation growth and blood vessel growth[3]. This article analyzes the possibility of using degradable magnesium alloy material as a scaffold material for rhinoplasty and suggests that this porous nasal scaffold can be prepared by 3D printing method. By using this method, a restorative nasal plastic effect can be obtained by guiding the regrowth of autologous nasal bone tissue. From a long-term perspective, the surgical results achieved by using magnesium alloy as a scaffold material for rhinoplasty are what the medical aesthetic community has been seeking. The use of biodegradable magnesium alloy as a rhinoplasty material is a new approach that is feasible and has potential for future economic and social benefits, but this approach needs further research and experiments to be validated in the future.

Functionalized Coatings on Degradable Magnesium Alloys for Orthopedic Implants: A Review

Functionalized Coatings on Degradable Magnesium Alloys for Orthopedic Implants: A Review

Novel Mg-Al-Sn-Ca with enhanced mechanical properties and high corrosion rate via sub-rapid solidification for degradable magnesium alloy

Coarse eutectic phases generally exist in cast alloys and have dramatic effects on mechanical properties and corrosion behaviors. Therefore, it would be convenient and feasible to modify the mechanical properties and corrosion behaviors via tuning the eutectic phases. Herein, sub-rapid solidification (SRS) is employed to modify the microstructure, especially the eutectic phase of Mg-4.5Al-1.5Sn-0.5Ca alloy. Our results reveal that SRS followed by homogenization treatment not only reduces the grain size of α-Mg matrix, but also spheroidizes and refines the large stick-like CaMgSn phase to ~1 µm. The pronounced structure modification leads to a superior strength-ductility synergy of SRS alloy, i.e., an ultimate tensile strength (UTS) of ~236 MPa and an elongation to failure (Ɛ f ) of ~24%, in comparison to the alloy prepared via conventional solidification, with an UTS of ~220 MPa and an Ɛ f of ~21%. Moreover, the homogenized SRS alloy exhibits homogeneous corrosion with increased corrosion rates of ~145 and ~2739 mm y −1 at 25 and 93 °C, respectively. It is mainly owing to the large contact area and short distance between CaMgSn and α-Mg matrix derived from the refined CaMgSn particles in SRS alloy. Our work proposes a novel degradable magnesium alloy and a processing method to prepare degradable magnesium alloy by structure modification, which is different from conventional addition of Cu, Fe, Ni elements. The novel degradable magnesium alloy shows great advantage in avoiding the residue of large amount of insoluble coarse eutectic phases, which would block the horizontal wells. • SRS followed by T4 treatment spheroidizes and refines the large stick-like CaMgSn. • Refined grains and CaMgSn in SRS alloy contribute to strength-ductility synergy. • Refined CaMgSn in SRS alloy yields uniform corrosion with increased corrosion rate.