Facile preparation of collagen fiber–glycerol-carboxymethyl cellulose composite film by immersing method

Using nanocellulose to strengthen and toughen collagen-based film: Effect of carboxymethyl content of nanocellulose and relative humidity

Collagen-based films possess numerous merits due to their biodegradability, edibility, and widespread existence. Although extensive research focuses on the mechanical strengthening of collagen films, the mechanisms underlying conformational changes of collagen during the film-forming stage and the impact of interface alterations on film's mechanical properties remain ill-defined. This work investigated the dynamic drying process of different-sized collagen fibers. It revealed that smaller size of collagen fiber exhibited a shorter gelatin stage and more rapid conformational transition. Subsequently, the effects of substitution degree (SD) of carboxymethylated nanocellulose (CNF) and relative humidity (RH) were analyzed on the mechanical behaviors of collagen-based film. When RH was in the range of 50 %-90 %, increasing RH and SD of CNF gradually weakens the interfacial strength between CNF and matrix, thereby increasing the toughness and decreasing strength of collagen-based film. The highest strength (110.76 ± 6.60 MPa) was achieved in COL/CNF-C2 film. Combined with water content and microstructure results, the transformation from brittle to ductile fracture could be observed in collagen-based films, ascribing the toughening of water molecules and hydration of CN with water. These results can provide a guidance for the actual production of collagen-based film and offer strategies for the adjustable mechanical properties of biopolymer films.

Facile fabrication of transparent and stretchable cellulose ionic gel paper for sustainable use in multifunctional sensors and optoelectronic devices

Although transparent paper derived from cellulose has been successfully demonstrated as an inexpensive, renewable and biodegradable substrate used for flexible electronics, the inherently stiff characteristic and intrinsic poor conductivity of the cellulose paper inevitably hinders its application in stretchable electronic devices. Herein, we report a new avenue for construction of highly stretchable, transparent, and ionic conductive cellulose gel paper via glycerol inducing plasticizing and CaCl2 initiating chelating, a facile casting and drying strategy. The renewable carboxymethyl cellulose is employed for its intrinsically abundant carboxyl groups for crosslinking with Ca2+ via ionic coordination bonds, benefiting the improvement of various performances. The resultant cellulose ionic gel paper (CIGP) displays high stretchability (tensile strain 320 % and strength 978 kPa at fracture), and transparency (over 90 % in 400 nm to 780 nm wavelength). In addition, the CIGP also has high ionic conductivity (82.78 mS/m), and displays highly reliable, sensitive and wide range strain sensing abilities to various stimuli. Significantly, the transparent CIGP with excellent sensing performances has been successfully integrated into multifunctional sensors and optoelectronic device, showing broad applications in flexible electronics.

Study on the barrier properties of carboxymethyl cellulose/montmorillonite mulch films regulated by magnetic field

To enhance the barrier performance of biomass films, carboxymethyl cellulose (CMC) was combined with montmorillonite (MMT) modified by stearyltrimethylammonium bromide (STAB) and loaded with Fe₃O₄ particles as a nano-filler, and a CMC/m-OMMT mulch film was fabricated using magnetic field orientation. The characterization of m-OMMT was conducted through Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and vibrating sample magnetometry (VSM), which confirmed the successful intercalation of STAB into the MMT structure, along with the effective loading of Fe₃O4 particles onto the MMT matrix. A comprehensive investigation into the mechanical properties of CMC/m-OMMT films revealed that, in the dry state, the films exhibited a tensile strength of 29 MPa and an elongation at break of 64 %. A series of barrier performance tests were conducted on the films. The findings demonstrated that the incorporation of MMT and the application of a magnetic field substantially enhanced the water contact angle, increasing it from 86° to 112°. Additionally, water vapor permeability increased by approximately 30 %, soil erosion was reduced by about 22 %, and UV resistance was notably improved by 94 %. Moreover, scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and biodegradation tests on the CMC/m-OMMT/40mT films revealed that the magnetic field effectively oriented the MMT nanosheets within the composite matrix. This study presents a novel approach for enhancing the barrier properties of biomass-based mulch films.

A carboxymethyl cellulose-based pH-responsive chlorine dioxide release film for strawberry preservation

Fruit spoilage caused by microorganisms results in huge economic losses and health risks worldwide every year. To develop an intelligent antimicrobial material capable of responding to the physiological activity of postharvest fruits and releasing antibacterial agents on demand, we fabricated a pH-responsive film for the release of chlorine dioxide (ClO2) using carboxymethyl cellulose (CMC) and sodium chlorite (NaClO2) via the solution casting method, with a CMC:NaClO2 ratio of 1:2 w/w. An acid environment simulated by 4 % acetic acid activated 43 % of ClO2 released by the film within 7 days. A 1 × 2 cm2 film in acid environment effectively inhibited the growth of Staphylococcus aureus (106 CFU/mL), Escherichia coli (105 CFU/mL), and Aspergillus niger on agar media (with colony diameters reduced from 60.00 mm to 19.07 mm). A 3 × 3 cm2 film reduced the decay incidence caused by A. niger in strawberry from 100 % (control) to only 20 % on day 6. Compared with the control group, no significant differences in color, total soluble solids (TSSs) or triable acidity (TA) were observed in the film-treated groups. The development of pH-responsive NaClO2-CMC films offers a practical and effective solution for extending the shelf-life and maintaining the quality of fruit.

Comparison of five preservation methods for fascia allograft

IMPORTANCE

Research on tissue preservation, including cortical bone, skin, nerves, and vessels in glycerol and cortical bone in honey, has shown positive results. On the other hand, relatively few studies have been performed on fascia preservation, and comparisons between different fascia preservation methods remain scarce.

OBJECTIVE

This in vitro study compared the biomechanical properties of five different methods of preserving fascia lata.

METHODS

The control group underwent biomechanical testing immediately after decellularization, while the other five groups were stored in glycerol, honey, deep freezer, lyophilizer, and liquid nitrogen for 30 days. The ultimate load, elongation at failure, and stiffness for each group were determined from a load-elongation curve.

RESULTS

A comparison of the ultimate load showed that the control group had the highest value, followed by the glycerol group. The glycerol group was the only group that did not show a significant difference from the control group, while all the other groups showed a significantly lower ultimate load. A comparison of elongation at failure revealed the glycerol group to have the highest value at failure among all groups and was significantly higher than the deep freeze, honey, and cryopreservation groups.

CONCLUSIONS AND RELEVANCE

Glycerol can be used as an effective method for preserving fascia allografts because the resulting allografts show a similar ultimate load to the control group and the highest mean elongation at failure.

Syzgium cumini seed/poly vinyl alcohol based water resistant biodegradable nano-cellulose composite reinforced with zinc oxide and silver oxide nanoparticles for improved mechanical properties

The current work explored a comparative study of biodegradable jamun seed/polyvinyl alcohol (JS) nanocomposites reinforced with varying concentrations of ZnO and Ag2O nano-fillers. The effect of spherical shaped ZnO and Ag2O nanoparticles (NPs) on the on structure, morphology, swelling and solubility, crystallinity and mechanical properties together with biodegradation performance of the composite films was fully studied. SEM results showed uniform distribution of ZnO and Ag2O nanofillers into the JS matrix and dense or compact nanocomposite films were formed. JS-ZnO and JS-Ag2O nanocomposites with 0.5 wt% ZnO and Ag2O content showed maximum crystallinity i.e. 11.3 and 9.58 %, respectively, as determined by XRD. When compared to the virgin JS film (8.41 MPa), the resultant JS-ZnO-0.5 and JS-Ag2O-0.5 nanocomposites showed significantly enhanced tensile strength (35.7 MPa, 29.2 MPa), elongation at break (15.42 %, 14.62 %) and Young's modulus (141 MPa, 126 MPa), respectively. Also, reduced swelling (120.4 % and 116.1 %) and solubility ratio (17.45 % and 18.42 %) was observed for JS-ZnO-0.5 and JS-Ag2O-0.5 nanocomposites, respectively. Biodegradation results showed that maximum degradation (88 %) was achieved for the JS film within 180 days of soil burial whereas JS-ZnO-0.1 and JS-Ag2O-0.1 nanocomposites showed 78 % and 72 % degradation within 180 days, respectively.

Development of hybrid polyvinylpyrrolidone/carboxymethyl cellulose/collagen incorporated oregano scaffolds via direct ink write printing for potential wound healing applications

Additive manufacturing can develop regenerative scaffolds for wound healing. 3D printing offers meticulous porosity, mechanical integrity, cell adhesion and cost-effectiveness. Herein, we prepared ink composed of carboxymethyl cellulose (CMC), polyvinylpyrrolidone (PVP), collagen, and oregano extract for the fabrication of tissue constructs. The blend was optimized to form a homogeneous ink and rheological characterization demonstrated shear thinning behavior. The scaffolds were printed using Direct Ink Write (DIW) at a flow speed of 4 mm3/s and a layer height of 0.18 mm. The fabricated scaffolds demonstrated an ultimate tensile strength (UTS) and toughness of 730 KPa and 2.72 MJ/m3, respectively. Scanning Electron Microscopy (SEM) revealed an average pore size of 300 ± 30 μm. Fourier transform infrared spectroscopy (FTIR) analysis confirmed that all materials were present. The contact angle of the composite scaffold was 68° ± 1°. Moreover, the scaffolds presented 82 % mass loss (degradation) in phosphate buffer saline (PBS) over 14 days. The composite scaffold exhibited inhibition zones of 9 mm and 12 mm against Staphylococcus aureus and Escherichia coli, respectively. The PVP/CMC/collagen/oregano 3D printed scaffolds exhibited excellent biocompatibility with the mesenchymal stem cells and humman dermal fibroblast cells, confirmed by water-soluble tetrazolium - 8 (WST-8) assay (test conducted for 7 days). The enhanced angiogenic potential of said scaffold was assesed by release of vascular endothelial growth factor followed by further validation through in-vivo CAM assay. Thus, confirming suitability for the potential wound healing application.

A novel biopolymeric composite edible film based on sunnhemp protein isolate and potato starch incorporated with clove oil: Fabrication, characterization, and amino acid composition

Composite edible films were developed by casting method using sunnhemp protein isolate (SHPI) and potato starch (PS) at various proportions (100:0, 90:10, 80:20; 70:30, 60:40, and 50:50) containing glycerol as a plasticizer and clove oil. All the edible films were evaluated for thickness, moisture content, solubility, swelling ratio, water activity. Further characterization of edible films was done on the basis of mechanical, optical, thermal and structural attributes along with morphology. Among all the films, composite film containing 50 % SHPI, 50 % PS and 1 % clove oil were having better characteristics. The solubility and WVP decreased, while the tensile strength and elongation at break of composite film increased with the inclusion of potato starch and clove oil. Intermolecular interactions in the composite film matrix were confirmed by FTIR and XRD analysis. SEM images confirmed the structural compactness and integrity of all the developed films. The amino acid composition of edible films indicated presence of most of the essential amino acids. The present finding of this research work shows that the utilization of sunnhemp protein in the development of biocomposite edible films represents an alternative opportunity of sustainable edible food packaging.

Collagen and gelatin: Structure, properties, and applications in food industry

Food-producing animals have the highest concentration of collagen in their extracellular matrix. Collagen and gelatin are widely used in food industry due to their specific structural, physicochemical, and biochemical properties, which enable them to improve health and nutritional value as well as to increase the stability, consistency, and elasticity of food products. This paper reviews the structural and functional properties including inherent self-assembly, gel forming, water-retaining, emulsifying, foaming, and thickening properties of collagen and gelatin. Then the colloid structures formed by collagen such as emulsions, films or coatings, and fibers are summarized. Finally, the potential applications of collagen and gelatin in muscle foods, dairy products, confectionary and dessert, and beverage products are also reviewed. The objective of this review is to provide the current market value, progress as well as applications of collagen and its derivatives in food industry.

Changes in Structures and Properties of Collagen Fibers during Collagen Casing Film Manufacturing

Collagen casing is an edible film, which is widely used in the industrial production of sausages. However, the detailed changes in the collagen fibers, from the raw material to the final collagen film, have rarely been reported. In this research, the changes in the collagen fibers during the manufacturing process, including the fiber arrangement, the triple-helix structure and the thermal stability, were investigated using scanning electron microscopy (SEM), thermogravimetric analysis (TGA), X-ray diffraction (XRD), differential scanning calorimetry (DSC) and Fourier-transform infrared (FTIR) spectroscopy. The relationship between the structure stability and the arrangement of the collagen fibers was also discussed. According to the SEM, XRD, TGA, DSC and FTIR results, the collagen fibers were depolymerized during the acid swelling and became uniformly aligned after the homogenization process. Degassing had no obvious effect on the triple-helix structure. Alkaline neutralization with ammonia destroyed the triple-helix structure, which could be partly reversed through the washing and soaking processes. During the final drying step, the depolymerized triple helix of the collagen fibers recombined to form new structures that showed decreased thermal stability. This study expands our knowledge about the behavior of collagen fibers during the industrial process of producing collagen biobased casings.

Collagen processing with mesoscale aggregates as templates and building blocks

Collagen presents a well-organized hierarchical multilevel structure. Microfibers, fibers, and fiber bundles are the aggregates of natural collagen; which achieve an ideal balance of mechanical strength and toughness at the mesoscopic scale for biological tissue. These mesostructured aggregates of collagen isolated from biological tissues retain these inherent organizational features to enable their use as building blocks for constructing new collagen materials with ideal mechanical performance, thermal and dimensional stability. This strategy is distinct from the more common bottom-up or molecular-level design and assembly approach to generating collagen materials. The present review introduces the hierarchical structure of biological collagen with a focus on mesostructural features. Isolation strategies for these collagen aggregates (CAs) are summarized. Recent progress in the use of these mesostructural components for the construction of new collagen materials with emerging applications is reviewed, including in catalysis, environmental applications, biomedicine, food packaging, electrical energy storage, and flexible sensors. Finally, challenges and prospects are assessed for controllable production of CAs as well as material designs.

Using celluloses in different geometries to reinforce collagen-based composites: Effect of cellulose concentration

Cellulose is frequently used to strengthen biocomposite films, but few literature systematically deliberates the effects of concentration of celluloses in different geometries on the reinforcement of these composites. Here we prepared three types of celluloses, including rod-like cellulose nanocrystalline (CNC), long-chain cellulose nanofiber (CNF) and microscopic cellulosic fines (CF). The effect of concentration of the three celluloses was examined on the barrier properties to water and light, thermostability, microstructure, and mechanical properties of collagen (COL) films. The addition of celluloses increased the watertightness and thermostability of composite films. Besides, FTIR showed a increased hydrogen bonding for COL/CNF and COL/CNC composite films, but decrease for COL/CF composites. As the concentration of CF and CNF increased, the strength of composites improved. The TS for COL/CNF (124 MPa) and COL/CF composites (113 MPa) were largely increased, compared with that of collagen ones (90 MPa). Considering the factors of crystallinity, hydrogen bonding, and interfacial tortuosity, COL/CNF composites possessed better mechanical behaviors than that of COL/CF and COL/CNC composites. Furthermore, Halpin-Kardos and Ouali models well predicted the modulus of COL/CNF composites when CNF was below and above percolation threshold (2.7 wt%), respectively.

A rethinking of collagen as tough biomaterials in meat packaging: assembly from native to synthetic

Due to the high moisture-associated typical rheology and the changeable and harsh processing conditions in the production process, packaging materials for meat products have higher requirements including a sufficient mechanical strength and proper ductility. Collagen, a highly conserved structural protein consisting of a triple helix of Gly-X-Y repeats, has been proved to be suitable packaging material for meat products. The treated animal digestive tract (i.e. the casing) is the perfect natural packaging material for wrapping meat into sausage. Its thin walls, strong toughness and impact resistance make it the oldest and best edible meat packaging. Collagen casing is another wisdom of meat packaging, which is made by collagen fibers from hide skin, presenting a rapid growth in casing market. To strengthen mechanical strength and barrier behaviors of collagen-based packaging materials, different physical, chemical, and biological cross-linking methods are springing up exuberantly, as well as a variety of reinforcement approaches including nanotechnology. In addition, the rapid development of biomimetic technology also provides a good research idea and means for the promotion of collagen's assembly and relevant mechanical properties. This review can offer some reference on fundamental theory and practical application of collagenous materials in meat products.

Tunable mechanical behavior of collagen-based films: A comparison of celluloses in different geometries

Collagen (Col) films were reinforced by celluloses in different geometries: microcrystalline cellulose (MCC), cellulosic fines (CF), cellulose nanofiber (CNF) and cellulose nanocrystals (CNC). The reinforcement mechanisms were investigated by the elastoplasticity and fracture appearance. Compared with the fracture stress of collagen film (67.5 MPa), the Col-CNF films effectively borne the stress (95.8 MPa) by intercrystalline fracture, ascribing the abundant hydrogen bonding and mechanical locking between cellulose and collagen. The toughness of Col-CF films was increased by the interfibrillar slippage of CF and pull-off of CF within the matrix, improving the strain-to-break from 8.37% to 12.13%. The films added with MCC and CNC weaken the mechanical behavior, due to the defects and lack of mechanical locking. Besides, the effects of celluloses' geometries on the thickness, density, water-tightness, thermal stability, crystallinity and FTIR of films were also investigated. These provide the evidence that the geometries of fillers diversely improve the behaviors of collagen film offering strategies for the film with adjustable mechanical properties.

Characterization and Evaluation of Commercial Carboxymethyl Cellulose Potential as an Active Ingredient for Cosmetics

Carboxymethyl cellulose is the most used water-soluble cellulose with applications in industries such as food, cosmetics, and tissue engineering. However, due to a perceived lack of biological activity, carboxymethyl cellulose is mostly used as a structural element. As such, this work sought to investigate whether CMC possesses relevant biological properties that could grant it added value as a cosmeceutical ingredient in future skincare formulations. To that end, CMC samples (Mw between 471 and 322 kDa) skin cell cytotoxicity, impact upon pro-collagen I α I production, and inflammatory response were evaluated. Results showed that samples were not cytotoxic towards HaCat and HDFa up to 10 mg/mL while simultaneously promoting intracellular production of pro-collagen I α I up by 228% relative to the basal metabolism, which appeared to be related to the highest DS and Mw. Additionally, CMC samples modulated HaCat immune response as they decreased by ca. 1.4-fold IL-8 production and increased IL-6 levels by ca. five fold. Despite this increase, only two samples presented IL-6 levels similar to those of the inflammation control. Considering these results, CMC showed potential to be a more natural alternative to traditional bioactive cosmetic ingredients and, as it is capable of being a bioactive and structural ingredient, it may play a key role in future skincare formulations.

Flexible Wearable Pressure Sensor Based on Collagen Fiber Material

Flexible wearable pressure sensors play a pivotal role in healthcare monitoring, disease prevention, and humanmachine interactions. However, their narrow sensing ranges, low detection sensitivities, slow responses, and complex preparation processes restrict their application in smart wearable devices. Herein, a capacitive pressure sensor with high sensitivity and flexibility that uses an ionic collagen fiber material as the dielectric layer is proposed. The sensor exhibits a high sensitivity (5.24 kPa−1), fast response time (40 ms), long-term stability, and excellent repeatability over 3000 cycles. Because the sensor is resizable, flexible, and has a simple preparation process, it can be flexibly attached to clothes and the human body for wearable monitoring. Furthermore, the practicality of the sensor is proven by attaching it to different measurement positions on the human body to monitor the activity signal.

Collagen Fibril-Assembled Skin-Simulated Membrane for Continuous Molecular Separation

A skin-simulated thin-film-composite membrane was fabricated using a vacuum-assisted interfacial polymerization method. A negatively charged surface-selective layer on a polyacrylonitrile (PAN) substrate was cross-linked using trimesoyl chloride to form polyamide and polyester with a three-layer structure that was similar to skin. The loading of collagen fibrils assembled on the membrane surface was varied, and a selective layer was obtained, of which the thickness, morphology, and hydrophilicity can be manipulated. The optimal membrane decorated with 0.5 mg of collagen fibril had a selective layer thickness of around 130 nm with pure water permeability up to 84.7 LMH bar^-1. Furthermore, the membrane exhibited impressive rejections toward dyes (Congo red with a molecular weight of 696.68 Da: 99.6%, reactive blue 19 with a molecular weight of 626.54 Da: 99.8%, and Coomassie blueG-250 with a molecular weight of 854.02 Da: 98.6%) while high permeations of Na₂SO₄ and NaCl were achieved. This facile strategy provides a useful guideline for constructing bionic membranes through biomaterials.

Heterologous expression, fermentation strategies and molecular modification of collagen for versatile applications

Collagen is a kind of high macromolecular protein with unique tissue distribution and distinctive functions in the body. At present, most collagen products are extracted from the tissues and organs of mammals or marine fish. However, this method exhibits several disadvantages, including low efficiency and serious waste generation, which makes it difficult to meet the current market demand. With the rapid development of synthetic biology and the deepening of high-density fermentation technology, the collagen preparation by biosynthesis strategy emerges as the times require. Co-expression with the proline hydroxylase gene can solve the problem of non-hydroxylated collagen, but the yield may be affected. Therefore, improving the expression through molecular modification and dynamic regulation of synthesis is an entry point for future research. Due to the defects in certain properties of the natural collagen, modification of properties would be benefit for meeting the requirements of practical application. In this paper, in-depth investigations on recombinant expression, fermentation, and modification studies of collagen are conducted. Also, it summarizes the research progress of collagen in food, medicine, and beauty industry in recent years. Furthermore, the future development trend and application prospect of collagen are discussed, which would provide guidance for its preparation and application.

Scale-up one-pot synthesis of waste collagen and apple pomace pectin incorporated pentapolymer biocomposites: Roles of waste collagen for elevations of properties and unary/ ternary removals of Ti(IV), As(V), and V(V)

Herein, hazardous solid particulate waste collagenic fibers (SWCFs) of leather industries were incorporated into apple pomace pectin (APPN)-grafted-pentapolymer, i.e., APPN-g-[sodium 2-methylidenebutanedioate(SMBD)-co-N-((3-(isopropylamino)-3-oxopropoxy) methyl) butyramide (CM1)-co-N-(hydroxymethyl)prop-2-enamide (NHMPE)-co-N-(hydroxymethyl)-4-(N-isopropylbutyramido)butanamide (CM2)-co-N-(propan-2-yl)prop-2-enamide NPYPE)/ PENP1], i.e., APPN-g-PENP1/ PENP2, prepared via one-pot facile polymerization of APPN and synthetic monomers, i.e., SMBD, NHMPE, and NPYPE, in aqueous medium, to fabricate an optimum multifunctional hybrid biocomposite adsorbent/ HCOM3. In PENP1, PENP2, and HCOM3, fourth/ CM1 and fifth/ CM2 multifunctional comonomers were anchored in situ. The structures of PENP1, PENP2, HCOM3, CM1, CM2, and metal-ion adsorbed HCOM3; APPN-grafting; SWCF incorporation; and surface properties were analyzed through NMR, XPS, FTIR, XRD, and SEM. The elevated adsorption efficiencies (AEs), reusability, thermostability, swelling, network durability, and crosslink density of HCOM3 were attributed to variable functionalities of SWCF/ APPN, explored by DLS and TGA, swelling, network, and thermodynamic parameters. Compared to SWCF, APPN, PENP1, and PENP2, the elevated AEs and reusability compelled HCOM3 as more suitable for scalable waste management. The maximum AEs, i.e., 171.79, 180.47, and 177.27 mg g^-1, for Ti(IV), As(V), and V(V) at pH_op = 7.0, 3.0, and 5.0, respectively, within 5-100 mg L^-1 and at 298 K for 25 mg HCOM3 deteriorated during ternary adsorption by the antagonistic effects.

Development of 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methyl-morpholinium chloride cross-linked carboxymethyl cellulose films

First example of the use of 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methyl-morpholinium chloride (DMTMM) as cross-linking agent for the development of carboxymethyl cellulose (CMC) films for food packaging is reported. Influence of different wt % of DMTMM and glycerol on the physical-mechanical properties of CMC films was investigated. The presence of DMTMM effectively improved moisture uptake, moisture content, water vapour permeability, water solubility of the films, oil resistance together with good biodegradability. Best compromise between high water resistance, vapour permeability and mechanical properties was accomplished with 5 wt % DMTMM and 50 wt % glycerol giving tensile strength and elongation at break of 52.25 ± 4.33 and 37.32 ± 2.04 respectively. DSC, TGA and SEM analysis further confirmed CMC cross-linking by DMTMM. All films prepared showed low opacity and high transparencies. Therefore, data reported show that DMTMM can efficiently cross-link CMC to produce films for food packaging.