Mechanical and thermal properties of novel polymerized NDGA-gelatin hydrogels

Insights on Chemical Crosslinking Strategies for Proteins

Crosslinking of proteins has gained immense significance in the fabrication of biomaterials for various health care applications. Various novel chemical-based strategies are being continuously developed for intra-/inter-molecular crosslinking of proteins to create a network/matrix with desired mechanical/functional properties without imparting toxicity to the host system. Many materials that are used in biomedical and food packaging industries are prepared by chemical means of crosslinking the proteins, besides the physical or enzymatic means of crosslinking. Such chemical methods utilize the chemical compounds or crosslinkers available from natural sources or synthetically generated with the ability to form covalent/non-covalent bonds with proteins. Such linkages are possible with chemicals like carbodiimides/epoxides, while photo-induced novel chemical crosslinkers are also available. In this review, we have discussed different protein crosslinking strategies under chemical methods, along with the corresponding crosslinking reactions/conditions, material properties and significant applications.

Nordihydroguaiaretic Acid in Therapeutics: Beneficial to Toxicity Profiles and the Search for its Analogs

Nordihydroguaiaretic acid (NDGA) is a plant lignan obtained from creosote bush, Larrea tridentata and is known to possess antioxidant, anticancer activities and is used in traditional medicine in North America and Mexico. However, its prolonged consumption leads to liver damage and kidney dysfunction. Despite its toxicity and side effects, there is little awareness to forbid its consumption and its use in the treatment of medical ailments has continued over the years. Several reports discuss its therapeutic efficiency and its medical applications have tremendously been on the rise to date. There has been a recent surge of interest in the chemical synthesis of NDGA derivatives for therapeutic applications. NDGA derivatives have been developed as better alternatives to NDGA. Although several NDGA derivatives have been chemically synthesized as evidenced by recent literature, there is a paucity of information on their therapeutic efficacies. This review is to highlight the medicinal applications of NDGA, its toxicity evaluations and discuss the chemical derivatives of NDGA synthesized and studied so far and suggest to continue research interests in the development of NDGA analogs for therapeutic applications. We suggest that NDGA derivatives should be investigated more in terms of chemical synthesis with preferred conformational structures and exploit their biological potentials with future insights to explore in this direction to design and develop structurally modified NDGA derivatives for potential pharmacological properties.

Modification of human pericardium by chemical crosslinking

Autologous and allogenic human pericardia used as biomaterials for cardiovascular surgery are traditionally crosslinked with glutaraldehyde. In this work, we have evaluated the resistivity to collagenase digestion and the cytotoxicity of human pericardium crosslinked with various concentrations of glutaraldehyde in comparison with pericardium crosslinked by genipin, nordihydroguaiaretic acid, tannic acid, and in comparison with unmodified pericardium. Crosslinking retained the wavy-like morphology of native pericardium visualized by second harmonic generation microscopy. The collagenase digestion products were analyzed using SDS-PAGE, capillary electrophoresis, and a hydroxyproline assay. Glutaraldehyde and genipin crosslinking protected the native pericardium efficiently against digestion with collagenase III. Only low protection was provided by the other crosslinking agents. The cytotoxicity of crosslinked pericardium was evaluated using xCELLigence by monitoring the viability of porcine valve interstitial cells cultured in eluates from crosslinked pericardium. The highest cell index, reflecting both the number and the shape of the monitored cells was observed in eluates from genipin. Crosslinking pericardium grafts with genipin therefore seems to be a promising alternative procedure to the traditional crosslinking with glutaraldehyde, because it provides similarly high protection against degradation with collagenase, without cytotoxic effects.

Thermal Conductivity of Polyacrylamide Hydrogels at the Nanoscale

A polymer network can imbibe copious amounts of water and swell, and the resulting state is known as a hydrogel. In many potential applications of hydrogels, such as stretchable conductors, ionic cables, and neuroprostheses, the thermal conductivities of hydrogels should be understood clearly. In the present work, we build molecular dynamics (MD) models of random cross-linked polyacrylamide hydrogels with different water volume fractions through a reaction method. On the basis of these models, thermal conductivities of hydrogels at the nanoscale are investigated by a none-equilibrium MD method. This work reveals that when the water fraction of hydrogels is under 85%, the thermal conductivity increases with the water fraction, and can be even higher than the thermal conductivities of both pure polymer networks and pure water because of the influence of the interface between polymer networks and water. However, when the water fraction in hydrogels is bigger than 85%, its thermal conductivity will decrease and get close to the water's conductivity. Accordingly, to explain this abnormal phenomenon, a 2-order-3-phase theoretical model is proposed by considering hydrogel as a 3-phase composite. It can be found that the proposed theory can predict results which agree quite well with our simulated results.

Core/shell PLGA microspheres with controllable in vivo release profile via rational core phase design

Highly soluble drugs tend to release from preparations at high speeds, which make them need to be taken at frequent intervals. Additionally, some drugs need to be controlled to release in vivo at certain periods, so as to achieve therapeutic effects. Thus, the objective of this study is to design injectable microparticulate systems with controllable in vivo release profile. Biodegradable PLGA was used as the matrix material to fabricate microspheres using the traditional double emulsification-solvent evaporation method as well as improved techniques, with gel (5% gelatine or 25% F127) or LP powders as the inner phases. Their physicochemical properties were systemically investigated. Microspheres prepared by modified methods had an increase in drug loading (15.50, 16.72, 15.66%, respectively) and encapsulation efficiencies (73.46, 79.42, 74.40%, respectively) when compared with traditional methods (12.01 and 57.06%). The morphology of the particles was characterized by optical microscope (OM) and scanning electron microscopy (SEM), and the amorphous nature of the encapsulated drug was confirmed by differential scanning calorimetry (DSC) and X-ray diffraction (XRD) analysis. To evaluate their release behaviour, the in vitro degradation, in vitro release and in vivo pharmacodynamics were subsequently studied. Traditional microspheres prepared in this study with water as the inner phase had a relatively short release period within 16 d when compared with modified microspheres with 5% gelatine as the inner phase, which resulted in a smooth release profile and appropriate plasma LP concentrations over 21 d. Thus this type of modified microspheres can be better used in drugs requiring sustained release. The other two formulations containing 25% F127 and LP micropowders presented two-stage release profiles, resulting in fluctuant plasma LP concentrations which may be suitable for drugs requiring controlled release. All the results suggested that drug release rates from the microspheres prepared by various methods were mainly controlled by either the porosity inside the microspheres or the degradation of materials, which could, therefore, lead to different release behaviours. This results indicated great potential of the PLGA microsphere formulation as an injectable depot for controllable in vivo release profile via rational core phase design. Core/shell microspheres fabricated by modified double emulsification-solvent evaporation methods, with various inner phases, to obtain high loading drugs system, as well as appropriate release behaviours. Accordingly, control in vivo release profile via rational core phase design.

Thermal Transport in Soft PAAm Hydrogels

As the interface between human and machine becomes blurred, hydrogel incorporated electronics and devices have emerged to be a new class of flexible/stretchable electronic and ionic devices due to their extraordinary properties, such as softness, mechanically robustness, and biocompatibility. However, heat dissipation in these devices could be a critical issue and remains unexplored. Here, we report the experimental measurements and equilibrium molecular dynamics simulations of thermal conduction in polyacrylamide (PAAm) hydrogels. The thermal conductivity of PAAm hydrogels can be modulated by both the effective crosslinking density and water content in hydrogels. The effective crosslinking density dependent thermal conductivity in hydrogels varies from 0.33 to 0.51 Wm-1K-1, giving a 54% enhancement. We attribute the crosslinking effect to the competition between the increased conduction pathways and the enhanced phonon scattering effect. Moreover, water content can act as filler in polymers which leads to nearly 40% enhancement in thermal conductivity in PAAm hydrogels with water content vary from 23 to 88 wt %. Furthermore, we find the thermal conductivity of PAAm hydrogel is insensitive to temperature in the range of 25⁻40 °C. Our study offers fundamental understanding of thermal transport in soft materials and provides design guidance for hydrogel-based devices.

In Vitro Enzymatic Degradation of Tissue Grafts and Collagen Biomaterials by Matrix Metalloproteinases: Improving the Collagenase Assay

Matrix metalloproteinase-1 and -8 are active during the wound healing and remodelling processes, degrading native extracellular matrix and implantable devices. However, traditional in vitro assays utilize primarily matrix metalloproteinase-1 to mimic the in vivo degradation microenvironment. Herein, we assessed the influence of various concentrations of matrix metalloproteinase- 1 and 8 (50, 100, and 200 U/mL) as a function of pH (5.5 and 7.4) and time (3, 6, 9, 12, and 24 h) on the degradation profile of three tissue grafts (chemically cross-linked Permacol, nonchemically cross-linked Permacol and nonchemically cross-linked Strattice) and a collagen biomaterial (nonchemically cross-linked collagen sponge). Chemically cross-linked and nonchemically cross-linked Permacol samples exhibited the highest resistance to enzymatic degradation, while nonchemically cross-linked collagen sponges exhibited the least resistance to enzymatic degradation. Qualitative and quantitative degradation analysis of all samples revealed a similar degradation profile over time, independently of the matrix metalloproteinase used and its respective concentration and pH. These data indicate that matrix metalloproteinase-1 and matrix metalloproteinase-8 exhibit similar degradation profile in vitro, suggesting that matrix metalloproteinase-8 should be used for collagenase assay.

Hyaluronan-Itaconic Acid-Glutaraldehyde Films for Biomedical Applications: Preliminary Studies

New hyaluronic acid–itaconic acid films were synthesized as potential materials with biomedical applications. In this work, we explored the homogeneous cross-linking reactions of hyaluronic acid using glutaraldehyde in the presence of itaconic acid and triacetin as plasticizers.

Biomechanical properties were assessed in terms of stability by measuring swelling in aqueous environments, investigating wettability using contact angle tests, and evaluating bioadhesive performance. The ductility of the materials was evaluated through stress-strain measurements and the morphology was explored by scanning electron microscopy.

The results show that the incorporation of itaconic acid improved most of the desirable properties, increasing adhesiveness and reducing wettability and swelling. The use of triacetin enhanced the strength, bioadhesiveness, and ductility of the material.

Antitumor and antimetastasis effects of macerating solutions from an injectable chitosan-based hydrogel on hepatocarcinoma

In our previous studies, injectable chitosan-based hydrogel (CH) was prepared and its application in surgery removal of tumor was studied. In this study, the antitumor and antimetastasis effects of the macerating solutions from CH were investigated. Our in vitro results showed that macerating solutions from CH significantly increased the proliferation of human normal liver L02 cells. In contrast, macerating solutions from CH showed significant inhibitory effects on the growth of human hepatoma Bel-7402 cells. In a mouse H22 tumor model, intraperitoneal injection of macerating solutions from CH decreased tumor growth and prevented tumor diffusion. Tumor weight was decreased dramatically in mice treated with macerating solutions from CH. The thymus index and spleen index were significantly increased by treatment with macerating solutions from CH. Administration of macerating solutions from CH also remarkably increased serum levels of TNF-α, IL-2, IFN-γ, and decreased serum VEGF content as compared with the control group treated with saline. The antimetastasis studies showed that the number of pulmonary nodules, pulmonary metastases index, and lymph nodes index were significantly decreased in experimental groups treated with macerating solutions from CH. This study provided more supporting data for the potential clinical application of CH after surgical removal of tumor.

Crosslinking strategies for preparation of extracellular matrix-derived cardiovascular scaffolds

Heart valve and blood vessel replacement using artificial prostheses is an effective strategy for the treatment of cardiovascular disease at terminal stage. Natural extracellular matrix (ECM)-derived materials (decellularized allogeneic or xenogenic tissues) have received extensive attention as the cardiovascular scaffold. However, the bioprosthetic grafts usually far less durable and undergo calcification and progressive structural deterioration. Glutaraldehyde (GA) is a commonly used crosslinking agent for improving biocompatibility and durability of the natural scaffold materials. However, the nature ECM and GA-crosslinked materials may result in calcification and eventually lead to the transplant failure. Therefore, studies have been conducted to explore new crosslinking agents. In this review, we mainly focused on research progress of ECM-derived cardiovascular scaffolds and their crosslinking strategies.

Tailoring the properties of gelatin films for drug delivery applications: influence of the chemical cross-linking method

Two types of chemically cross-linked gelatin films were prepared and characterized. The first type of films was cross-linked with 1-ethyl-3-(3-dimethyl aminopropyl)carbodiimide hydrochloride (EDC) under heterogeneous conditions and are named Gel-E. In the second type of films, gelatin was previously functionalized with methacrylamide side groups by the reaction with methacrylic anhydride and for that is named Gel-MA. The modified gelatin was subsequently cross-linked by a photoinitiated radical polymerization. These films were characterized relatively to their degree of cross-linking, buffer uptake capacity, resistance to hydrolytic and proteolytic degradation, and mechanical and thermal properties. Results show that the employed cross-linking method, together with the degree cross-linking, dictate the final properties of the films. Gel-E films have significant lower buffer uptake capacities and higher resistance to collagenase digestion when compared to Gel-MA films. Additionally, Gel-E films exhibit higher values of stress at break and lower strains at break. Moreover, the films properties could be modified by varying the extent of the chemical cross-linking, which in turn could be controlled by varying the concentration of EDC, for the first type of films (Gel-E), or by using gelatins with different degrees of functionalization, in the case of the second type of films (Gel-MA).

Immobilized lentivirus vector on chondroitin sulfate-hyaluronate acid-silk fibroin hybrid scaffold for tissue-engineered ligament-bone junction

The lack of a fibrocartilage layer between graft and bone remains the leading cause of graft failure after anterior cruciate ligament (ACL) reconstruction. The objective of this study was to develop a gene-modified silk cable-reinforced chondroitin sulfate-hyaluronate acid-silk fibroin (CHS) hybrid scaffold for reconstructing the fibrocartilage layer. The scaffold was fabricated by lyophilizing the CHS mixture with braided silk cables. The scanning electronic microscopy (SEM) showed that microporous CHS sponges were formed around silk cables. Each end of scaffold was modified with lentiviral-mediated transforming growth factor-β3 (TGF-β3) gene. The cells on scaffold were transfected by bonded lentivirus. In vitro culture demonstrated that mesenchymal stem cells (MSCs) on scaffolds proliferated vigorously and produced abundant collagen. The transcription levels of cartilage-specific genes also increased with culture time. After 2 weeks, the MSCs were distributed uniformly throughout scaffold. Deposited collagen was also found to increase. The chondral differentiation of MSCs was verified by expressions of collagen II and TGF-β3 genes in mRNA and protein level. Histology also confirmed the production of cartilage extracellular matrix (ECM) components. The results demonstrated that gene-modified silk cable-reinforced CHS scaffold was capable of supporting cell proliferation and differentiation to reconstruct the cartilage layer of interface.

Keeping an eye on decellularized corneas: a review of methods, characterization and applications

The worldwide limited availability of suitable corneal donor tissue has led to the development of alternatives, including keratoprostheses (Kpros) and tissue engineered (TE) constructs. Despite advances in bioscaffold design, there is yet to be a corneal equivalent that effectively mimics both the native tissue ultrastructure and biomechanical properties. Human decellularized corneas (DCs) could offer a safe, sustainable source of corneal tissue, increasing the donor pool and potentially reducing the risk of immune rejection after corneal graft surgery. Appropriate, human-specific, decellularization techniques and high-resolution, non-destructive analysis systems are required to ensure reproducible outputs can be achieved. If robust treatment and characterization processes can be developed, DCs could offer a supplement to the donor corneal pool, alongside superior cell culture systems for pharmacology, toxicology and drug discovery studies.

Potential of collagen cross-linking therapies to mediate tendon mechanical properties

Collagen cross-links are fundamental to the mechanical integrity of tendon, with orderly and progressive enzymatic cross-linking being central to healthy development and injury repair. However, the nonenzymatic cross-links that form as we age are associated with increased tendon brittleness, diminished mechanical resistance to injury, and impaired matrix remodeling. Collagen cross-linking thus sits at the center of tendon structure and function, with important implications to age, disease, injury, and therapy. The current review touches on these aspects from the perspective of their potential relevance to the shoulder surgeon. We first introduce the most well-characterized endogenous collagen cross-linkers that enable fibrillogenesis in development and healing. We also discuss the glycation-mediated cross-links that are implicated in age- and diabetes-related tendon frailty and summarize work toward therapies against these disadvantageous cross-links. Conversely, we discuss the introduction of exogenous collagen cross-links to augment the mechanical properties of collagen-based implants or native tendon tissue. We conclude with a summary of our early results using exogenous collagen cross-linkers to prevent tendon tear enlargement and eventual failure in an in vitro model of partial tendon tear.

Osteoid-mimicking dense collagen/chitosan hybrid gels

Bone extracellular matrix (ECM) is a 3D network, composed of collagen type I and a number of other macromolecules, including glycosaminoglycans (GAGs), which stimulate signaling pathways that regulate osteoblast growth and differentiation. To model the ECM of bone for tissue regenerative approaches, dense collagen/chitosan (Coll/CTS) hybrid hydrogels were developed using different proportions of CTS to mimic GAG components of the ECM. MC3T3-E1 mouse calvaria preosteoblasts were seeded within plastically compressed Coll/CTS hydrogels with solid content approaching that of native bone osteoid. Dense, cellular Coll/CTS hybrids were maintained for up to 8 weeks under either basal or osteogenic conditions. Higher CTS content significantly increased gel resistance to collagenase degradation. The incorporation of CTS to collagen gels decreased the apparent tensile modulus from 1.82 to 0.33 MPa. In contrast, the compressive modulus of Coll/CTS hybrids increased in direct proportion to CTS content exhibiting an increase from 23.50 to 55.25 kPa. CTS incorporation also led to an increase in scaffold resistance to cell-induced contraction. MC3T3-E1 viability, proliferation, and matrix remodeling capability (via matrix metalloproteinase expression) were maintained. Alkaline phosphatase activity was increased up to two-fold, and quantification of phosphate mineral deposition was significantly increased with CTS incorporation. Thus, dense Coll/CTS scaffolds provide osteoid-like models for the study of osteoblast differentiation and bone tissue engineering.

A highly elastic tissue sealant based on photopolymerised gelatin

Gelatin is widely used as a medical biomaterial because it is readily available, cheap, biodegradable and demonstrates favourable biocompatibility. Many applications require stabilisation of the biomaterial by chemical crosslinking, and this often involves derivatisation of the protein or treatment with cytotoxic crosslinking agents. We have previously shown that a facile photochemical method, using blue light, a ruthenium catalyst and a persulphate oxidant, produces covalent di-tyrosine crosslinks in resilin and fibrinogen to form stable hydrogel biomaterials. Here we show that various gelatins can also be rapidly crosslinked to form highly elastic (extension to break >650%) and adhesive (stress at break >100 kPa) biomaterials. Although the method does not require derivatisation of the protein, we show that when the phenolic (tyrosine-like) content of gelatin is increased, the crosslinked material becomes resistant to swelling, yet retains considerable elasticity and high adhesive strength. The reagents are not cytotoxic at the concentration used in the photopolymerisation reaction. When tested in vivo in sheep lung, the photopolymerised gelatin effectively sealed a wound in lung tissue from blood and air leakage, was not cytotoxic and did not produce an inflammatory response. The elastic properties, thermal stability, speed of curing and high tissue adhesive strength of this photopolymerised gelatin, offer considerable improvement over current surgical tissue sealants.

Crosslinking effect of Nordihydroguaiaretic acid (NDGA) on decellularized heart valve scaffold for tissue engineering

Decellularized heart valve scaffolds possess many desirable properties in valvular tissue engineering. However, their current applications were limited by short durability, easily structural dysfunction and immunological competence. Although crosslinking with chemical reagents, such as glutaraldehyde (GA), will enhance the mechanical properties, the low long-term stability and cytotoxicity of the scaffolds remains potential problem. Nordihydroguaiaretic acid (NDGA) is a bioactive natural product which is able to crosslink collagen and was proven to be effective in preparation of scaffold for tendon tissue engineering. In this paper, NDGA crosslinked decellularized heart valve scaffolds demonstrated higher tensile strength, enzymatic hydrolysis resistance and store stability than the non-crosslinked ones. Its mechanical properties and cytocompability were superior to that of GA-crosslinked heart valve matrix. Below the concentration of 10 microg/ml, NDGA has no visible cytotoxic effect on both endothelial cells (EC) and valvular interstitial cells (VIC) and its cytotoxicity is much less than that of GA. The LC50 (50% lethal concentration) of NDGA on ECs and VICs are 32.6 microg/ml and 47.5 microg/ml, respectively, while those of GA are almost 30 times higher than NDGA (P < 0.05). ECs can attach to and maintain normal morphology on the surface of NDGA-crosslinked valvular scaffolds but not GA-crosslinked ones. This study demonstrated that NDGA-crosslinking of decellularized valvular matrix is a promising approach for preparation of heart valve tissue engineering scaffolds.

Hydrogels Based on Carboxymethylcellulose and Gelatin for Inclusion and Release of Chloramphenicol

Hydrogels based on carboxymethylcellulose (CMC) and gelatin (GEL) crosslinked with glutaraldehyde were used to obtain interpenetrated— interconnecting polymer networks. They are designed to obtain controlled release polymeric drug systems. CMC and GEL were chosen for their biocompatibility and nontoxicity, which are compulsory conditions for polymers used in biomedical applications. By modifying the parameters of the crosslinking reaction, the obtained networks presented different crosslinking degrees and hence different swelling capacities. These properties determined the quantity of drug able to be loaded (0.25 g per gram of hydrogel). We obtained systems for which biologically active matter release was controlled by diffusion. The kinetics were zero-order during the major part of release period (∼500 min). These systems improve the bactericide activity compared with free drugs.

Development of a Silk Cable-Reinforced Gelatin/Silk Fibroin Hybrid Scaffold for Ligament Tissue Engineering

The objective of this study was to develop a silk cable-reinforced gelatin/silk fibroin hybrid scaffold for ligament tissue engineering. The scaffold was fabricated by lyophilizing the cross-linked gelatin and silk fibroin mixture with braided silk cables. Scanning electronic microscopy (SEM) observation showed that microporous gelatin/silk fibroin sponges formed around silk cables mimicked the microstructures of ligament extracellular matrix (ECM). The silk cables significantly increased the tensile strength of the scaffold to meet the mechanical requirements for ligament tissue engineering. The scaffold possessed good cell adhesion property, and when mesenchymal stem cells (MSCs) were seeded on it, cells proliferated profusely. After 2 weeks of culture, seeded MSCs were distributed uniformly throughout the scaffold and were highly viable. Occurrence of cell death during culture was not significant. Deposition of collagen on the scaffold was found to increase with time. Differentiation of MSCs into ligament fibroblasts was verified by expressions of ligament ECM specific genes including collagen type I, collagen type III, and tenascin-C in mRNA and protein level. Immunohistochemistry stains also confirmed the production of key ligament ECM components on the scaffold. The results demonstrate that silk cable-reinforced gelatin/silk fibroin scaffold possesses the appropriate mechanical properties and has enlarged surface area. It is also capable of supporting cell proliferation and differentiation for ligament tissue engineering.

Modification of sericin-free silk fibers for ligament tissue engineering application

Biomedical application of silk requires the removal of sericin that is the gumming material of native silk fibers. This is because sericin can elicit an adverse immune response after implantation in the human body. However, the removal of sericin causes the silk fiber to fray and weakens its structural property, making it very difficult to knit or braid them into a scaffold for ligament tissue engineering applications. The aim of this study was to replace sericin with gelatin using NDGA as a cross-linking agent to biomimic the natural structure of native silk fibers. The physical properties and biocompatibility of the modified and native silk fibers were compared by in vitro and in vivo models. The mechanical and swelling properties of sericin-free silk fibers were greatly increased after modification with gelatin. Both modified and native silk fibers were shown to be nontoxic by in vitro cytotoxicity tests. The in vivo study demonstrated that the modified silk fibers, after 4 weeks' subcutaneous implantation in rats, caused little or no inflammatory reaction as compared with native silk fibers. The superior mechanical properties and lower inflammatory potential of modified silk fibers make them a promising candidate for ligament tissue engineering applications.

Biomimetic evolutionary analysis: testing the adaptive value of vertebrate tail stiffness in autonomous swimming robots

For early vertebrates, a long-standing hypothesis is that vertebrae evolved as a locomotor adaptation, stiffening the body axis and enhancing swimming performance. While supported by biomechanical data, this hypothesis has not been tested using an evolutionary approach. We did so by extending biomimetic evolutionary analysis (BEA), which builds physical simulations of extinct systems, to include use of autonomous robots as proxies of early vertebrates competing in a forage navigation task. Modeled after free-swimming larvae of sea squirts (Chordata, Urochordata), three robotic tadpoles (`Tadros'), each with a propulsive tail bearing a biomimetic notochord of variable spring stiffness, k (N m-1), searched for, oriented to, and orbited in two dimensions around a light source. Within each of ten generations, we selected for increased swimming speed, U (m s-1) and decreased time to the light source, t (s),average distance from the source, R (m) and wobble maneuvering, W (rad s-2). In software simulation, we coded two quantitative trait loci (QTL) that determine k: bending modulus, E (Nm-2) and length, L (m). Both QTL were mutated during replication, independently assorted during meiosis and, as haploid gametes, entered into the gene pool in proportion to parental fitness. After random mating created three new diploid genotypes, we fabricated three new offspring tails. In the presence of both selection and chance events(mutation, genetic drift), the phenotypic means of this small population evolved. The classic hypothesis was supported in that k was positively correlated (r2=0.40) with navigational prowess, NP, the dimensionless ratio of U to the product of R, t and W. However, the plausible adaptive scenario, even in this simplified system, is more complex, since the remaining variance in NP was correlated with the residuals of R and U taken with respect to k, suggesting that changes in k alone are insufficient to explain the evolution of NP.

Characterizing the viscoelastic properties of thin hydrogel-based constructs for tissue engineering applications

We present a novel indentation method for characterizing the viscoelastic properties of alginate and agarose hydrogel based constructs, which are often used as a model system of soft biological tissues. A sensitive long working distance microscope was used for measuring the time-dependent deformation of the thin circular hydrogel membranes under a constant load. The deformation of the constructs was measured laterally. The elastic modulus as a function of time can be determined by a large deformation theory based on Mooney-Rivlin elasticity. A viscoelastic theory, Zener model, was applied to correlate the time-dependent deformation of the constructs with various gel concentrations, and the creep parameters can therefore be quantitatively estimated. The value of Young's modulus was shown to increase in proportion with gel concentration. This finding is consistent with other publications. Our results also showed the great capability of using the technique to measure gels with incorporated corneal stromal cells. This study demonstrates a novel and convenient technique to measure mechanical properties of hydrogel in a non-destructive, online and real-time fashion. Thus this novel technique can become a valuable tool for soft tissue engineering.

Nitrocinnamate-functionalized gelatin: synthesis and "smart"hydrogel formation via photo-cross-linking

Gelatin having p-nitrocinnamate pendant groups (Gel-NC) was prepared via an efficient one-pot synthesis, yield >87%. (1)H NMR data indicated that 1 mol of gelatin was modified with 18 +/- 6 mol of the photosensitive group. Upon exposure to low-intensity 365 nm UV light and in the absence of photoinitiators or catalysts, Gel-NC cross-linked within minutes into a gelatin-based hydrogel as monitored by UV-vis spectroscopy. The degree of swelling of this biodegradable hydrogel in aqueous solutions responded to changes in Gel-NC concentration levels, the ionic strength of the aqueous solutions, and photo-cross-linking time. Topography changes associated with phase transition resulting from "photocleavage" of the hydrogel network with 254 nm UV light were studied with AFM. Both Gel-NC and its hydrogel expressed low toxicity to human neonatal fibroblast cells. In addition, gelatin-based microgels were prepared via the photo-cross-linking of Gel-NC within inverse micelles.

Characterizing the viscoelastic properties of thin hydrogel-based constructs for tissue engineering applications

We present a novel indentation method for characterizing the viscoelastic properties of alginate and agarose hydrogel based constructs, which are often used as a model system of soft biological tissues. A sensitive long working distance microscope was used for measuring the time-dependent deformation of the thin circular hydrogel membranes under a constant load. The deformation of the constructs was measured laterally. The elastic modulus as a function of time can be determined by a large deformation theory based on Mooney-Rivlin elasticity. A viscoelastic theory, Zener model, was applied to correlate the time-dependent deformation of the constructs with various gel concentrations, and the creep parameters can therefore be quantitatively estimated. The value of Young's modulus was shown to increase in proportion with gel concentration. This finding is consistent with other publications. Our results also showed the great capability of using the technique to measure gels with incorporated corneal stromal cells. This study demonstrates a novel and convenient technique to measure mechanical properties of hydrogel in a non-destructive, online and real-time fashion. Thus this novel technique can become a valuable tool for soft tissue engineering.