Mechanical Properties Of Swollen Nanocomposite Hydrogels And Cell Sheet Detachment From Gel Surface

The gelation process of poly N-isopropylacrylamide(PNIPAm)-hectorite Laponite nanocomposite hydrogel (NC gel), large strain mechanical properties of NC gel after swelling in water, cell culture and automatic cell sheet detachment from NC gel surface was studied in the present work. First, the interaction between NIPAm molecules and Laponite platelets as well as the polymerization process of NC gel were studied through fluorescence probes; second, the mechanical properties of swollen NC gels under large strain were investigated by uniaxial compression and elongation, the swelling behavior of NC gels in different solvents and solutions was also studied; third, cell culture and cell sheet detachment on NC gel surface with the introducing of biocompatible component were also indagated. The main contents and results are as follows:1. The adsorption of NIPAm molecules on Laponite platelets was systematically studied through fluorescence probes pyrene, 1-pyrenemethylamine hydrochloride (PyMA·HCl) and 1-naphthalenemethanamine (1-NpMA). The adsorption of fluorophores on Laponite platelets was confirmed by increasing ionic strength of Laponite suspension and separating Laponite from the suspension through centrifugation. The adsorption of NIPAm molecules on Laponite platelets was proved with fluorescence excimer, anisotropic and non-radiative energy transfer (NRET) methods.2. The polymerization process of NC gels was monitored by adding fluorescence probes into the reaction suspension. The transparency of reaction suspensions with different NIPAm concentrations all had a sudden decrease during the polymerization process but could recover in several minutes, the fluorescence spectra also changed during which period, the relative strength of fluorescence excimer increased obviously after the recovery of transparency. It could be concluded from fluorescence spectra that polymer chains formed during polymerization was adsorbed around Laponite platelets, so the fluorescence probes adsorbed on the Laponite platelets was compressed together to form excimers. This finding is very important in understanding the crosslinking structure of NC gels.3. Mechanical behavior of swollen PNIPAm-Laponite NC gel was studied with compression, elongation compared with that of as-prepared. Results showed that the swollen NC gels behaved like rubber elasticity and no fracture was observed during compression. The compression stress-strain curve was described well with the Mooney-Rivlin equation. The strain hardening was strong with the increase of Laponite concentration and weakened when swollen. There was no fracture during compression process, the swollen NC gels were found to be stretched up to strain of 450% while chemically crosslinked gels could only be stretched for 100%. The Mooney-Rivlin equation failed to describe the tensile results at large strains, whereas both the tensile and compressive results were well described with the model of parallel combination of the UCM model with the Gent strain-hardening model. The stress-strain hysteresis was observed with loading-unloading circles of either compression or elongation and the hysteresis was enhanced with increasing Laponite concentration in the NC gels and weakened when swollen, which was also the origin of the high fracture toughness of the NC gels. The repeat of stretch-recover process indicate that swollen NC gels could bear repeat load without fracture, which made NC gel a potential material in artificial cartilage and muscle. The effective network chain density of the swollen NC gels measured was the same as that calculated from the as-prepared NC gel and swelling ratio, indicating that the NC gel network was merely expanded and the cross-linkers were not destroyed during swelling. The number of the effective network chains attached to one Laponite XLS platelet was about 165 and almost a constant independent of the NC gel composition, confirming that Laponite acted as multifunctional cross-linker.4. The swelling behavior in different organic solvents or their aqueous solutions and the mechanical properties after swelling of nonionic PNIPAm NC gels and ionic PNIPAm/SA NC gels was investigated. Swelling ratios or mechanical properties of the two kinds of NC gels hardly changed after immersing in nonpolar solvent hexane. PNIPAm NC gel first deswelled and then re-swelled with the increase of ethanol concentration in ethanol aqueous solutions, which was called reentrant phase transition; while PNIPAm NC gel dissolved gradually in acetone aqueous solutions. PNIPAm/SA NC had no reentrant phase transition in ethanol aqueous solutions. The swelling ratio increased with increasing SA content in water rich region, but decreased in ethanol rich region. PNIPAm/SA NC gels could also dissolve or break up in acetone aqueous solutions. This findings are helpful in the application of NC gels in different solutions. 5. Semi-interpenetrating polymer network (semi-IPN) NC gel was successfully synthesized by in situ polymerization of the monomer NIPAm in an aqueous hectorite Laponite suspension containing the polysaccharide alginate (AL). This semi-IPN NC gel reserved thermo-responsibility and high mechanical performance of the PNIPAm-Laponite NC gel. Cell cultures of fibroblasts (L929), human lung adenocarcinoma epithelial cells (A549), and human cervical cancer cells (HeLa) on the semi-IPN NC gels showed improved cell compatibility with addition of AL. By lowering temperature, a rapid cell sheet detachment within 15 min was observed from the surface of the semi-IPN NC gel containing 0.2 w/v% of AL without trypsin treatment. The roughness of surface texture of the semi-IPN NC gel containing AL accelerated the cell sheet detachment, due to faster water penetration. The detached cell sheet was seeded and proliferated again, showing a good cell viability, which indicates that promising applications in tissue engineering and other biomedical fields exist.