Hydrogels PNIPAAm

Fig. 9 Swelling behaviour of the temperature-sensitive hydrogel PNIPAAm...

PNIPAAm-based hydrogel Hydrophobic interaction Temperature [30]... 

In this work, we generated new PMAA/PNIPAAm IPN hydrogels with both pH and temperature sensitivities by the interpenetration of the pH-sensitive and temperature-sensitive polymer networks. 

DSC were conducted on PMAA/PNIPAAm IPN hydrogels swollen at different pH values. The results show that the difference in pH has great influence on the LCST transitions of the IPN hydrogels, as shown in Figure 5. 

At pH 4.3, there is no significant transition detected around 32°C. Transition temperatures are detected and increase as pH increases. This is because, at low pH, the aggregation of PMAA decreases the mobility of the PNIPAAm network as well as the water uptake of the IPN, resulting in drastically lowering the temperature sensitivity of the IPN hydrogel. However, at higher pH value, the swollen PMAA allows the PNIPAAm to have a higher mobility, which makes the IPN more temperature sensitive. 

Dirk Kuckling of Chemisorbed Thin PNIPAAm Hydrogel Layers... 

Fig.3 Volume degree of swelling of the photo cross-linked PNIPAAm hydrogel layer with varying DMAAm content plotted as a function of temperature in PBS buffer ( - NIPAAm5, o - NI-PAAmlO, A - NTPAAm20. V - NIPAAm30, 0 - NIPAAm40)...

By raising the temperature above 7(, all hydrogel layers collapsed to different 1/0P values. The collapsed state was also found to be dependent on the mol-% of DMAAm in hydrogel layers. Higher the DMAAm mol-% in hydrogel layers, larger is the l/0p in the collapsed state. It could be interpreted from these results that the DMAAm, which increases the swelling (below Tc) also results in a partially collapsed state at temperatures above Tc. This is due the hydrophilic nature of DMAAm, which retains water even after the PNIPAAm chains are collapsed. 

The thin NIPAAm hydrogel layers were covalently attached to the surface, therefore the swelling was confined to one direction perpendicular to the substrate. The degree of anisotropy was calculated from ratio of l/0p and swelling ratio. This ratio was found to be between 1.2-1.3. Such a study describing the effect of hydrophilic comonomer on Tc of PNIPAAm provides an opportunity to formulate thermoresponsive hydrogel layers with desired transition temperature and swelling behavior for various applications. 

To mimic the macromolecular-based ECM in biological tissue, the cell adhesion and proliferation properties of hydrogels are critical parameters. However, various hydrogels that originate from natural resources, such as alginate [87], chitosan [88, 89], and hyaluronic acid [90], and that are synthetically created, such as poly (7V-isopropylacrylamide) (PNIPAAm) [91], PEO [92], PVA [93], and poly(ethylene glycol) (PEG) [94], show a poor cellular viability without modification with cell adhesive proteins or peptides, such as collagen, laminin, fibronectin, and the RGD (Arg-Gly-Asp) sequence. 

Fig. 4 The equilibrium swollen normalized gel volume V q of ionic PNIPAAm hydrogels as function of the numberof charges per network chain (see (3)) (a) Reduced modulus Gr of ionic PNIPAAm hydrogels as function of the normalized gel volume (b) The mole fractions x,- of AMPS Na are indicated in the figure. Reprinted from Gundogan et al. (2002) with permission of American Chemical Society...

Fig. 5 Variation of the volume ratio V oilV (equilibrium swollen gel volume in solution / equilibrium swollen gel volume in water) of PNIPAAm, PDMAAm, and TBA/AAm (60/40 by mole) hydrogels filled symbols) and PAAm hydrogels open symbols) with the volume fraction

The lower critical solution temperature (LCST) of the pure, weakly cross-linked PNIPAAm gel is 34°C determined by the swelling method (Shibayama et al. 1994). In order to increase the mechanical properties hydrogels with higher cross-linking densities can be prepared. In Fig. 2 the temperature-dependent swelling behaviour... 

In Fig. 4 the pH-dependent degrees of swelling of PNIPAAm hydrogels with 5 mol-% of the AAmPA comonomer are shown. Increased comonomer content increased the step height of the pH curve (Kuckling et al. 2003a). The pH sensitivity... 

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