Results and Discussion
Stable and well-characterized series of curves are generated from our equation, demonstrating the deswelling of the biopolymer (i.e., type I collagen) gel matrix with a temperature increase. Under a fixed crosslinking condition, the gel elasticity-associated coefficientα appears to be a major factor in determining the deswelling behavior (Fig. 2). The gel shrinkage becomes more sensitive to the normalized temperature change at T/Θ < 0.95, as decreasing α value on the order of magnitude range of α ≤ 109. When α exceeds ~109, the volume change curves are no longer an injective function in the given range of normalized temperature. For the subsequent analyses, we assume α = 1.0 × 107 as an optimal elastic condition.
Next, we find that the doping ratio of thermoresponsive pNIPAM chains (A = -6.4 and Θ = 315 K 13) in the type I collagen gel matrix is another critical parameter to tune the overall temperature range of the volume phase transition. Fig. 3 demonstrates that the phase transition temperature is tunable in a wide range (~50 degrees) from room temperature (Troom ~22 °C), by varying the mixing ratio of pNIPAM (rpolym ) from 0.3 to 0.5. [Note that the curves for only four values of rpolymhas been plotted in different colors in the Fig. 3. The transition temperature tuning can be achieved in a fully continuous manner in this range.]
In a fixed doping ratio (rpolym ) of the thermosensitive component pNIPAM, it is found that the crosslinking degree of the network (represented by the degree of polymerization between crosslinks, Nx ) is the parameter which controls the temperature sensitivity of the gel matrix. For each transition curve at different rpolym value in Fig. 3, a higher gel volume change occurs for less crosslinked networks (i.e., larger Nx ; displayed as a series of curves with decreasing saturation), when compared in a given temperature domain. The condition of rpolym = 0.35 andNx = 1 has selected as an optimized physicochemical parameter set under the consideration of (1) the onset of the phase transition at a temperature slightly higher than the body temperature (Tbody ~37 °C) and (2) the transition temperature range spanning a few degrees aboveTbody , which corresponds to unharmful hyperthermia for human tissues in short time scales14.
Under the selected parameter values, the effect of collagen molecular structure on the transition temperature range of the gel matrix has been investigated (Fig. 4). Because the information on the conformation of type I collagen is reflected in the Flory-Huggins interaction parameter between biopolymer and water, χbio , we plot for the four different values of χbio that are chemically plausible in acellular in vitro situations. In Fig. 4, the curve for gel matrix made of intact collagen type I (χbio = 0.161 15), which is a composite of a main helical rod and extrahelical peptides at both terminals, is presented with the parameter set chosen in Fig. 3 (blue solid line). If the gel comprises the pronase-digested collagen type I (χbio = 0.107 15), which has no extrahelical structures to be closer to a rod particle, the phase transition range decreases about by 8 degrees (blue dashed line). When the input value of χbio is for gelatin (χbio = 0.48 for coil state16and χbio = 0.49 for coexistence of coil and helical states16,17), the resulting plots (cyan solid and dashed lines, respectively in Fig. 4) illustrate that the transition temperature ranges appear to be above the temperature threshold for denaturation from type I collagen to gelatin (~65 °C18), which partially validates the effectiveness of our model in a biophysical basis.
For bioengineering applications, the representative examples of the optimized matrix design are suggested for type I collagen and gelatin gels with different doping ratios of pNIPAM (Fig. 5). Assuming the volume ratio of the total polymers in a swollen hydrogel is about 0.1 (with Vi/VNIPA = 20), thenrpolym = 0.35 for collagen type I andrpolym = 0.48 for gelatin are found to be suitable for inducing gel deswelling through local heating in the viable temperature range for tissues in vivo and in vitro . In these conditions, the gel matrices are fully swelled atTbody and completes deswelling at slightly augmented temperatures (i.e., ~8 degrees aboveTbody ) and vice versa . For instance, this property can be utilized for controlled release of drugs in human body or as miniaturized tissue-resembling actuators in organ-on-a-chip platforms for cell cultures10.