Stimuli-responsive injectable hydrogels based on weak supramolecular interactions may represent safer alternatives to chemically reactive adhesive hydrogels for biomedical applications where weak to moderate adhesion is required. We investigated the linear and nonlinear rheological properties as well as the adhesive properties of two thermoresponsive graft copolymers with inverse topologies, poly(N-isopropylacrylamide)-g-poly(N,N-dimethylacrylamide) (PNIPAM-g-PDMA) and PDMA-g-PNIPAM. Except for their topologies, these copolymers are analogous in terms of chemistry, architecture (graft), and monomer composition (50–50 wt %). Over a wide range of concentrations, they both form injectable homogeneous solutions at room temperature and turn into soft and sticky viscoelastic hydrogels close to body temperature. We find that the linear viscoelastic properties of these two hydrogels are not discernible far above the thermal transition temperature. However, the PNIPAM-g-PDMA hydrogel having long thermoresponsive backbones shows a strain-hardening behavior in large strains both in probe tack tests and in shear. The inverse topology, PDMA-g-PNIPAM, showed no hardening and simply softened until failure. This distinction was observed regardless of the polymer concentration (in the entangled regime). We attribute the hardening to a continuous, load-bearing nanostructure from strong hydrophobic PNIPAM associations, while the softening is due to the easy pullout of short PNIPAM grafts from separate hydrophobic clusters bridged by PDMA backbones. The findings of this work highlight the importance of macromolecular design in determining the nanostructure and thereby the mechanical performance of soft hydrogels for specific applications.

中文翻译：

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特定于拓扑的可注射粘性水凝胶

基于弱分子间相互作用的可刺激刺激的可注射水凝胶可代表需要弱至中度粘附力的生物医学应用中化学反应性粘合水凝胶的更安全替代品。我们研究了两种具有相反拓扑结构的热响应性接枝共聚物的线性和非线性流变特性以及粘合特性，聚（N-异丙基丙烯酰胺）-g-聚（N，N-二甲基丙烯酰胺）（PNIPAM- g -PDMA）和PDMA- G-PNIPAM。除了它们的拓扑结构，这些共聚物在化学，结构（接枝）和单体组成（50-50 wt％）方面类似。在很宽的浓度范围内，它们都可以在室温下形成可注射的均质溶液，并在接近人体温度的条件下转变为柔软且粘稠的粘弹性水凝胶。我们发现，这两种水凝胶的线性粘弹性特性在远高于热转变温度时无法辨别。然而，具有长的热响应性主链的PNIPAM- g- PDMA水凝胶在探针粘性测试和剪切中均在大应变中显示出应变硬化行为。逆拓扑，PDMA- g-PNIPAM，没有硬化，只是软化直到失效。不管聚合物浓度如何（在纠缠态），都观察到这种区别。我们将硬化归因于来自强疏水性PNIPAM缔合的连续的，承重的纳米结构，而软化归因于短PNIPAM嫁接物易于从由PDMA骨架桥接的单独疏水性簇中拉出。这项工作的发现突出了大分子设计在确定纳米结构中的重要性，从而确定了软水凝胶在特定应用中的机械性能。