Designing smart and adaptive materials that can change their properties depending on external stimuli in a controlled and reversible manner constitutes an important frontier of material science and engineering. In recent years, smart materials with tunable mechanical properties have gained significant research attention from the perspective of cutting-edge applications in the field of regenerative medicine, tissue engineering, drug delivery, and soft robotics [1], [2], [3], [4], [5]. To this end, temperature-responsive hydrogels formed by aqueous poly(N-isopropylacrylamide) (PNIPAM) have been widely studied due to their easy tunability over an accessible range of temperatures, close to the normal human body temperature [6], [7], [8], [9]. At temperatures below the lower critical solution temperature (LCST) of PNIPAM, the hydrophilic amide groups form hydrogen bonding with the surrounding water molecules and trap them in the structure. Above LCST, the hydrophobic interactions among the propyl groups become more favourable compared to the hydrogen bonding interactions when the trapped water is released from the structure and the polymer chains collapse. This results in a volume phase transition (VPT) of PNIPAM [6], [10].

The thermoresponsive properties of PNIPAM have been exploited in materials design, that include: synthetic biocompatible material formed by an interpenetrating network of polyisocyanide (PIC) and PNIPAM that shows a huge, reversible mechanical stiffening response [11]; composite formed by PNIPAM and biopolymer fibrin shows significant stiffening effect when the temperature crosses the VPT [12]; cytocompatible cell scaffolds formed by PNIPAM grafted collagen demonstrating a variation of shear moduli over several orders of magnitude [13]; PNIPAM grafted collagen nanofibrils for improved drug delivery applications [14], [15]. Due to their abundance and biological importance, collagen has become one of the most commonly used materials for designing various biocomposites [3], [4], [16], [17], [18], [19].

Understanding the reversible switching of the composite system offers new principles for materials design, which is especially relevant in light of recent efforts to create synthetic bio-compatible materials with tunable properties. Since collagen is the main structural protein in many tissues, reinforcing collagen with different materials can pave the way towards various biological applications mentioned above. Alterations in the mechanical properties of collagen are linked with various pathological conditions like osteoporosis, arthritis, and cardiovascular diseases. Thus, developing techniques for effectively controlling mechanical properties can provide new diagnostic and therapeutic applications [20], [21], [22]. Although PNIPAM hydrogels have been widely used in various fields mentioned above, the low mechanical strength of pure PNIPAM hydrogels may limit their use in some of these applications. This is because the mechanical properties of hydrogels depend on the degree of swelling characterized by the swelling ratio of the particles. When the hydrogel is highly swollen, the polymer chains become more flexible, leading to reduced mechanical strength. To overcome this problem, one can design suitable composite systems with desired properties. Thus, designing such composite systems can be an effective approach to improve the mechanical properties of the hydrogels, which can expand their potential applications [23], [24] in various fields mentioned above.

In spite of many detailed studies, how the tunability of shear moduli of the composite materials depends on the network architecture of the biopolymers remains poorly addressed. Recent experimental studies highlight that for the same monomer concentration, the network architecture significantly affects the linear and non-linear mechanics of collagen networks [25], [26], [27]. Thus, probing the architecture-dependent mechanics can help not only in optimizing the performance of these biocomposites but may also provide design strategies for synthetic biocompatible systems.

Here we study the switching response of biocomposites formed by embedding PNIPAM hydrogel particles in the collagen networks using shear rheology technique. We tune the network architecture of collagen by varying the polymerization temperature [28] and compare the switching response of composite for the same swelling-deswelling ratio of the microgel particles. Using laser scanning confocal microscopy, we also demonstrate an interesting clustering-declustering dynamics of PNIPAM particles inside the collagen mesh structure that is intimately linked to the observed switching response of the composite.