Nature-Inspired Hydrogels with Soft and Stiff Zones that Exhibit a 100-Fold Difference in Elastic Modulus

Abstract

Many biological materials, such as the squid beak and the spinal disc, have a combination of stiff and soft parts with very different mechanical properties, for example, the elastic modulus (stiffness) of the stiffest part of the squid beak is about 100 times that of the softest part. Researchers have attempted to mimic such structures using hydrogels but have not succeeded in synthesizing bulk gels with such large variations in moduli. Here, we present a general approach that can be used to form hydrogels with two or more zones having appreciably different mechanical characters. For this purpose, we use a technique developed in our lab for creating hybrid hydrogels with distinct zones. For the soft zone of the gel, we form a polymer network using a conventional acrylic monomer [N,N′-dimethylacrylamide (DMAA)] and with laponite (LAP) nanoparticles as the cross-linkers. For the stiff zone, we combine DMAA, LAP, and a methacrylated silica precursor ([3-(methacryloyloxy)-propyl]trimethoxy-silane). When this mixture is polymerized, nanoscale silica particles (∼300 nm in diameter) are formed, and these serve as additional crosslinks between the polymer chains, making this network very stiff. The unique character of each zone is preserved in the hybrid gel, and different zones are covalently linked to each other, thereby ensuring robust interfaces. Rheological measurements show that the elastic modulus of the stiff zone can be more than 100 times that of the soft zone. This ratio of moduli is the highest reported to date in a single, continuous gel and is comparable to the ratio in the squid beak. We present different variations of our soft−stiff hybrid gels, including multizone cylinders and core−shell discs. Such soft−stiff gels could have utility in bioengineering, such as in interfacing stiff medical implants with soft tissues.

Gradient Hydrogels—The State of the Art in Preparation Methods

“…Multi-layered hydrogels with a gradual change in stiffness were described by Gharazi et al [5]. The key feature of their preparation process was to use different types of nanoparticles as additional cross-linking agents in an acrylic-based hydrogel.…”

“…(b) The cylinder is stretched to show that zones 3 and 4 are negligibly extended, zone 2 is extended as it, and zone 1 is extended a lot. Reprinted from[5] with permission.…”

Gradient Hydrogels—Overview of Techniques Demonstrating the Existence of a Gradient

“…Gharazi et al prepared hydrogels consisting of several distinct zones progressively differing in their stiffness [ 10 ]. In other words, a steep mechanical gradient was formed.…”

“…Sharp gradients like those observed for zonal hydrogels designed by Gharazi et al are relatively easily checkable [ 10 ]. The materials of individual (inter-gradient) zones can be separately subjected to mechanical tests.…”

The aging effect on the enhancement of thermal stability, mechanical stiffness and fluorescence properties of histidine-appended naphthalenediimide based two-component hydrogels

“…Low molecular weight gels comprising of an amino acid or a peptide moiety are good building blocks for making supramolecular smart materials. Biocompatibility and biodegradable nature of these materials reinforced their usability in various research field including drug delivery [5][6][7][8] , tissue engineering 9,10 , waste water management 11,12 , catalysis [13][14][15] , biosensors [16][17] and others [18][19][20][21][22] . The gelator molecules are self-assembled by using various non-covalent interactions including hydrogen bonding interactions, π-π interactions, van der Waals interaction and others to form an entangled three dimensional network structure with lot of void spaces occupied by many solvent molecules under a suitable condition to form gels.…”