Nature-Inspired Hydrogels with Soft and Stiff Zones that Exhibit a 100-Fold Difference in Elastic Modulus Salimeh Gharazi, † Brady C. Zarket, † Kerry C. DeMella, and Srinivasa R. Raghavan* Department of Chemical & Biomolecular Engineering, University of Maryland, College Park, Maryland 20742, United States * S Supporting Information 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 cross- links 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. KEYWORDS: hybrid, nanocomposite, silica nanoparticles, compressive modulus, shear modulus ■ INTRODUCTION Biological materials are frequently in a gel state, that is, they exhibit the properties of elastomeric solids while containing a large fraction of liquid (water) within them. 1-3 These include aquatic invertebrate animals like squids, octopuses, sponges, and jellyfish, as well as terrestrial invertebrates like worms and snails. Although these creatures may have hard or stiff elements within them, for the most part, they are soft and gel-like. In our bodies also, many tissues, organs, or other body parts are gel- like and compliant. 4,5 From the viewpoint of a materials engineer, such soft objects bring to mind polymer hydrogels, which are water-swollen networks of polymer chains cross- linked by chemical or physical bonds. 6-9 Hydrogels are easily formed in the lab by free-radical polymerization of monomers and cross-linkers. In recent years, researchers working on hydrogels have begun to recognize the remarkable properties of biological gels and attempted to mimic them. For example, gels with the mechanical resilience and toughness of cartilage 10 or the responsive properties of sea cucumbers 11 have been reported. One additional feature of biological gels is their hybrid or multisegmented nature. 12,13 That is, although a given gel may appear to be a single, homogeneous unit, it may actually have many connected segments. Individual segments may differ in their chemical or biochemical composition (i.e., each part may have its own type of cells or extracellular matrix) or in their micro- or nanostructure (e.g., cells may be oriented into chains in one segment but not others). These chemical or structural differences are often reflected in the macroscopic properties of the various segments, specifically their mechanical properties. Two examples help to illustrate the mechanical differences within a biological soft material, and these are (a) the squid beak 14-16 and (b) the spinal disc. 4,5 Schematics of these structures are provided in Figure S1 (Supporting Information section.) Both of these are fully organic materials, that is, they do not contain inorganic minerals in them. Yet, in the squid beak, the tip (rostrum) is very stiff, with an elastic modulus around 5 GPa, whereas the base is much softer, with a modulus around 50 MPa (Figure S1a). Thus, the ratio in moduli between the stiff and soft ends is about 100. Between these Received: August 16, 2018 Accepted: September 17, 2018 Published: September 28, 2018 Research Article www.acsami.org Cite This: ACS Appl. Mater. Interfaces 2018, 10, 34664-34673 © 2018 American Chemical Society 34664 DOI: 10.1021/acsami.8b14126 ACS Appl. Mater. Interfaces 2018, 10, 34664-34673 Downloaded via UNIV OF MARYLAND COLG PARK on October 20, 2018 at 11:07:24 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.