Microstructured thermo-responsive double network granular hydrogels

Abstract

Many hydrogels respond to external stimuli such as changes in temperature, pH, or salt concentrations by changing their degree of swelling, and hence mechanical properties, rendering them attractive actuators. Unfortunately, response rates of many of these hydrogels are limited because they rely on the diffusion of water, which is relatively slow within the gel. Here, we introduce thermo-responsive granular hydrogels which combine accelerated response rates with load-bearing properties. To accelerate the response to temperature changes, we formulate poly(N-isopropylacrylamide) (PNIPAM) microgels with connected pores by leveraging phase separations. To impart the porous hydrogel load-bearing properties, we formulate them as thermo-responsive double network granular hydrogels (TDNGHs). We demonstrate that the granular structure combined with the open micropores located within the microfragments increase the response-rate of these gels 3-fold compared to that of bulk counterparts. Moreover, the granular material exhibits 18-fold enhanced work of fracture compared to the bulk. The granular structure adds an additional benefit: it renders them 3D printable. We co-process thermo-responsive hydrogels with a non-responsive counterpart to fabricate a bilayer, which lifts up to 85% of its weight if heated and 3D print a butterfly as a bilayer structure that bends its wings when exposed to elevated temperatures.

Fig. 1. Schematic illustration of the production of porous microfragments. (a) The formation of PNIPAM in water results in homogeneous hydrogels whereas if formed in a mixture of DMSO and water, PNIPAM exhibits micropores with diameters of order of a few micrometers. (b) Schematic illustration of (i) the fragmentation of bulk hydrogels to produce (ii) microfragments that are (iii) filtered to remove particles with dimensions above 150 μm and subsequently soaked in an aqueous solution containing precursors for the 2nd network. The ink is cast or 3D printed before the (iv) 2nd network is formed to solidify the structure. (v) If heated above the LCST, the bilayer structure bends.

Fig. 2. Visualization of homogeneous and microporous PNIPAM microfragments. (a) Optical images of (i) homogeneous and (ii) microporous microfragments formed in aqueous solutions containing 40 mol% and (iii) 60 mol% DMSO. (b) Scanning electron microscopy images of bulk homogeneous and microporous PNIPAM hydrogels. (c) The average diameter of the different types of microfragments (n = 100). (d) Swelling ratio of the homogeneous PNIPAM gel and samples with small and large micropores by weight ( grey) and volume ( orange). The sample possessing small micropores display the highest swelling ratio.

Fig. 3. Rheological properties and printability of jammed PNIPAM microfragments. (a) Photograph of jammed PNIPAM fragments that (i) are homogeneous and (ii) contain small micropores. (b) Frequency, (c) amplitude sweeps, and (d) shear recovery curves of jammed fragments that are homogeneous ( grey) and contain small ( orange) and large micropores ( blue). (e) Printability of the jammed fragments. The spreading coefficient as a function of the porosity of microfragments (n = 25). The minimum pressure required to print the jammed microfragments is shown on the right. The best shape fidelity is achieved with homogeneous PNIPAM and PAMPS fragments. (f) Optical micrographs of printed grids composed of jammed PNIAPM microfragments (i) that are homogeneous (ii) contain small, and (iii) large micropores and (iv) homogeneous PAMPS microfragments.

Fig. 4. Influence of microfragments on the mechanical properties of DNGHs. (a) Stress–strain curves of TDNGHs fabricated from PNIPAM microfragments that are homogeneous (grey) and contain small (orange) and large micropores (blue). PAM is used as a 2nd network. (b) Work of fracture ( grey) and Young's Modulus ( orange) of TDNGHs made of microfragments that are homogeneous and contain small and large micropores (n = 3). (c) Young's moduli of TDNGHs measured at 22 °C ( grey) and 60 °C ( orange) (n = 3).

Fig. 5. Temperature-induced deswelling of TDNGHs. (a) Deswelling of TDNGHs fabricated from PNIPAM microfragments that are homogeneous ( grey) and contain small ( orange) and large micropores ( blue) connected by a PNIPAM 2nd network as a function of time at 70 °C. The highest deswelling rate is achieved within samples containing microporous PNIPAM microfragments. (b) Photographs of TDNGHs at 22 °C and 70 °C. (c) Deswelling of TDNGHs fabricated with PNIPAM microfragments that are homogeneous ( grey) and contain small ( orange) and large micropores ( blue) connected through a PAM 2nd network as a function of time kept at 70 °C. (d) Ashby plot of normalized deswelling rates as a function of the Young's modulus of PNIPAM-based hydrogels reported in the literature. Blue symbols represent DIW 3D printable systems and orange symbols represent non-printable counterparts.

Fig. 6. Degree of actuation of TDNGHs. The bending angle as a function of time after the bilayers have been actuated in a water bath of 70 °C. The active layer is composed of TDNGHs made of microfragments that are homogeneous ( grey) and contain small ( orange) and large ( blue) micropores. The inset is a photograph encompassing the definition of the bending angle, ∠AOB.

Fig. 7. Proof-of-concept of thermally actuated TDNGHs. (a) (i) Bilayer composed of an inert layer made of PAMPS microfragments and a responsive layer composed of PNIPAM microfragments with small micropores. (ii) If immersed in an aqueous solution at 60 °C, the bilayer starts to bend after 10 minutes to lift a 2 g weight after (iii) 30 and (iv) 50 minutes. (b) A TDNGH made with PNIPAM microfragments with small micropores at (i) room temperature, where it breaks when loaded with 50 g and (ii) at 70 °C where it holds the load that constitutes 50-times the weight of the sample itself. (c) Bilayer DNGHs immersed in water at 70 °C after (i) 0 and (ii) 30 minutes when the 3D printed ball is kicked to the left due to bending. (d) 3D printed butterfly consisting of a bilayer. The inert DNGH is made of PAMPS microfragments in a PAM 2nd network, and the active layer is made of PNIPAM microfragments with small micropores connected with a PNIPAM 2nd network (i top view and ii side view) after printing at room temperature and (iii and iv) immersed in water at 70 °C where it starts to bend the wings after 10 minutes.