Printing Double-Network Tough Hydrogels Using Temperature-Controlled Projection Stereolithography (TOPS)

Abstract

We report a new method to shape double-network (DN) hydrogels into customized 3D structures that exhibit superior mechanical properties in both tension and compression. A one-pot prepolymer formulation containing photo-cross-linkable acrylamide and thermoreversible sol-gel κ-carrageenan with a suitable cross-linker and photoinitiators/absorbers is optimized. A new TOPS system is utilized to photopolymerize the primary acrylamide network into a 3D structure above the sol-gel transition of κ-carrageenan (80 °C), while cooling down generates the secondary physical κ-carrageenan network to realize tough DN hydrogel structures. 3D structures, printed with high lateral (37 μm) and vertical (180 μm) resolutions and superior 3D design freedoms (internal voids), exhibit ultimate stress and strain of 200 kPa and 2400%, respectively, under tension and simultaneously exhibit a high compression stress of 15 MPa with a strain of 95%, both with high recovery rates. The roles of swelling, necking, self-healing, cyclic loading, dehydration, and rehydration on the mechanical properties of printed structures are also investigated. To demonstrate the potential of this technology to make mechanically reconfigurable flexible devices, we print an axicon lens and show that a Bessel beam can be dynamically tuned via user-defined tensile stretching of the device. This technique can be broadly applied to other hydrogels to make novel smart multifunctional devices for a range of applications.

Keywords: additive manufacturing; digital micromirror; double-network hydrogel; mechanically reconfigurable soft devices; projection stereolithography.

Figure 1

(A) Schematic diagram of DN gels formed by photo-cross-linking and thermoreversible sol–gel transition. Acrylamide in the presence of the cross-linker MBAA and photoinitiator LAP formed the first network through irreversible photo-cross-linking. κ-Carrageenan formed the second network by reversible physical cross-linking via sol–gel transition. (B) Schematic setup of the TOPS hydrogel for printing 2D and 3D DN gel structures. (C) The 3D model of the sample holder was used as a computational domain consisting of a PDMS dish with a copper plate. Temperature distribution over the PDMS layer at the plane corresponding to section A-A′ at t = 90 seconds when the steady state was reached.

Figure 2

(A, B) Figure and plot depicting the lateral resolution of printed structure using DN gels (scale bar—200 μm). (C) Plot showing the axial resolution of the printed DN gel structure. (D) Schematic of 2D printing of the planar structure and computer-generated digital mask for printing chemical structure of acrylamide and logo of the BioInspired Institute. (E, F) 2D-printed acrylamide structure before the development and logo of the BioInspired Institute after the development of structure (scale bar—4 mm). (G) 3D CAD model, corresponding computer-generated digital mask, and 3D-printed structure of a Mayan pyramid (scale bar—5 mm). (H) CAD model, digital masks, and 3D-printed lattice structure. The printed structure was dipped into ethanol for 30 minutes to remove tartrazine and enhance the contrast for imaging (scale bar—5 mm).

Figure 3

(A) (i) Schematic and printed dog-bone structure using a hybrid gel structure with a laser power of 2.17 mW/cm² and an exposure time of 60 s. (ii) Photographs showing the performance of dog-bone structure during tensile testing, and the structure stretched 9.5 times its original length. (iii) Stress–strain plot obtained from dog-bone structures printed using the DN gel, acrylamide-only gel, and carrageenan-only gel. Ultimate stress and ultimate strain of the fabricated structures are also depicted. (B) (i) 3D-printed hollow-lattice geometry with a structure for studying the tensile performance (scale bar—5 mm). (ii) Photographs showing the tensile performance of the structure (scale bar—8 mm). (C–F) Ultimate stress and ultimate strain of DN gel structures printed by varying exposure times, and amounts of the photoinitiator, cross-linker (MBAA), and κ-carrageenan.

Figure 4

(A) Solution to the necking behavior: (i) comparison of the stress–strain plot of the structure with necking behavior (red), without necking behavior (blue), and without necking with restored stress (green); (ii, iii) photographs showing the necked and un-necked structure during stretching. (B) Stress plotted during three cycles of loading and reloading: (i) with the strain of (480%); (ii) with the strain of (800%). (C) (i) FTIR spectra of molded and cast κ-carrageenan, TOPS-printed PAAm, and DN gels; (ii) FTIR spectra, zoomed in, acquired from as-printed, dry, and swollen DN gel structures before and after stretching.

Figure 5

(A) Stress–strain plot obtained from the cylindrical stud structure printed using acrylamide-only, κ-carrageenan-only, and acrylamide/κ-carrageenan hybrid gels. Inset shows a strain–stress plot of the κ-carrageenan structure. (B) Demonstration of compression and recoverability of a 3D-printed Mayan pyramid structure (scale bar—5 mm). (C) Force–strain plot for 3 compression cycles of the pyramid structure. (D) Compressive stress–strain plot for structures printed using two different concentrations (4 and 8 wt %) of κ-carrageenan in the hybrid gel. (E) Stress–strain plot for DN gel structures printed by varying laser exposure time at fixed laser intensity. Inset shows the change in the modulus of structures printed using different exposure times.

Figure 6

(A) Transmission spectra of the structure printed using the DN gel structure. The printed structure is placed on the top of the BioInspired Institute logo to demonstrate the high transmissivity of the DN gel structure (scale bar—1 cm). (B) CAD design, computer-generated digital masks, and 3D-printed axicon lens using TOPS (scale bar—2 mm). (C) Characterization of the annular ring while the axicon lens is static (scale bar—2 cm). (D) Optical setup to characterize the annular ring of the axicon lens during dynamic stretching. (E) A screenshot was obtained from Video V4 showing the tunability of the axicon lens (scale bar—3 cm). (F) Schematic showing the zero-order Bessel beam generation by the axicon and experimentally obtained annular rings before and after stretching the axicon lens (scale bar—2 cm). ω_b = radius of the central lobe, z_b = length of Bessel region, D₀ = diameter of incident beam.

Figure 7

(A) Comparison of performance of TOPS-printed acrylamide/κ-carrageenan structures with existing technology of hydrogel fabrication in terms of lateral resolution and ultimate strain. (B) Comparison of performance of TOPS-printed acrylamide/κ-carrageenan structures with hydrogels and double-network hydrogels in terms of fracture energy and modulus of elasticity. (C) Plot depicting the mechanical performance (in terms of strain and strain) and comparison of printing performance (in terms of resolution) of TOPS-printed acrylamide/κ-carrageenan structures with other 3D-printed DN hydrogels. Note: all other studies only report lateral (XY) resolution. Since we printed 3D hollow structures with overhangs and undercuts, we have also reported the z (depth) resolution.^,,−,,,,−