•Thiol-ene-epoxy photochemistry for patterning diverse mechanical properties•Materials possess a stable three-orders-of-magnitude difference in Young’s modulus•Demonstration of a low-cost, 3D-printed multimodulus wearable braille display Numerous research efforts seek to realize “augmented humanity,” whereby technology enhances human performance by closely interfacing engineered devices with our anatomy. A key challenge to these efforts is mechanical mismatch—electronic devices are composed of rigid materials, but our natural tissues are soft. Co-depositing disparate materials at high resolution in a single manufacturing step is often difficult to implement, and abruptly joining soft-stiff materials leads to delamination. This new thiol-ene-epoxy framework offers photopatterned control over reaction conversion and, consequently, local stiffness. Unlike previous multimaterial photopolymers that rely on “under-cured” material, this chemistry creates persistent and continuous mechanical gradients spanning three orders of magnitude in modulus comparable with gradients in animal physiology. Such capabilities enable new applications, as demonstrated by a low-cost, 3D-printed braille display that can be worn on a single finger. Seamless multimaterial construction is a common motif in animal physiology. Such continuous mechanical gradients remain challenging to reproduce in engineered systems, as current resin chemistries typically result in a single fixed set of properties. As an alternative to single-property materials, we introduce a thiol-ene-epoxy-based photothermal reaction scheme that produces multimaterials by altering the polymer microstructure within a single resin. In this system, the photodosage during the first stage of processing dictates the extent of conversion for each subsequent reaction. As a result, our photochemistry can exhibit a diverse range of soft (Young’s modulus, E ∼ 400 kPa; elongation, dL/L0 ∼ 300%) and stiff (E ∼ 1.6 GPa; dL/L0 ∼ 3%) mechanical properties. Furthermore, we pattern photostable and mechanically robust modulus gradients (d[Er, stiff/Er, soft]/dx > 1,000 mm−1) that exceed those found in squid beaks and human knee entheses. We demonstrate the ability to build intricate multimaterial architectures including a soft, wearable braille display. Seamless multimaterial construction is a common motif in animal physiology. Such continuous mechanical gradients remain challenging to reproduce in engineered systems, as current resin chemistries typically result in a single fixed set of properties. As an alternative to single-property materials, we introduce a thiol-ene-epoxy-based photothermal reaction scheme that produces multimaterials by altering the polymer microstructure within a single resin. In this system, the photodosage during the first stage of processing dictates the extent of conversion for each subsequent reaction. As a result, our photochemistry can exhibit a diverse range of soft (Young’s modulus, E ∼ 400 kPa; elongation, dL/L0 ∼ 300%) and stiff (E ∼ 1.6 GPa; dL/L0 ∼ 3%) mechanical properties. Furthermore, we pattern photostable and mechanically robust modulus gradients (d[Er, stiff/Er, soft]/dx > 1,000 mm−1) that exceed those found in squid beaks and human knee entheses. We demonstrate the ability to build intricate multimaterial architectures including a soft, wearable braille display. Across length scales, biology combines soft and stiff structures to control mechanical performance and enable higher functionality. As a prime example, load-bearing activity in vertebrates relies on the connection of rigid bones and soft muscle tissue.1Vogel S. Comparative Biomechanics: Life’s Physical World Second Edi. 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Here, we introduce a sequential thiol-ene-epoxy framework as a new framework for creating stable multimaterials capable of mimicking a wide range of mechanical performance available in natural tissues and commercial polymers. Specifically, our approach utilizes photoinitiated thiol-ene reactions followed by thiol-epoxy step-growth polymerization and then epoxy homopolymerization at elevated temperatures. This unique design creates a reactive sequence whereby the photoirradiation dosage during processing determines the thiol and ene functional group conversion (x) in the first stage, which determines the amount of “soft” thiol-epoxy network created in the second stage, which consequently determines the amount of “stiff” epoxy network formed in the third stage. Thus, the applied photodosage directly dictates the extent of “soft” and “stiff” segments within the printed part. This precise control yields a diverse range of polymers from soft elastomers (Esoft ∼ 400 kPa, dL/L0 ∼ 300%) to glassy thermosets (Estiff ∼ 1.6 GPa, dL/L0 ∼ 3%). Unlike other photopatterned multimaterials, these sequential reactions always result in the full consumption of readily reacted groups during processing, which imparts environmental stability (Estiff/Esoft > 103 after the equivalent of ∼1,000 h of solar irradiation) to our final material. Not only do these mechanical properties approximate those found in commodity polymers (acrylics, polyurethanes, silicones, hydrogels) and anatomy (tendons, ligaments, skin, gastrointestinal tissues), but the photodosage-controlled, continuous stiffness gradients (0 ≤ d[Er,stiff/Er,soft]/dx ≤ 1,300 mm−1) produced from our system can also approximate those found in healthy human knee entheses and squid beaks. To demonstrate the capabilities that such biomimetic photopatterned multimaterials offer engineered systems, we use this chemistry to rapidly (tprint < 10 min) 3D print a soft wearable braille display. Designing a materials chemistry that can selectively produce soft and stiff materials requires consideration of the phenomenological origin of the mechanical properties. Solid polymeric materials are ensembles of macromolecular chains held together by both intra- and interchain interactions. At the microscale, the density and relative strength of these interactions determine how the polymer network deforms under a given load which, in turn, dictates the observed macro-scale mechanical properties. Thus, polymer properties highly depend on both backbone structure and chain arrangements. As a result, creating a stable, one-pot multimaterial chemistry is particularly challenging: the initial chemical composition of the formulation cannot change, and intentionally leaving unreacted precursors threatens long-term stability (e.g., the reactions may proceed after processing to alter properties). Instead, we leverage different polymerization mechanisms to yield drastically different crosslinking densities, viscoelastic properties, and, therefore, mechanical performance. Our resin (Figure 1A) combines a triallyl (-ene) species, a thiol mixture (9:1 M ratio of di- and tetra-thiol molecules), and a diepoxy. For simplicity, we use a 1:1:1 stoichiometric ratio between ene, thiol, and epoxide functional groups. This composition creates a three-stage curing chemistry: two step-growth reactions (thiol-ene and thiol-epoxy polymerizations) and a chain-growth polymerization (epoxy homopolymerization). As shown in Figure 1B, the first stage is a radical initiated thiol-ene photopolymerization that rapidly forms a loosely crosslinked, percolated network during printing. The second stage continues to build a soft network based on the step-growth thiol-epoxy polymerizations initiated by a thermally latent imidazole (Technicure LC-80) under modest temperatures (∼80°C). During the higher-temperature (∼120°C) third stage, the anionic homopolymerization of the rigid diepoxy species creates a stiff, highly crosslinked network, as reported in previous literature.34Konuray A.O. Fernández-Francos X. Ramis X. Analysis of the reaction mechanism of the thiol-epoxy addition initiated by nucleophilic tertiary amines.Polym. Chem. 2017; 8: 5934-5947https://doi.org/10.1039/c7py01263bCrossref Scopus (47) Google Scholar Key to this design is that this sequential reaction is effectively controllable—we can create separate curing “stages” during processing by simply applying different stimuli or leveraging differential reaction rates. The proposed stoichiometry and sequence of these reactions enables precise control over each stage of curing. As shown in Figure 1C, the photodosage (He) applied during the first stage dictates the percent conversion (x) of ene and thiol functional groups. In the second stage, the remaining molar fraction (1 − x) of thiols acts as a limiting reagent and reacts stoichiometrically with epoxide functional groups (i.e., 1:1 thiol/epoxide) through stepwise addition. Lastly, during the third stage, the residual epoxides (1 − [1 − x] = x) homopolymerize in a chain-growth fashion. When fully processed, our sequential reaction motif should consume all the thiols and epoxides regardless of x to minimize post-print aging due to continued polymerization. Thus, our chemical design suggests that varied photoexposure can produce stable but distinct local network microstructures and, consequently, multimaterial properties. We use Fourier transform infrared (FTIR) spectroscopy to infer the consumption of chemical groups over time (ene, thiol, and epoxide) and verify the control over the proposed three reactions (see experimental procedures and Figures S1 and S2 for more information). As shown in Figure 1C, the conversion of thiol and ene groups, x, depends on the applied photodosage (He) as described by the typical first-order reaction kinetic model shown in Equation 135Cramer N.B. Davies T. O’Brien A.K. Bowman C.N. Mechanism and modeling of a thiol-ene photopolymerization.Macromolecules. 2003; 36: 4631-4636https://doi.org/10.1021/ma034072xCrossref Scopus (186) Google Scholar:[x=1−e−0.02He].(Equation 1) Note that termination steps (radical recombination and chain transfer) rapidly quench any free radicals as evidenced by the thiol-ene conversion quickly plateauing after cessation of light (Figure S3).36Reddy S.K. Cramer N.B. Bowman C.N. Thiol-vinyl mechanisms. 2. Kinetic modeling of ternary thiol-vinyl photopolymerizations.Macromolecules. 2006; 39: 3681-3687https://doi.org/10.1021/ma0600097Crossref Scopus (73) Google Scholar Thus, control of the light engine (e.g., light-emitting diode [LED] projector) allows for spatiotemporal control of the chemical reaction. In Figures 1D–1F, the photodosage is varied during the first reactive stage followed by thermal curing at 80°C and 120°C. In the absence of light irradiation (Figure 1D), no thiol-ene addition occurs (x ∼ 0), which enables nearly all thiols to undergo nucleophilic addition to epoxides in the second step. Consequently, only a few remaining epoxides are available to undergo homopolymerization in the third stage. As shown in Figure 1E, at intermediate photodosages (He = 62.5 mJ cm−2), the photopolymerization consumes thiol and ene groups evenly (x ∼ 0.52). As expected, the residual thiols serve as a limiting reagent and consume equimolar epoxide functional groups in the second stage. The epoxide conversion appears to increase slightly even after the thiol conversion approaches 1. This increase suggests that additional reactions (i.e., epoxy-epoxy homopolymerization) can occur, albeit more slowly, at this temperature. However, differential scanning calorimetry (DSC) (Figure S4) confirms that the thiol-epoxide reaction remains dominant until full consumption of thiols: thiol-epoxy reactions are almost 10-fold faster than epoxide homopolymerization at 80°C, which is consistent with previous literature.37Fernández-Francos X. Konuray A.-O. Belmonte A. De La Flor S. Serra À. Ramis X. Sequential curing of off-stoichiometric thiol–epoxy thermosets with a custom-tailored structure.Polym. Chem. 2016; 7: 2280-2290https://doi.org/10.1039/c6py00099aCrossref Scopus (86) Google Scholar At the elevated temperatures of the third stage, the epoxy homopolymerization accelerates and approaches full conversion. Alternatively, a longer first-stage photodosage (He = 175 mJ cm−2) consumes 98% of the ene and thiol groups (Figure 1F). With few remaining thiols, the second-stage thiol-epoxy addition is minimal, and nearly all epoxides homopolymerize during the final step. In total, these experiments, as well as photorheology and thermal rheology (Figure S5), indicate that the sequential framework behaves as expected. After demonstrating the ternary reaction motif, we characterized the tensile properties resulting from different initial photodosages (see experimental procedures). Figure 2A contains the average (N ≥ 7) stress-strain plots for each photodosage (note the log-linear scaling). No photodosage (He ∼ 0 mJ cm−2), when only the loosely crosslinked thiol-epoxy network forms (x ∼ 0), produces a soft (E ∼ 410 kPa), stretchable material with ∼300% elongation (dL/L0). In contrast, at He ≥ 250 mJ cm−2 the resin sequentially forms a thiol-ene network (x ∼ 1) interpenetrated by a rigid epoxy homopolymer. When compared with the x ∼ 0 variant, this material is over 3,800 times stiffer (E ∼ 1.6 GPa) with 100-fold decrease in ultimate elongation (dL/L0 ∼3%). The resin exhibits intermediate behavior at the other photodosages—in general, as the applied photodosage and conversion (x) increases, the final material becomes stiffer and less stretchable. Our single-resin chemistry exhibits a profound diversity in mechanical properties that span a range comparable with that of commercial polymers and soft biological tissues. Figure 2B contextualizes this ternary sequential chemistry by comparing the modulus and ultimate elongation of biomaterials, commonly used polymers, and previous multimaterial photochemistries. 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