Here, we present a method to contract dextran hydrogels using a covalent secondary crosslink strategy using g-irradiation as an external stimulus. We use chemically crosslinked dextran-based hydrogels that are modified with extra alkyne functionalities on the dextran backbone. We started from dextran (MW = 500 kDa) that is randomly modified with terminal alkyne side chain groups (degree of substitution = 36%, Figure ) using propargyl glycidyl ether chemistry. We then formed a hydrogel by chemically crosslinking a fraction of the alkyne moieties (theoretical maximum is 8%) via copper catalyzed azide alkyne cycloaddition with a bis-azide-PEG19 crosslinker (Figure ). This procedure afforded transparent, self-supporting hydrogels with a storage modulus (G') of 1.6 * 10 3 Pa and tan d (G''/G') of 8.0 * 10 -3 (Figure ). Next, we exposed centimeter-sized gel cubes to g-irradiation from a 60 Co source, at doses up to 14.4 kGy. We observed that the gel cubes would shrink considerably from the start of the experiment with increasing dose, and that these contracted gels had an increased stiffness (Figure and). Control hydrogels remained virtually unchanged during the course of the 24-hour experiment, whereas the irradiated hydrogels linearly reduced in weight (Figure ). The contraction rate of hydrogels swollen in PB is lower (Figure , blue data) compared to the contraction rate of hydrogels swollen in demineralized water (Figure , black data). After 24 hours irradiation (14.4 kGy) the weight reduction of the PB swollen gels and the demineralized water swollen gels is 39% and 74%, respectively. Figure and 2G show a set of photographs that illustrate the volume reduction. Figure shows a transparent water swollen hydrogel at the start of the g-irradiation experiment (left) and the contracted hydrogel after 24 hours of g-irradiation (right). During hydrogel contraction, the gels remain transparent and contract in all three dimensions equally, holding their cubic shape. A similar but less pronounced effect is observed for the PB swollen hydrogel (Figure , left the hydrogel at t = 0 and Photographs taken before (left) and after 24 h irradiation (right). H) SEM image taken before irradiation. I) SEM image taken after right at t = 24 hours g-irradiation). We then conducted a frequency sweep experiment on the rheometer to determine the rheological properties of the hydrogel before and after g-irradiation. The storage modulus (G') increased a factor 3-fold (PB swollen) or 2-fold (H2O swollen) after 24 hours of irradiation, indicating a more elastic material which is probably a result of additional crosslink formation (Figure and S3). Additionally, the hydrogels were analyzed by scanning electron microscopy (SEM) before and after g-irradiation. SEM analysis revealed that the hydrogels have multiple morphologies which have different pore sizes (Figure ). To demonstrate the effect of g-irradiation on the micro-scale sized pores, a similar area in a non-irradiated hydrogel and an g-irradiated hydrogel are shown in Figure , I. In the non-irradiated control hydrogel, the pore structure has a more open character compared to the g-irradiated hydrogel. This implies that additional crosslinks are formed induced by g-irradiation which is in agreement with the increased hydrogel stiffness (Figure and). To get more insight in the mechanism of g-irradiation induced contraction, a control experiment was conducted in which solutions of unmodified dextran (500 kDa, 10 wt% in H2O) and alkyne modified dextran were subjected to girradiation ( 60 Co source, 0.6 kGy/h) (Figure ). No changes could be observed for the unmodified dextran solution. In contrast, we found that the alkyne modified dextran solution gels overnight (14.4 kGy), which implies that the alkyne functionalities are crucial for hydrogel formation, and thus likely play a role in the observed contraction and increased stiffness. Crosslink formation may occur via the formation of reactive terminal alkyne radicals, which could be generated directly by g-irradiation or indirectly via the reaction products which emerge from water radiolysis. The main products of water radiolysis are hydrated electrons, HO• (hydroxyl radical), H2, H2O2 and the H2O• (hydroperoxyl radical).