## 3 Results
### Rheological Properties of DNA Hydrogels  
Polymeric hydrogels generally demonstrate robust mechanical strength because of their dense, entangled, and crosslinked networks with small mesh sizes. Unlike these conventional hydrogels, pure DNA hydrogels are more thixotropic and can display poor mechanical strength [46]. Although the mechanical properties of DNA hydrogels can be fine-tuned by adjusting the type and concentration of initial DNA tiles with different numbers of branches, even the toughest DNA hydrogel only exhibits a storage modulus of a few thousand Pa [46]. Because of this property, the applications of DNA hydrogels are limited to only certain fields. To explore further enhancement of DNA hydrogels' mechanical strength, we implemented two strategies: modifying the DNA building blocks and fortifying the structure with novel nanomaterials that confer mechanical rigidity. We performed small-amplitude oscillatory rheology to understand the gelation properties of pure and DNA hydrogel composites. The storage modulus (G') and loss modulus (G\") represent the elastic and viscous contributions to the total stress. Viscoelastic materials with solid-like properties are formed due to internal crosslinks within the materials; crosslinking can come from chemical bonds or physical-chemical interactions between individual molecules [47; 48]. Using the testing conditions described in the methods session, we performed oscillation measurements on hydrogels formed by Y-shaped DNA tiles, X-shaped DNA tiles, DNA-SWNT conjugates, DNA-MWNT conjugates, and, finally, DNA-AuNP conjugates with spacers of three different lengths. The goals of this group of tests were to study the influence of length of spacers on different types of crosslinkers, as well as to compare the influence of different crosslinkers. For all tested samples, we saw a higher storage modulus (G') than loss modulus (G'') across the tested angular frequencies, as shown in Figure 3, demonstrating solid-like behavior, which is typically observed for hydrogels constructed with DNA [49; 50]. As Figure 3a shows, when constructing pure DNA hydrogels with the same crosslinkers (X or Y), using shorter spacers gives more solid-like hydrogels as indicated by the higher storage modulus. The X-shaped DNA tiles also construct more solid-like hydrogels than the Y-shaped DNA tiles with all types of spacers. However, when using conjugates as crosslinkers (see below), longer spacers construct more solid-like hydrogels, opposite to the behavior observed from pure DNA hydrogels. The mechanical strengths of pure hydrogels are also improved by integrating nanomaterials. With the same spacers (SI), DNA-SWNT conjugates also construct more solid-like hydrogels than DNA-MWNT conjugates, and both DNA-CNT conjugates make more solid-like hydrogels than DNA-AuNP conjugates (Figure 4). Figure 4: Storage modulus and loss modulus (G’, G”) vs. angular frequencies showing the rheological properties of (**a**) hydrogel composites constructed with DNA-SWNT conjugates, X- (or Y-) shaped DNA tiles, and long spacer (Sl); and (**b**) hydrogel composites constructed with DNA-MWNT conjugates, X- (or Y-) shaped DNA tiles, and Sl. Figure 3: Storage modulus and loss modulus (G’, G”) vs. angular frequencies showing the rheological properties of (**a**) pure DNA hydrogels, (**b**) DNA–SWNT and DNA–MWNT hydrogels, and (**c**) DNA–AuNP hydrogels constructed with spacers of different lengths. Our next set of tests was to use mixed crosslinkers, combining DNA-CNT conjugates and DNA tiles. The objective of these tests was to show the influence of different crosslinker compositions and to see how mixed crosslinkers of different length scales change the mechanical properties of the final hydrogels. Specifically, we substituted 25%, 50%, and 75% of the DNA-CNT conjugates from the previous test with X- or Y-shaped DNA tiles while keeping the total concentration of sticky ends from all crosslinkers the same. Only long spacer (Sl) was used for these mixed crosslinker tests.