## 5 Conclusions  
We designed and built pure DNA hydrogels as well as composites using DNA/nanomaterial conjugate crosslinkers, DNA tile crosslinkers, and linear DNA spacers. We characterized the nanomaterial networks in the composites and examined their mechanical and electrical behaviors. We found that shorter spacers form more solid-like hydrogels when combined with pure DNA crosslinkers, while longer spacers construct more solid-like hydrogels when assembled with DNA/nanomaterial crosslinkers. We obtained hydrogel composites with significantly higher mechanical strength by combining DNA-CNT conjugates and up to 25% DNA tiles as crosslinkers. In addition, dried networks from both DNA-SWNT and DNA-AuNP conjugates and hydrogels show non-linear electrical behaviors. By comparing the conductivities of dehydrated networks from conjugates and hydrogels, we showed the ability of DNA self-assembly to integrate and connect percolating networks with nanotubes and nanoparticles. These initial examples of biomolecular functionality by design suggest that the basic concepts of DNA self-assembly can effectively be used to create more complicated materials. Potentially, crosslinkers such as DNA-wrapped CNTs can be used to create more sophisticated conjugates and nanostructures. We can design DNA to realize control in nanoelectronics morphology through connection and arrangement of nanomaterials. These materials have potential for applications in 3D integrated circuits and hardware with shorter production time, lower cost, lower power consumption, and higher energy efficiency. Eventually, electronic hardware utilizing 3D integration and assembled using DNA nanotechnology may achieve computing capabilities in certain operations beyond the performance currently achieved by circuits fabricated using traditional lithography techniques.  
The following are available online at [[https://www.mdpi.com/2076-341](https://www.mdpi.com/2076-341)]([https://www.mdpi.com/2076-341](https://www.mdpi.com/2076-341)) 7/11/5/2245/s1, Figure S1: Photos of (a) SWNT and (b) MWNT dispersions in deionized (DI) water without (left) and with (right) DNA modification. SWNTs and MWNTs without DNA modification settle at the bottom within 15 min after sonication; Figure S2: Photos of pure and DNA hydrogel composites constructed by SI with (from left to right) Y-shaped DNA tiles, X-shaped DNA tiles, DNA-SWNT conjugates, DNA-MWNT conjugates, and DNA-AuNP conjugates; Figure S3: UV-Vis spectrum of as-synthesized AuNPs. A peak \(\lambda_{\text{max}}\) at 519 nm wavelength proves that these are well-formed AuNPs with an average diameter of 13 nm. UV-Vis spectrum was recorded on a Thermo Scientific NanoDrop 2000c Spectrophotometer (Waltham, USA); Figure S4: The band of pure AuNPs (left) appears violet under visible light, while the band of DNA-AuNP conjugates (right) retains a red color and shows a much better mobility; Figure S5: Gold electrodes of 200 um spacing for electrical characterization; Table S1: Sequences of ssDNA for the preparation of crosslinkers and spacers; Table S2: Concentrations and ratios of crosslinkers and spacers to prepare for hydrogel.  
Conceptualization, T.H.L., A.K., and M.G.; methodology, T.H.L., A.K., L.C.H., and M.G.; formal analysis, M.G., A.M., and A.K.; investigation, M.G., A.M., and A.K.; Writing--Original draft preparation, M.G.; Writing--Review and editing, M.G., A.K., T.H.L., L.C.H., and A.M. All authors have read and agreed to the published version of the manuscript.  
This research was partially funded by the US National Science Foundation through grant NSF-CISE-CCF 1748459 to T.H.L.  
Not Applicable.  
The authors declare no conflict of interest.