Embedding near-infrared fluorescent single-walled carbon nanotubes within freeform fabricated hydrogels via continuous digital light processing

Abstract

Photoluminescent single-walled carbon nanotubes (SWCNTs) are emerging materials for developing bioimaging agents and sensors. Free-standing architectures which include 3D printed freeform hydrogels can be leveraged as versatile platforms for sensors. However, a key challenge in developing 3D-printed SWCNT-based optical probes and sensors is preserving their optical properties under demanding processing conditions, such as UV irradiation and in situ free radical polymerization used in 3D printing processes. This research leveraged a continuous digital light processing (CDLP) technique to fabricate fluorescent SWCNT-embedded freeform hydrogels. We found that integrating an aqueous suspension of SWCNTs did not inhibit the creation of 3D fabricated hydrogel objects. However, the photoluminescence of SWCNTs in 3D-printed objects was adversely impacted by the SWCNT functionalization type, hydrogel ink formulations, and the process parameters during CDLP. Upon screening the formulation and the CDLP conditions, we identified that SWCNTs non-covalently functionalized with poly(styrene-co-styrene sulfonate) (PS-co-PSS) maintained their characteristic NIR emission in the 3D printing ink formulation and upon exposure to the UV-curing process. More importantly, no apparent leaching of SWCNTs was observed after two weeks of soaking in deionized water. By tuning the hydrogel ink formulation, structures with desirable application-specific properties could be achieved, such as the hydrogel 9–70 composition (9 wt% crosslinker-to-monomer ratio and 70 wt% water) exhibiting stretchability increase of up to ∼142%. Both the hydrated and the dehydrated samples of key hydrogel ink formulations showed no evidence of cracking after 24 hours. This successful integration of NIR fluorescent SWCNTs into complex, freeform hydrogel architectures via CDLP represents a fundamental advance toward creating robust, portable, and versatile optical sensing platforms.