Additive Manufacturing of Catalytically Active Living Materials.

Living materials, which are composites of living cells residing in a polymeric matrix, are designed to utilize the innate functionalities of the cells to address a broad range of applications such as fermentation and biosensing. Herein, we demonstrate the additive manufacturing of catalytically active living materials (AMCALM) for continuous fermentation. A multi-stimuli-responsive yeast-laden hydrogel ink, based on F127-dimethacrylate, was developed and printed using a direct-write 3D printer. The reversible stimuli-responsive behaviors of the polymer hydrogel inks to temperature and pressure are critical, as they enabled the facile incorporation of yeast cells and subsequent fabrication of 3D lattice constructs. Subsequent photo-cross-linking of the printed polymer hydrogel afforded a robust elastic material. These yeast-laden living materials were metabolically active in the fermentation of glucose into ethanol for 2 weeks in a continuous batch process without significant reduction in efficiency (∼90% yield of ethanol). This cell immobilization platform may potentially be applicable toward other genetically modified yeast strains to produce other high-value chemicals in a continuous biofermentation process.

Production of Materials with Spatially-Controlled Cross-Link Density via Vat Photopolymerization.

We describe an efficient method to produce objects comprising spatially controlled and graded cross-link densities using vat photopolymerization additive manufacturing (AM). Using a commercially available diacrylate-based photoresin, 3D printer, and digital light processing (DLP) projector, we projected grayscale images to print objects in which the varied light intensity was correlated to controlled cross-link densities and associated mechanical properties. Cylinder and bar test specimens were used to establish correlations between light intensities used for printing and cross-link density in the resulting specimens. Mechanical testing of octet truss unit cells in which the properties of the crossbars and vertices were independently modified revealed unique mechanical responses from the different compositions. From the various test geometries, we measured changes in mechanical properties such as increased strain-to-break in inhomogeneous structures in comparison with homogeneous variants.

3D-printed mechanochromic materials.

We describe the preparation and characterization of photo- and mechanochromic 3D-printed structures using a commercial fused filament fabrication printer. Three spiropyran-containing poly(ε-caprolactone) (PCL) polymers were each filamentized and used to print single- and multicomponent tensile testing specimens that would be difficult, if not impossible, to prepare using traditional manufacturing techniques. It was determined that the filament production and printing process did not degrade the spiropyran units or polymer chains and that the mechanical properties of the specimens prepared with the custom filament were in good agreement with those from commercial PCL filament. In addition to printing photochromic and dual photo- and mechanochromic PCL materials, we also prepare PCL containing a spiropyran unit that is selectively activated by mechanical impetus. Multicomponent specimens containing two different responsive spiropyrans enabled selective activation of different regions within the specimen depending on the stimulus applied to the material. By taking advantage of the unique capabilities of 3D printing, we also demonstrate rapid modification of a prototype force sensor that enables the assessment of peak load by simple visual assessment of mechanochromism.