Additive manufacturing, commonly referred to as 3D printing (3DP), has ushered in a new era of advanced manufacturing that is seemingly limited only by imagination. In actuality, the fullest potentials of 3DP can only be realized through innovative breakthroughs in printing technologies and build materials. Whereas equipment for 3DP has experienced considerable development, molecular-scale programming of function, adaptivity, and responsiveness in 3DP is burgeoning. This review aims to summarize the state-of-the-art in stimuli-responsive materials that are being explored in 3DP. First, we discuss stimuli-responsiveness as it is used to enable 3DP. This highlights the diverse ways in which molecular structure and reactivity dictate energy transduction that in turn enables 3D processability. Second, we summarize efforts that have demonstrated the use of 3DP to create materials, devices, and systems that are in their final stage stimuli-responsive. This section encourages the artistic license of advanced manufacturing to be applied toward leveraging, or enhancing, energy transduction to impart device function across multiple length scales.
New advancements in 3D printing enable manufacturing a solid part with spatially controlled and varying material properties; this research seeks to establish techniques for finding optimal designs that use this new technology for the greatest structural benefit. We describe the use of a sequential quadratic programming based optimization solver to find an optimal distribution of material properties that minimize strain energy gradients, as calculated using finite element analysis. This design method is applied to the case of a flat thin plate with a hole, and has been proven to successfully reduce strain energy gradients and therefore stress concentrations. The optimally designed plates are 3D printed using a novel technology that uses vat polymerization technology. The computational model is validated with experiments. Enabling design engineers to customize material properties around geometric discontinuities will provide greater flexibility in reducing stress concentrations without modifying geometry or adding additional supports.
We used digital light processing additive manufacturing (DLP-AM) to produce mechanochemically responsive test specimens from custom photoresin formulations, wherein designer, flex activated mechanophores enable quantitative assessment of the total mechanophore activation in the specimen. The manufactured object geometries included an octet truss unit cell, a gyroid lattice, and an “8D cubic lattice”. The mechanophore activation in each test specimen was measured as a function of uniaxial compressive strain applied to the structure. Full shape recovery after compression was exhibited in all cases. These proof-of-concept results signify the potential to use flex activated mechanophore for nondestructive, quantitative volumetric assessment of mechanochemistry in test specimens with complex geometries. Additionally, the integration of DLP-AM with flex activated mechanophore build materials enabled the creation of customizable, three-dimensional mechanochemically responsive parts that exhibit small molecule release without undergoing irreversible deformation or fracture.
A series of photoresins suitable for production of elastomeric objects via digital light processing additive manufacturing are reported. Notably, the printing procedure is readily accessible using only entry-level equipment under ambient conditions using visible light projection. The photoresin formulations were found to be modular in nature and straightforward adjustments to the resin components enabled access to a range of compositions and mechanical properties. Collectively, the series includes silicones, hydrogels, and hybrids thereof. Printed test specimens displayed maximum elongations of up to 472% under tensile load, tunable swelling behavior in water, and Shore A hardness values from 13.7 to 33.3. A combination of the resins was used to print a functional multi-material three-armed pneumatic gripper. These photoresins could be transformative to advanced prototyping applications such as simulated human tissues, stimuli-responsive materials, wearable devices, and soft robotics.
We describe an efficient method to produce objects comprising spatially controlled and graded crosslink 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 crosslink densities and associated mechanical properties. Cylinder and bar test specimens were used to establish correlations between light intensities used for printing and crosslink 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.
This paper aims to explore and demonstrate the ability to integrate entry-level additive manufacturing (AM) techniques with responsive polymers capable of mechanical to chemical energy transduction. This integration signifies the merger of AM and smart 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.