3D printing offers unique opportunities to combine materials, geometries, and functionalities, but integrating multiple materials remains a significant challenge. Vat polymerization, a polymer chemistry-based approach, presents a pathway to overcome this limitation. Existing techniques in vat polymerization leverage light's characteristics to pattern materials, including using multiple wavelengths and orthogonal chemistries, manipulating light intensity and oxygen inhibition, or incorporating photochromic molecules. However, these methods often have limitations in terms of complexity or nanoscale control. This research proposes and demonstrates a new approach utilizing polymerization-induced phase separation (PIPS) within the vat polymerization process (3D PIPS) to achieve precise control over material placement in 3D printed objects. By carefully adjusting resin formulation to balance gelation kinetics, crosslinking density, and material diffusion rates, the method allows for the creation of functional coatings, gradients, and composites. The ability to generate nanoscale material phases within macroscale 3D designs is a significant advantage compared to other nanoscale printing techniques. This work explores the relationship between resin formulation and PIPS outcomes, providing insights for controlling material placement and creating functional 3D objects with tailored properties. The potential applications are extensive, ranging from conductive structures in electronics to objects with embedded antimicrobial agents. The ultimate goal is to enhance the design freedom and functional complexity achievable through 3D printing, leading to advancements in areas such as structural electronics, soft robotics, and the Internet of Things.

The authors review existing methods for multi-material 3D printing, highlighting the limitations of current approaches in vat polymerization. Existing techniques utilize light-based control to achieve spatial variations in material properties, including the use of two wavelengths for orthogonal polymerizations, light intensity modulation for crosslinking density control, and photochromic molecules for bio-inspired structures. The authors compare their approach to established techniques such as two-photon polymerization, localized electroplating, and metal ion reduction, emphasizing the advantages of 3D PIPS in achieving both macro- and micro-scale designs with nanoscale material phases. The use of phase separation in holographic polymerization is also reviewed, laying the groundwork for the adaptation to 3D printing.

The researchers investigate the influence of resin formulation on polymerization-induced phase separation (PIPS) in vat polymerization. They employ photoresins containing a silver precursor (a mixture of silver neodecanoate (AgND) and 2-ethyl-2-oxazoline) as a functional, non-polymerizable component. Different concentrations of polyethylene glycol (PEG) diacrylates with varying PEG spacer lengths (DA-170, DA-250, DA-575, DA-700) are used as crosslinkers, combined with 2-ethylhexyl acrylate (EHA) as a monomer. A 25 wt.% silver precursor concentration is chosen based on previous studies showing optimal electrical conductance. Cylinders (1.5 mm diameter, 2 cm length) are 3D printed using these resins and subsequently sintered at 210 °C to convert the silver precursor to metallic silver. The resulting morphologies are characterized using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). EDS line scans across cross-sections of the cylinders determine silver concentration profiles. The effect of crosslinker concentration on silver distribution, surface silver content, and electrical resistance is analyzed. Gelation time (delay time, td) is measured using phase contrast optical microscopy to determine the time required for a resin to form a gel. The relationship between td, crosslinker molecular weight, and surface silver content is explored. To investigate the influence of crosslinking density on silver diffusivity during polymerization, coarse-grained Langevin dynamics simulations are conducted. These simulations track the displacement of a probe molecule (representing silver neodecanoate) in systems with different crosslinker lengths (L = 3, 6, and 9). Finally, three applications are demonstrated: strain sensors, dipole antenna arrays, and antimicrobial objects, highlighting the versatility of the 3D PIPS method. For strain sensors, truss structures with graded silver compositions are 3D printed and tested for piezoresistive properties. Dipole antennas are fabricated, and their radiation pattern and gain are measured using an anechoic chamber. Antibacterial activity is evaluated using a halo inhibition zone test and bacterial growth kinetic studies.

The study demonstrates that the spatial distribution of silver within 3D-printed objects can be precisely controlled by manipulating resin formulation. Resins with low crosslinker concentrations and slow gelation kinetics lead to silver accumulation at the surface, forming a distinct coating. Conversely, high crosslinker concentrations and rapid gelation result in a more uniform silver distribution throughout the object. Intermediate conditions produce silver concentration gradients. The length of the PEG spacer in the diacrylate crosslinker significantly influences silver distribution, with longer spacers producing more gradual gradients. EDS analysis shows a clear correlation between crosslinker concentration, surface silver content, and electrical resistance. Gelation time measurements reveal that longer gelation times allow for greater silver migration to the surface. Langevin dynamics simulations support the experimental findings, showing that tighter networks formed by shorter crosslinkers impede silver diffusion. The 3D PIPS method is successfully applied to create functional 3D objects. Piezoresistive strain sensors exhibit tunable sensitivity based on resin formulation, achieving gauge factors comparable to 2D sensors. Dipole antenna arrays demonstrate efficient 2.4 GHz wave transmission, with radiation patterns closely matching theoretical predictions. 3D printed objects with embedded silver nanoparticles show significant antibacterial activity against E. coli. This confirms that the controlled phase separation allows the creation of functional coatings and graded materials with various compositions.

The findings demonstrate that 3D PIPS offers a powerful and versatile method for creating functional 3D-printed objects with controlled material placement. The ability to tune the morphology from a distinct coating to a homogeneous composite or a graded distribution opens new avenues for designing and fabricating complex, multifunctional materials. The successful application in creating strain sensors, antennas, and antimicrobial objects validates the practical potential of this approach. The tunable sensitivity of the strain sensors demonstrates the advantages of controlling phase separation for tailoring material properties. The close agreement between measured and theoretical radiation patterns of the 3D-printed antennas highlights the accuracy and precision achievable with this method. The antimicrobial properties of the 3D-printed objects with embedded silver nanoparticles showcase potential applications in medical and other fields. The use of simulations complements the experimental results, providing valuable insights into the underlying mechanisms governing material distribution during polymerization. The simplicity and versatility of 3D PIPS make it a promising technique for widespread adoption in various applications.

This study successfully demonstrates the efficacy of 3D PIPS as a single-step method to generate functional coatings and compositionally graded materials for 3D-printed objects. The approach offers precise control over material placement by harnessing the dynamics of polymerization-induced phase separation. The successful fabrication of functional devices such as strain sensors, antennas, and antimicrobial objects highlights its significant potential for creating complex, multi-material 3D printed parts. Future research could explore the use of different functional materials, optimize resin formulations for specific applications, and further refine the predictive modeling of material distribution during 3D PIPS.

The current study focuses primarily on silver as a functional material. While the principles of 3D PIPS are broadly applicable, further research is needed to investigate the effectiveness with a wider range of materials. The simulations, while insightful, are simplified representations of a complex process. More sophisticated models incorporating the polymerization kinetics more accurately would be beneficial for enhancing predictive capabilities. The antibacterial tests were conducted with E. coli; further studies are necessary to assess the efficacy against other bacteria and viruses.