Rapid synchronized fabrication of vascularized thermosets and composites

Abstract

Bioinspired vascular networks transport heat and mass in hydrogels, microfluidic devices, self-healing and self-cooling structures, filters, and flow batteries. Lengthy, multistep fabrication processes involving solvents, external heat, and vacuum hinder large-scale application of vascular networks in structural materials. Here, we report the rapid (seconds to minutes), scalable, and synchronized fabrication of vascular thermosets and fiber-reinforced composites under ambient conditions. The exothermic frontal polymerization (FP) of a liquid or gelled resin facilitates coordinated depolymerization of an embedded sacrificial template to create host structures with high-fidelity interconnected microchannels. The chemical energy released during matrix polymerization eliminates the need for a sustained external heat source and greatly reduces external energy consumption for processing. Programming the rate of depolymerization of the sacrificial thermoplastic to match the kinetics of FP has the potential to significantly expedite the fabrication of vascular structures with extended lifetimes, microreactors, and imaging phantoms for understanding capillary flow in biological systems.

Fig. 1. Overview of synchronized polymerization and vascularization concept.

a Schematic representation of the coordinated fabrication of a microvascular structure. ➀A sacrificial polymer template is embedded inside a liquid or gelled host matrix. ➁A self-propagating exothermic polymerization reaction transforms the host into a solid matrix and concurrently depolymerizes the sacrificial template into small molecules through a localized increase in temperature. ➂A durable thermoset with vasculature mirroring the starting template is manufactured under standard ambient conditions within minutes. b Scheme for FP of DCPD monomer (1) into crosslinked pDCPD (2) using a second-generation Grubbs’ catalyst (GC2) and tributyl phosphite inhibitor (TBP). c Scheme for photoacid generator (PAG) catalyzed thermal depolymerization of PPC (3) into PC monomer (4).

Fig. 2. Experimental characterization and thermochemical modeling of the coordinated vascularization process.

a Optical image of propagating FP reaction in a DCPD gel (α₀ = 0.25) in tandem with depolymerization of an embedded PPC (1% PAG) template to create vascular pDCPD (t = 31 s after initiation). b Front velocity (black triangles) and maximum front temperature (red squares) during FP as a function of α₀ of the DCPD gel. Successful VaSC through complete depolymerization of PPC depends on the amount of heat released during FP, which decreases with increasing α₀. Sacrificial templates undergo complete depolymerization for α₀ ≤ 0.25. Partial depolymerization results in clogged microchannels for 0.30 ≤ α₀ ≤ 0.35 and FP is no longer possible for α₀ ≥ 0.40. Error bars represent one standard deviation from the mean (n = 3). c Simulation of a DCPD gel (α₀ = 0.25) showing the spatial distribution of the degree of cure (α) of DCPD and the degree of depolymerization (β) of the sacrificial template during FP (t = 31 s after initiation). d Predicted spatial variation of α and β in the direction of the propagating front (t = 31 s after initiation). Successful polymerization and vascularization are defined by α and β reaching 0.90 or higher, as shown by the α₀ = 0.25 case (orange lines). The α₀ = 0.35 case (blue lines) shows successful polymerization but unsuccessful vascularization, corroborating experimental observations.

Fig. 3. Advanced manufacturing of microvascular structures.

a–f On-demand modulation of the vascular architecture in free-standing elastomeric DCPD gels (α₀ = 0.25). a Optical image of a helical sacrificial template (diameter = 384 ± 10 μm) embedded in a DCPD gel prior to FP. b μCT reconstruction of the resulting microchannel (diameter = 404 ± 12 μm) filled with a radiocontrast fluid for the sample region encompassed by the dashed box in a. c Microchannel inside a horseshoe-shaped pDCPD structure traces the curvature of the matrix. d–f Concurrent polymerization and vascularization under oscillatory loading create sinusoidal microchannels. Representative optical images of FP under oscillatory strain (frequency = 0.25 Hz, amplitude = 4 mm) and resulting vascularized structure are shown in d and e, respectively. f µCT reconstructions of microchannels filled with a radiocontrast fluid show an increase in out-of-plane deformation for decreasing oscillation frequency. g Optical image and micrograph of the cross section of a void-free composite with a circular microchannel (dashed orange line) fabricated in 30 s under ambient conditions.

Fig. 4. Synchronized manufacturing of a structure with hierarchical vascular network.

a An Impatiens leaf. b 3D-printed PPC (1% PAG) template referencing veins of leaf in a. c Simultaneous polymerization and vascularization of DCPD gel (α₀ = 0.25) with the embedded 3D-printed PPC (1% PAG) template. Depolymerization products escape through the central vein, completing the vascularization process in 80 s. d Optical image of the vascular structure and its µCT reconstruction (inset) show interconnectivity in the resulting microchannels.