Print-parameter control of cure state in DLP-printed biocarbon/ABS-like photopolymer composites: A proof-of-concept study of microstructure and acoustic impedance toward ultrasound-visible tissue markers

Abstract

Image-guided breast biopsy relies on an implanted marker that must remain locatable across ultrasound, mammography and MRI, yet current devices force a compromise: metallic markers corrupt MRI and fade on ultrasound, whereas polymer markers are MRI-friendly but carry a fixed, mould-set acoustic response. This work addresses a narrower, process-level question: in a fixed-composition particulate photopolymer composite, how far do the digital light processing (DLP) print parameters alone govern the cure state, microstructure and bulk acoustic impedance of the printed part? Biocarbon from pyrolysed woody biomass (mean particle size ≈ 15 µm) was dispersed at 2.5 wt% in an ABS-like acrylate resin (the highest loading at which the process would run reliably) and printed at two layer thicknesses (0.025 and 0.050 mm) over exposure times of 50–200 s, with neat resin (18 s) as reference. That carbon absorbs strongly at the 405 nm curing wavelength is an established constraint for photoabsorbing fillers in vat photopolymerisation rather than a question addressed here; the study instead tests how far the resulting absorption-limited cure can be offset by raising the delivered radiant exposure and shortening the optical path through the layer. Tensile testing, scanning electron microscopy, light microscopy, three-dimensional topography, automated surface analysis and X-ray micro-computed tomography were applied across ten conditions. The tensile modulus of the 0.025 mm series rose monotonically from ≈ 0.9–1.2 GPa between 50 and 200 s, whereas the 0.050 mm series remained low and largely exposure-insensitive. Properties governed by surface and void formation (three-dimensional relief and micro-CT porosity) did not share this monotonic behaviour but passed through intermediate optima; this difference is reported rather than smoothed over, and is itself the more informative result. Pulse-echo measurements at 4 MHz placed the acoustic impedance in a narrow band (Z ≈ 2.77–3.00 MRayl); across the whole set this spread corresponds to a difference of approximately 1 dB in reflected intensity at a marker–tissue interface, which is a small acoustic contrast. A single ex vivo B-mode acquisition in turkey breast is presented as a feasibility demonstration that a marker of this geometry and composition returns a detectable, well-localised echo; it is not offered as evidence ranking formulations. Monomer conversion was not determined spectroscopically or calorimetrically, and no claim about a measured degree of cure is made. Within these bounds the study shows that the print recipe is a substantive and under-used lever on the state of a filled photopolymer, and sets out the specific measurements required to extend the work toward a qualified device.