Biodegradable iron-based scaffolds : developing a replication method using additive manufacturing

Abstract

Orthopaedic trauma patients may require a load bearing scaffold to assist their recovery. Ideally such a scaffold would be biodegradable, with its degradation rate matching that of bone growth and with pore diameter in the range of 100 µm to 800 µm. Research being carried out on iron-based scaffolds suggests that this can be achieved. This work is aimed to develop a reliable fabrication process for biodegradable iron scaffolds, based on the replication method combined with stereolithography (SLA) 3D printing. The replication method is a powder metallurgy technique which uses a perishable polymer template that is coated with a slurry containing the desired iron-based final material. Instead of using said slurry, a dry coating technique was developed which made use of the inherent tackiness of the 3D printed polymer templates, to attach the powder. The metallic coated polymer template is then heat treated at a low temperature to partially sinter the powder coating to form an interconnected lattice. This is then followed by a high temperature heat treatment to completely burn away the polymer template and fully sinter the metallic scaffold implant. In this work, the technique was developed further by incorporating SLA 3D printing to produce the polymeric templates thus making it possible to produce patient specific scaffolds at a very low price. Two template types were developed namely, cubic and gyroid type templates. To develop this adapted replication method, the SLA 3D printing polymer was analysed using dynamic mechanical analysis, differential scanning calorimetry and furnace heat treatments, to determine the softening and degradation temperatures. The 3D printed templates were analysed using optical microscopy and scanning electron microscopy to analyse their strut and pore size. Coated templates were subsequently analysed using weighted coating mass uptake and X-ray Microscopy. Scanning electron microscopy with electron dispersive spectroscopy was employed to characterise the powder used and the final heat-treated iron lattices. For both template types, the minimum achievable pore and strut size was 600 µm and 420 µm respectively. The optimal pore and strut size was set to 1000 µm and 700 µm, to minimise pore clogging for gyroid templates and to cater for the shrinkage experienced during heat treatment. The best heat treatment achieved used milled iron powder (particle diameter about 1.5 µm), coated using the dry coating method and heat treated with the first dwell at 175°C for 2 hours and a final dwell at 1120°C for 3 hours.