3D inkjet printing provides a vital tool to deposit biomaterials with high precision (in the micrometer range) to address the complexity required for 3D scaffolds fabrication. However, its implementation is hindered by the lack of inks that exhibit properties optimal for tissue regeneration, and meet the properties for the demanding inkjet printing process. We address this challenge by engineering a hydroxyapatite nanoparticle ink (nHAp) as well as a UV-curable biodegradable polymer (BDP) based on thiol-yne click chemistry.
nanoXIM (Fluidinova), is a synthetic aqueous hydroxyapatite suspension containing rod-shaped nanoparticles. It has been used to produce bone regenerative materials and is biocompatible and osteoinductive. However, its suitability as an inkjet ink has not been proved yet.
The BDP system comprises of 4 monomers; a trifunctional thiol crosslinker, an aliphatic di-alkyne ether, a viscosity modifier and a phosphorus-based monomer. This phosphorus monomer is based on a highly versatile system and we developed a monomer library which enables us to create BDPs with a range of properties (mechanical, degradation behaviour).
Several formulations of nHAp and polymer inks were developed and their viscosity and surface tension were systematically evaluated using a viscosimeter or rheometer and a Drop Shape Analyzer. Those formulation meeting the properties for the inkjet process were then tested in a lab printer (DIMATIX DMP2800). In case of the polymer ink, the formulation was also tested with an industrial printhead (Pixodro LP50 with a Spectra SL128).
The stability of nHAp formulation was evaluated by means of long-term sedimentation behavior using Turbiscan equipment. The solid content was determined gravimetrically. The degradability of the monomers and cured BDP is analyzed by NMR spectroscopy and mass loss tests.
nanoXIM resulted in a promising material for inkjet inks after tuning its viscosity, surface tension and agglomeration behaviour with additives. nHAp ink was stable for at least three weeks. Some phase separation was observed which could be avoided by using recirculating ink supply systems. nHAp solid contents of the inks varied from 2 to 8 %wt. The ink can be used as a coating, therefore different patterns were successfully inkjet printed on top of the BDP.
3D features including pillars, holes and walls were successfully printed with the UV-curable polymer formulation without phosphorus monomers using an industrial printhead. Degradation tests confirmed that the addition of phosphorus monomers/mixed formulations lead to a degradable polymer formulation.
Two new experimental inks were successfully developed and their suitability for an inkjet process was demonstrated. Already proven biocompatible nHAp suspensions were formulated into an inkjet ink and used on a lab scale printing process. The potential for upscaling was demonstrated opening the way to the use in an inkjet industrial process. Several new BDP ink formulations were tested. One of them shows very good performance in a 3D inkjet printing process using industrial printheads. The degradation behaviour can be tuned with the addition of phosphorous monomers. Compatibility of the two inks is demonstrated. Further development of the use in a 3D multi-material process is ongoing.