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In this study, the hydraulic reactivity and cement formation of baghdadite (Ca\(_{3}\)ZrSi\(_{2}\)O\(_{9}\)) was investigated. The material was synthesized by sintering a mixture of CaCO\(_{3}\), SiO\(_{2}\), and ZrO\(_{2}\) and then mechanically activated using a planetary mill. This leads to a decrease in particle and crystallite size and a partial amorphization of baghdadite as shown by X-ray powder diffraction (XRD) and laser diffraction measurements. Baghdadite cements were formed by the addition of water at a powder to liquid ratio of 2.0 g/ml. Maximum compressive strengths were found to be ~2 MPa after 3-day setting for a 24-h ground material. Inductively coupled plasma mass spectrometry (ICP-MS) measurements showed an incongruent dissolution profile of set cements with a preferred dissolution of calcium and only marginal release of zirconium ions. Cement formation occurs under alkaline conditions, whereas the unground raw powder leads to a pH of 11.9 during setting, while prolonged grinding increased pH values to approximately 12.3.
Poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-co-TrFE)) is an electroactive polymer with growing interest for applications in biomedical materials and flexible electronics. In this study, a solvent-free additive manufacturing technique called melt electrowriting (MEW) has been utilized to fabricate well-defined microperiodic structures of the copolymer (P(VDF-co-TrFE)). MEW of the highly viscous polymer melt was initiated using a heated collector at temperatures above 120 °C and required remarkably slow collector speeds below 100 mm min\(^{-1}\). The fiber surface morphology was affected by the collector speed and an increase in β-phase was observed for scaffolds compared to the unprocessed powder. Videography shows vibrations of the P(VDF-co-TrFE) jet previously unseen during MEW, probably due to repeated charge buildup and discharge. Furthermore, piezo-force microscopy measurements demonstrated the electromechanical response of MEW-fabricated fibers. This research therefore achieves the melt electrohydrodynamic processing of fibers with micrometer resolution into defined structures with an important electroactive polymer.
The Multiweek Thermal Stability of Medical-Grade Poly(ε-caprolactone) During Melt Electrowriting
(2022)
Melt electrowriting (MEW) is a high-resolution additive manufacturing technology that places unique constraints on the processing of thermally degradable polymers. With a single nozzle, MEW operates at low throughput and in this study, medical-grade poly(ε-caprolactone) (PCL) is heated for 25 d at three different temperatures (75, 85, and 95 °C), collecting daily samples. There is an initial increase in the fiber diameter and decrease in the jet speed over the first 5 d, then the MEW process remains stable for the 75 and 85 °C groups. When the collector speed is fixed to a value at least 10% above the jet speed, the diameter remains constant for 25 d at 75 °C and only increases with time for 85 and 95 °C. Fiber fusion at increased layer height is observed for 85 and 95 °C, while the surface morphology of single fibers remain similar for all temperatures. The properties of the prints are assessed with no observable changes in the degree of crystallinity or the Young's modulus, while the yield strength decreases in later phases only for 95 °C. After the initial 5-d period, the MEW processing of PCL at 75 °C is extraordinarily stable with overall fiber diameters averaging 13.5 ± 1.0 µm over the entire 25-d period.
The current study aims to extend the material platform for anisotropically structured calcium phosphates to low-temperature phases such as calcium-deficient hydroxyapatite (CDHA) or the secondary phosphates monetite and brushite. This is achieved by the phase conversion of highly porous α-tricalcium phosphate (α-TCP) scaffolds fabricated by ice-templating into the aforementioned phases by hydrothermal treatment or incubation in phosphoric acid. Prior to these steps, α-TCP scaffolds are either sintered for 8 h at 1400 °C or remain in their original state. Both nonsintered and sintered α-TCP specimens are converted into CDHA by hydrothermal treatment, while a transformation into monetite and brushite is achieved by incubation in phosphoric acid. Hydrothermal treatment for 72 h at 175 °C increases the porosity in nonsintered samples from 85% to 88% and from 75% to 88% in the sintered ones. An increase in the specific surface area from (1.102 ± 0.005) to (9.17 ± 0.01) m2 g−1 and from (0.190 ± 0.004) to (2.809 ± 0.002) m2 g−1 due to the phase conversion is visible for both the nonsintered and sintered samples. Compressive strength of the nonsintered samples increases significantly from (0.76 ± 0.11) to (5.29 ± 0.94) MPa due to incubation in phosphoric acid.
The human body has very good self-healing capabilities for numerous different injuries to a variety of different tissues. This includes the main human mechanical framework, the skeleton. The skeleton is limited in its healing without additional aid by medicine mostly by the defect size. When the defect reaches a size above 2.5 cm the regeneration of the defect ends up faulty. Here is where implants, defect fillers and other support approaches developed in medicine can help the body to heal the big defect still successfully.
Usually sturdy implants (auto-/allo-/xenogenic) are implanted in the defect to bridge the distance, but for auto- and allogenic implants a suitable donor site must be found and for all sources the implant needs to be shaped into the defect specific site to ensure a perfect fit, the best support and good healing. This shaping is very time consuming and prone to error, already in the planning phase. The use of a material that is moldable and sets in the desired shape shortly after applying negates these disadvantages. Cementitious materials offer exactly this property by being in a pasty stage after the powder and liquid components have been mixed and the subsequently hardening to a solid implant. These properties also enable the extrusion, and therefore may also enable the injection, of the cement via a syringe in a minimal invasive approach.
To enable a good injection of the cement modifications are necessary. This work aimed to modify commonly used calcium phosphate-based cement systems based on α-TCP (apatitic) and β-TCP (brushitic). These have been modified with sodium phytate and phytic acid, respectively. Additionally, the α-TCP system has been modified with sodium pyrophosphate, in a second study, to create a storable aqueous paste that can be activated once needed with a highly concentrated sodium orthophosphate solution.
The powder phase of the α-TCP cement system consisted of nine parts α-TCP and one part CDHA. These were prepared to have different particle sizes and therefore enable a better powder flowability through the bimodal size distribution. α-TCP had a main particle size of 20 μm and CDHA of 2.6 μm. The modification with sodium phytate led to an adsorption of phytate ions on the surface of the α-TCP particles, where they started to form complexes with the Ca2+ ions in the solution. This adsorption had two effects. The first was to make the calcium ions unavailable, preventing supersaturation and ultimately the precipitation of CDHA what would lead to the cement hardening. The second was the increase of the absolute value of the surface charge, zeta potential, of the powder in the cement paste. Here a decrease from +3 mV to -40 mV could be measured. A strong value for the zeta potential leads to a higher repulsion of similarly charged particles and therefore prevents powder agglomeration and clogging on the nozzle during injection. These two modifications (bimodal particles size distribution and phytic acid) lead to a significant increase in the paste injectability. The unmodified paste was injectable for 30 % only, where all modified pastes were practically fully injectable ~90 % (the residual paste remained in the nozzle, while the syringe plunger already reached the end of the syringe).
A very similar observation could be made for the β-TCP system. This system was modified with phytic acid. The zeta potential was decreased even stronger from -10 ± 1.5 mV to -71.5 ± 12 mV. The adsorption of the phytate ions and subsequent formation of chelate complexes with the newly dissolved Ca2+ ions also showed a retarding effect in the cements setting reaction. Where the unmodified cement was not measurable in the rheometer, as the reaction was faster than the measurement setup (~1.5 min), the modified cements showed a transition through the gel point between 3-6 min. This means the pastes stayed between 2 and 4 times longer viscous than without the modification. Like with the first cement system also here the effects of the phytate addition showed its beneficial influence in the injectability measurement. The unmodified cement was not injectable at all, due to the same issue already encountered at the rheology measurements, but all modified pastes were fully injectable for at least 5 min (lowest phytate concentration) and at least 10 min (all other concentrations) after the mixing of powder and liquid.
The main goal of the last modification with sodium pyrophosphate was to create a paste that was stable in aqueous environment without setting until the activation takes place, but it should still show good injectability as this was the desired way of application after activation. Like before also the zeta potential changed after the addition of pyrophosphate. It could be lowered from -22 ± 2mV down to -61 to -68 ± 4mV (depending on the pyrophosphate concentration). The pastes were stored in airtight containers at room temperature and checked for their phase composition over 14 days. The unmodified paste showed a beginning phase conversion to hydroxyapatite between 7 and 14 days. All other pastes were still stable and unreacted. The pastes were activated with a high concentrated (30 wt%) sodium orthophosphate solution. After the activation the pastes were checked for their injectability and showed an increase from -57 ± 11% for the unmodified paste to -89 ± 3% (practically fully injectable as described earlier) for the best modified paste (PP005).
It can be concluded that the goal of enabling full injection of conventional calcium phosphate bone cement systems was reached. Additional work produced a storage stable paste that still ensures full injectability. Subsequent work already used the storable paste and modified it with hyaluronic acid to create an ink for 3D extrusion printing. The first two cement systems have also already been investigated in cell culture for their influence on osteoblasts and osteoclasts. The next steps would have to go more into the direction of translation. Figuring out what properties still need to be checked and where the modification needs adjustment to enable a clinical use of the presented systems.