Laser powder bed fusion (LPBF) faces challenges when fabricating intricate structures with thin elements, comparable to the laser beam size, and thus requires precise deposition of melt spots. We devised a method for creating a Fe-15Mn-0.8C filigree structure via LPBF with a pulsed Gaussian laser beam. Initially, single melt spots were deposited to determine the optimal exposure time depending on laser power and laser spot size. Arrays of single melt tracks were then deposited, varying laser spot size, power, and point distance between spots, with the depth and width of each melt pool quantified. Through correlation analyses, key parameters were identified: exposure time and laser power for uniform melt spots, and laser power and point distance for melt pool depth. Utilizing the normalized enthalpy criterion, optimal parameter values were determined. We successfully LPBFfabricated fully dense, 80 mu m diameter pillars representing the simplest filigree structures and finally a complex stent structure with 99.5 % relative density. This approach can be adapted to other materials and additive manufacturing technologies using pulsed laser beams, streamlining experimentation.

The in vitro corrosion of Fe-30Mn-1C in a Ca-modified Hanks' balanced salt solution was investigated for biodegradable cardiovascular stents with a rotating disc electrode. A blood flow-related approach was developed to adjust laminar flow conditions. Electrochemical monitoring revealed that intermediate flow caused maximum corrosion, while static and high flow conditions led to minimal corrosion. Surface analysis indicated the formation of a non-crystalline, nanoscopic degradation layer, whereby thickness, porosity and layer-disrupting defects are strongly affected by flow conditions. The interplay between laminar flow and the degradation layer was crucial for the steel degradation and discussed by employing a hydrodynamics-based corrosion scheme.

Fully degradable stents that temporarily support healing tissue can solve potential long-term complications associated with permanent implants. In this study, we investigate austenitic high-manganese high-carbon steels as promising candidates for such biodegradable stent applications. The microstructural characteristics and mechanical properties of the Fe-Mn-C steels were systematically analysed to assess their suitability for this purpose. A series of four austenitic Fe-Mn-C steels with varying manganese (15 and 30 wt%) and carbon (0.6-1.0 wt%) content were processed by vacuum induction casting, annealing and hot forging. Microstructural analysis was performed, and the hot forged materials were further evaluated using ultrasound, macro hardness, and (stopped) quasi-static tensile tests to assess stent-related parameters. In contrast to the as-cast state, the hot forged steels exhibited high chemical homogeneity, as was found by energy-dispersive X-ray spectroscopy (EDXS). Electron backscatter diffraction (EBSD) revealed comparability between the four steels regarding their annealing twin fractions and grain size distributions. An austenitic microstructure was revealed for all modifications by transmission X-ray diffraction (XRD), except Fe-15Mn-0.6C, which displayed a minor epsilon-martensite fraction. Reducing manganese from 30 to 15 wt% resulted in favourable effects, including a higher Young's modulus, lowered offset yield strength, and an increased ultimate tensile strength due to larger strain hardening while preserving a high total elongation. However, reducing the carbon content to 0.6 wt% diminished the mechanical performance due to reduced solid solution strengthening and the epsilon-martensite formation. Among the investigated steels, the novel Fe-15Mn-0.8C alloy exhibited the most attractive combination of the studied properties for potential stent applications.

Biodegradable medical implants promise to benefit patients by eliminating risks and discomfort associated with permanent implantation or surgical removal. The time until full resorption is largely determined by the implant's material composition, geometric design, and surface properties. Implants with a fixed residence time, however, cannot account for the needs of individual patients, thereby imposing limits on personalization. Here, an active Fe-based implant system is reported whose biodegradation is controlled remotely and in situ. This is achieved by incorporating a galvanic cell within the implant. An external and wireless signal is used to activate the on-board electronic circuit that controls the corrosion current between the implant body and an integrated counter electrode. This configuration leads to the accelerated degradation of the implant and allows to harvest electrochemical energy that is naturally released by corrosion. In this study, the electrochemical properties of the Fe-30Mn-1C/Pt galvanic cell model system is first investigated and high-resolution X-ray microcomputed tomography is used to evaluate the galvanic degradation of stent structures. Subsequently, a centimeter-sized active implant prototype is assembled with conventional electronic components and the remotely controlled corrosion is tested in vitro. Furthermore, strategies toward the miniaturization and full biodegradability of this system are presented.

Implant-related infections or inflammation are one of the main reasons for implant failure. Therefore, different concepts for prevention are needed, which strongly promote the development and validation of improved material designs. Besides modifying the implant surface by, for example, antibacterial coatings (also implying drugs) for deterring or eliminating harmful bacteria, it is a highly promising strategy to prevent such implant infections by antibacterial substrate materials. In this work, the inherent antibacterial behavior of the as-cast biodegradable Fe69Mn30C1 (FeMnC) alloy against Gram-negative Pseudomonas aeruginosa and Escherichia coli as well as Gram-positive Staphylococcus aureus is presented for the first time in comparison to the clinically applied, corrosion-resistant AISI 316L stainless steel. In the second step, 3.5 wt % Cu was added to the FeMnC reference alloy, and the microbial corrosion as well as the proliferation of the investigated bacterial strains is further strongly influenced. This leads for instance to enhanced antibacterial activity of the Cu-modified FeMnC-based alloy against the very aggressive, wild-type bacteria P. aeruginosa. For clarification of the bacterial test results, additional analyses were applied regarding the microstructure and elemental distribution as well as the initial corrosion behavior of the alloys. This was electrochemically investigated by a potentiodynamic polarization test. The initial degraded surface after immersion were analyzed by glow discharge optical emission spectrometry and transmission electron microscopy combined with energy-dispersive X-ray analysis, revealing an increase of degradation due to Cu alloying. Due to their antibacterial behavior, both investigated FeMnC-based alloys in this study are attractive as a temporary implant material.

In the last decade, additive manufacturing technologies like laser powder bed fusion (LPBF) have emerged strongly. However, the process characteristics involving layer-wise build-up of the part and the occurring high, directional thermal gradient result in significant changes of the microstructure and the related properties compared to traditionally fabricated materials. This study presents the influence of the build direction (BD) on the microstructure and resulting properties of a novel austenitic Fe-30Mn-1C-0.02S alloy processed via LPBF. The fabricated samples display a {011} texture in BD which was detected by electron backscatter diffraction. Furthermore, isolated binding defects could be observed between the layers. Quasi-static tensile and compression tests displayed that the yield, ultimate tensile as well as the compressive yield strength are significantly higher for samples which were built with their longitudinal axis perpendicular to BD compared to their parallel counterparts. This was predominantly ascribed to the less severe effects of the sharp-edged binding defects loaded perpendicular to BD. Additionally, a change of the Young’s modulus in dependence of BD could be demonstrated, which is explained by the respective texture. Potentiodynamic polarization tests conducted in a simulated body fluid revealed only slight differences of the corrosion properties in dependence of the build design.