Laser beam melting of immiscible FeMn-AgX for adapted bioresorbability (eBook)

Journal of Alloys and Compounds 871 (2021) 159544

Novel AgCa and AgCaLa alloys for Fe-based bioresorbable implants with adapted degradation

Jan Tobias Krügera,1, Kay-Peter Hoyer , Viviane Filorb, Sudipta Pramanika, Manfred Kietzmannb, Jessica Meißnerb, Mirko Schapera

a Chair of Materials Science, Paderborn University, Mersinweg 7,33100 Paderborn, Germany

b Department of Pharmacology, Toxicology and Pharmacy, University of Veterinary Medicine Hanover, Foundation, Buenteweg 17, 30559 Hanover, Germany

ARTICLE INFO

Article history:

Received 3 November 2020

Received in revised form 18 February 2021

Accepted 13 March 2021

Available online 19 March 2021

Keywords:

Silver alloys

Corrosion

Biomedical application

Laser beam melting

Bioresorbable metal

Cytocompatibility

Iron-based alloys

ABSTRACT

Resorbable implants are highly beneficial to reduce patient burden since they need not be removed after a defined period. Currently, magnesium (Mg) and polymers are being applied as bioresorbable materials. However, for some applications the insufficient mechanical properties and high degradation rate of Mg cause the need for new materials. Iron (Fe)-based alloys are promising due to their biocompatibility and good mechanical properties, but their degradation rate is too low and needs to be adapted eg. via alloying with manganese (Mn). Besides, phases with high electrochemical potential lead to increased degradation of residual material with lower potential based on the galvanic coupling. Here, silver (Ag) is promising for the formation of such phases due to its high electrochemical potential (+0.8 V vs. SHE), immiscibility with Fe, biocompatibility, and anti-bacterial properties. Since remaining silver particles can lead to adverse consequences as thrombosis, these particles need to dissolve after the matrix material. Thus a silver alloy with high electrochemical potential, biocompatibility, and adjusted degradation behavior is required as an additive for iron-based bioresorbable materials. Several silver alloying systems are possible, but regarding the electrochemical potential and degradation behavior of binary alloys, calcium (Ca) and lanthanum (La) are best-suited considering their biocompatibility. Accordingly, this research addresses AgCa and AgCaLa alloys as additives for iron-based degradable materials with adapted degradation behavior

© 2021 Elsevier B.V. All rights reserved.

1. Introduction

Iron (Fe)-based alloys are in focus as bioresorbable materials for implants, such as stents or osteosynthesis plates [1-4]. Since iron is essential for the human body and shows only negligible toxicity it fulfills the requirements for biocompatibility 5,6]. Besides, the degradation rate is moderate, so the degradation rate is not needed to be reduced as for Mg-based alloys [1]. Also advantageous regarding the degradation in comparison to Mg-based degradable alloys is the reduced generation of gaseous corrosion products [7,8]. Furthermore, iron is characterized by superior mechanical properties, accordingly especially load-bearing implants can be designed with more filigree and less material is inserted in the human body. This, in combination with the slow degradation of iron-based alloys, is promising for the addressed application [9,10]. However, a modification of these alloys is necessary due to the insuffident degradation rate, as iron-based materials can show properties that are a prerequisite for permanent implants [1,2,6]. Furthermore, the high stiffness compared to the human bone needs to be adapted regarding disadvantageous bone growth and stress shielding [11]. Hence, iron-based alloys with balanced properties have to be developed.

An increasing degradation rate can be achieved by alloying iron with manganese (Mn). Two effects are possible; on the one hand, manganese leads to a more negative electrochemical potential, on the other hand, phases with different potentials evolve [12-14]. Furthermore, the addition of manganese results in better plasticity combined with good strength, which is advantageous for stents that need to be formable [9,14,15]. Further benefits of iron alloys with adequate manganese content are the paramagnetic properties. Hence magnetic resonance tomography (MRT) can be applied for the post-implantation diagnostic [14,16]. Due to the formation of passivating layers, the degradation rate of high manganese steel still needs to be increased [12,13,17]. The formation of local areas with different electrochemical potentials enhances the material dissolution, based on the formation of galvanic cells and the resulting anodic dissolution of the material with a less noble potential 18,19]. Areas with different potentials can be achieved by different phases, respectively by the formation of a multi-phase structure. Subsequently, to further accelerate the degradation of iron-manganese alloys, phases with a noble potential need to be generated. Therefore, iron insoluble elements with noble potential can be selected or the formation of appropriate intermetallic phases can be promoted. Consequently, the addition of elements with higher electrochemical potential should increase degradation, and elements with the highest possible potentials, Le., precious metals, are best suited. Thus, different precious metals are examined as an additive for ironbased biodegradable materials. Here, Huang et al. [19] showed that silver, gold (Au), palladium (Pd), and platinum (Pt) act as effective cathodes to increase the corrosion rate of Fe-based alloys. Similar effects are reported for the addition of palladium to high manganese steel [12,17]. Accordingly, a promising approach is to add silver, as it is biocompatible and anti-bacterial [21]. Silver should increase the degradation rate due to its positive electrochemical potential as the difference in potential referred to iron is 1.32 V [18,22]. The effectiveness of silver is confirmed by various studies showing enhanced degradation rates and a more uniform degradation for iron alloys containing silver phases [18,20,22,23].

The immiscibility of elements like iron and silver on the one hand promotes the evolution of phases with high electrochemical potential, but on the other hand, makes the processing of such an alloy challenging. In this regard, new manufacturing methods have to be considered, which allow the processing of conventionally immiscible material systems. With additive manufacturing it is possible to overcome these disadvantages and also, to allow unique processing conditions, like unprecedented design-freedom and individualized geometries [24,25]. In previous work, the processability of a Fe-MnAg alloy via laser beam melting (LBM) was proven [18,22]. The mechanical properties, as well as the corrosion behavior of the prepared alloys, are promising. The silver phases remain after the dissolution of the surrounding iron-based matrix. In terms of biomedical applications, the release of silver must be addressed, as an enrichment of silver in the biological environment should be avoided. The influence of silver is discussed controversially as both, the advantages and disadvantages have to be taken into account [26]. Silver acts antibacterial and is already used in medicine for example for the therapy of wounds and in the past for the therapy of diseases like syphilis [26,27]. Apart from that, a large intake of silver can lead to argyria [27,28]. Due to these effects, the amount of silver intake is the key factor for biocompatibility. For example, a stent with a weight of 0.02 g or an osteosynthesis plate with a weight of 2.0 g made of an iron alloy containing 5.0 wt-% silver would release 0.001 g respectively 0.1 g silver. This amount of silver is tolerable as the World Health Organization (WHO) proposes that a lifetime intake of up to 10 g silver is uncritical 29,30].

Nevertheless, the silver phases released during the degradation of the implant in the tissue may lead to adverse consequences as thrombosis or tissue damage due to sharp edges. Silver particles, especially nanoparticles, are critically discussed in the literature [31,32]. Therefore, the silver phases have to be modified in terms of biocompatibility and self-degradation to enable the phagocytosis of the released particles.

To identify the most promising alloying concepts for degradable silver alloys, various binary silver alloys containing magnesium (Mg), calcium (Ca), manganese (Mn), zinc (Zn), germanium (Ge), lanthanum (La), cerium (Ce), neodymium (Nd) and/or ytterbium (Yb) under conditions similar to the conditions presented in this work have been investigated in preliminary work (unpublished research). In preliminary work [33], first promising alloy systems with calcium have been presented. Alloys with 6.0 wt.-% and 10.0 wt-% calcium show promising degradation behavior [33]. Based on the low corrosion resistance of elementary calcium and the presence of different intermetallic phases such as Ag2Ca and AgCa with different electrochemical potential, the Ag-Ca-system was addressed [33,34]. Calcium is an obvious element due to good biocompatibility as it is vital for the human...