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Zn-Mg and Zn-Ag degradation mechanism under biologically relevant conditions
KTH, School of Engineering Sciences (SCI), Applied Physics. (Surface physics)ORCID iD: 0000-0001-6332-0501
KTH, School of Engineering Sciences (SCI), Applied Physics.ORCID iD: 0000-00003-1631-4293
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Zinc alloys form a promising new class of biodegradable metals that combine suitable mechanical properties with the favorable degradation properties of pure zinc. However, the current understanding of the influence of alloying elements on the corrosion of zinc alloys, in biologically relevant media, is limited. We studied the degradation of three alloys, Zn 4 wt% Ag, Zn 0.5 wt% Mg and Zn 3 wt% Mg by in situ electrochemical impedance spectroscopy (EIS). After exposure for 1h or 30 days the samples were characterized by infrared spectroscopy and scanning electron microscopy (SEM). The presence of secondary phases in the alloy microstructure induced selective corrosion and increased degradation rate. An increase in surface inhomogeneity was evident by EIS analysis both at short (hours) as well as long immersion times (days). The microgalvanic corrosion of the Zn-Ag alloy resulted in enrichment of the AgZn3 phase at the sample surface. The enrichment of Ag and potential release of AgZn3 particles may result in complications during the tissue regeneration. The Zn-Mg alloy surface was depleted of the Mg-rich phase after 8-12 days. The selective dissolution caused local precipitation of2corrosion products and a thicker corrosion layer with larger pore size consistent with increased corrosion rate.

National Category
Biomaterials Science
Identifiers
URN: urn:nbn:se:kth:diva-215665OAI: oai:DiVA.org:kth-215665DiVA: diva2:1148839
Funder
Swedish Research Council
Note

QC 20171017

Available from: 2017-10-12 Created: 2017-10-12 Last updated: 2017-10-19Bibliographically approved
In thesis
1. Investigation of corrosion properties of metals for degradable implant applications
Open this publication in new window or tab >>Investigation of corrosion properties of metals for degradable implant applications
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [sv]

Nedbrytbara metaller utgör en ny klass av biomaterial med potential attersätta permanenta material i tillfälliga applikationer. Detta för att minskarisken för långvariga biverkningar. I den pågående forskningen för att utvecklanya nedbrytbara metaller är screening av nya material genom in vitro testmetoderett attraktivt alternativ för att undvika onödiga, tidskrävande ochdyrbara djurstudier.Denna avhandling fokuserar på in vitro-testning av zink- och magnesiumbaserademetaller. Inverkan av faktorer såsom sammansättningen av testlösningen,buffersystemet, belastning samt mikrostruktur hos legeringar undersöktes.Genom att använda elektrokemiska in situ tekniker såsom impedansspektroskopi(EIS) är det möjligt att studera gränssnittet mellan metall ochlösning och karakterisera egenskaperna hos den korroderande ytan. Ex situytkaraktäriseringstekniker som svepelektronmikroskopi och infraröd spektroskopianvändes sedan för att komplettera resultaten av de elektrokemiskamätningarna.Korrosionen av zink i Ringer’s lösning fanns vara närmare in vivo korrosionän korrosionen i fosfatbuffrad saltlösning (PBS). Ringers lösning är därför denföredragna testmiljön för långsiktig utvärdering av zinkbaserade metallerDet biologiska buffersystemet (CO2/H2CO3) bör företrädesvis användasför att stabilisera pH-värdet på testlösningen vid magnesiumnedbrytning. NärHEPES användes för att stabilisera pH ökade korrosionshastigheten på grundav bildning av mindre skyddande skikt av korrosionsproduktMöjligheten att använda helblod och plasma som mer kliniskt relevantatestmiljöer utvärderades och befanns producera reproducerbara resultat.Bildning av ett korrosionsskikt bestående av både organiskt och oorganisktmaterial detekterades på zink i både plasma och helblod.När zink prover i helbod utsattes för belastning förhindrade korrosionsskiktetbildningen av mikrosprickor och förtidigt brott av provet. Det varvidare möjligt att detektera tidig sprickbildning på grund av belastning avMagnesium AZ61-legering med EIS.Adsorption av organiska species på zinkytan under anodisk polariseringökar yttäckningen av Zn-joner i helblod. Den ökade yttäckningen leder sedantill utfällningen av ett skyddande skikt av zinkfosfater och en minskadkorrosionshastighet vid högre potentialer.Korrosion av Zn-Mg och Zn-Ag legeringar i Ringers lösning befanns skevia selektiv upplösning. Lokal utfällning av korrosionsprodukter och bildningav ett poröst, mindre skyddande skikt av korrosionsprodukter hittades påZn-Mg legeringar. Den selektiva upplösningen av Zn-Ag legering orsakade enanrikning av AgZn3 vilket kan påverka biokompatibiliteten av ett implantatmed tiden.

Abstract [en]

Degradable metallic implants are a new class of biomaterials with potentialto replace permanent materials in temporary applications to reduce therisk of long term adverse effects.This thesis focuses on in vitro testing of zinc and magnesium based metals.As new degradable metals are developed screening of new materials within vitro test methods is an attractive option to avoid unnecessary, time consumingand expensive animal studies. The influence of factors such as ioniccomposition of the test solution, buffer system, strain and alloy compositionwas investigated. By employing electrochemical in situ techniques such asimpedance spectroscopy it is possible to study the metal-solution interfaceand determine the properties of the corroding surface. Ex situ surface characterizationtechniques such as scanning electron microscopy and infraredspectroscopy were then used to complement the results of the electrochemicalmeasurements.The importance of appropriate selection of the test solution is highlightedin this work. Zinc was found to corrode in Ringer’s solution by a mechanismcloser to in vivo corrosion than in a phosphate buffered saline solution(PBS).Ringer’s solution is therefore the more appropriate test environment for longterm evaluation of zinc based metals.When evaluating the corrosion of Zn-Mg and Zn-Ag alloys in Ringer’ssolution selective dissolution was found to occur for both types of alloys. Localprecipitation and formation of a porous, less protective, layer of corrosionproducts was found for Zn-Mg alloys. The selective dissolution of Zn-Agalloy caused an enrichment of AgZn3 on the surface which may affect thebiocompatibility of the alloy.The use of HEPES to maintain the pH of the test solution increasedthe corrosion rate of magnesium due to formation of a less protective layerof corrosion products. Magnesium corrosion should therefore preferably bestudied in solutions where the pH is maintained by the biological buffer systemCO2/H2CO3.In addition to saline solutions human whole blood and plasma were evaluatedas more clinically relevant in vitro environments. They were found toproduce reproducible results and to be suitable for short term experiments.Formation of a corrosion product layer comprised of both organic and inorganicmaterial was detected on zinc in both plasma and whole blood.During anodic polarization the adsorption of organic species on the zincsurface was found to increase the surface coverage of Zn ions in whole blood.The increased surface coverage then allowed for precipitation of a protectivelayer of Zn5(PO4)3 and a subsequent decrease in corrosion rate at higherpotentials.When subjecting zinc samples to strain the organic/inorganic corrosionproduct formed in whole blood was observed by impedance spectroscopy toprevent micro cracking and premature failure.The cracking of magnesium alloy samples under applied strain was alsocharacterized by impedance. Changes in surface properties due to crack initiation

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2017. 80 p.
Series
TRITA-FYS, ISSN 0280-316X ; 2017:58
Keyword
biodegradable, metal, zinc, magnesium, corrosion, electrochemistry
National Category
Bio Materials Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-215970 (URN)978-91-7729-528-0 (ISBN)
Public defence
2017-11-10, sal C (Sven-Olof Öhrvik), Electrum, kistagången 16, kista, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Research Council
Note

QC 20171019

Available from: 2017-10-19 Created: 2017-10-19 Last updated: 2017-11-02Bibliographically approved

Open Access in DiVA

The full text will be freely available from 2018-10-01 14:30
Available from 2018-10-01 14:30

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