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1. IntroductionUltra high molecular weight polyethylene (PE-UHMW) is avery frequently used material in artificial joint replacementsystems. It provides easy usability,extreme high mechanical wear resistance,high biocompatibility and is to acertainextent self-lubricating. However,the number of revisionsurgeries is still high,sometimes necessary because of therapid shelf life aging process and in vivo degradation due tofew selected protein and carbohydrate species, e.g. albuminand hyaluronic acid . It is known that the major abundantcomponents of synovial fluid, e.g. hyaluronic acid,albumin orimmunoglobulin,interact with PE-UHMW and tend to adsorbon the surface in model fluid systems.However,the interaction of the whole synovial proteome in the presence of lipidsetc. with PE-UHMW has never been investigated.Because thesynovial fluid composition is very similar to blood plasma(except for the high hyaluronic acid content)it is also highlyaffected by the patient’s age,sex,life style and pathologicalstatus . Therefore it is of high interest,which proteinsexcept albumin and immunoglobulin in fact adsorb on PE-UHMW before a deeper understanding of the pathologicalprocess can be obtained and/or material related selective adsorption effects can be gathered.Different compositions of PE-UHMW,Vitamin E dopedmaterial, highly cross-linked PE-UHMW and materials under-going different kinds of sterilization strategies have alreadybeen tested in clinical studies for improving the materials’properties and reducing protein adsorption.Highly abundantglycoproteins have been shown to adsorb on PE-UHMW insimulating models and adsorption has been investigated tooccur unspecific and concentration independent . In thesame study model fluids containing only one glycoproteinof interest revealed that the formed protein layer enhancesthe lubrication and friction behavior of PE-UHMW.However,besides the selected glycoproteins also other high abundanceand acute phase proteins occurring during inflammation andoxidative stress, e.g. during rheumatic diseases,are relevantfor adsorption,friction behavior and consequently materialmodification.The presented study focuses on the unbiased identificationand localization of proteins present in synovial fluid adsorbing on different varieties of PE-UHMW material.To answerboth questions,protein identity and spatial distribution,massspectrometry imaging(MSI) by means of matrix assisted laserdesorption/ionization time-of-flight(MALDI-TOF)examination was correlated to SDS PAGE analysis of adsorbed proteins.MSI isawell-established method for the localization andidentification of analytes of interest within an untargetedapproach. To date MSI has been applied to a huge varietyof biological samples and it has to be mentioned thatsurface analysis of biomaterials is an exponentially growingfield gaining more importance for the analysis of lubricationinteraction and material modifications.One limiting fact of MSI is that protein identification inthe high molecular mass range is only possible based onthe tentative assignment of proteins to measured m/z values, respectively the molecular weight of detected molecules. To corroborate protein assignment after MSI analysis,SDS PAGE analysis was chosen,providing the possibility tocompare synovial protein patterns to patterns of protein compounds adsorbed on PE-UHMW of different compositionswith respect to overall detected proteins.In parallel MS-based protein identification after in-gel digestion is feasible.Protein identification for adsorbed molecules was furthermore verified by on-tissue (“on polymer”)digestion. Thehydrophobic surface of PE-UHMW supports the preservationof protein localization during trypsin application in aqueous buffer systems,however,diffusion is acritical point formechanical wear,oxidation and material modification.Synovial fluid(SF),the major lubricating system in the jointcompartment, directly interacts with the material surface,leading to biomolecule adsorption and diffusion possibly altering the polymer characteristics and stability.Themajor components have already been investigated,howeverit has not been determined yet,whether modified PE-UHMWsurface areas lead to enhanced biomolecule adsorption anddiffusion or vice versa. Furthermore,protein layer formation
on PE-UHMW has so far not been investigated except for a
both enzyme and matrix application.To obtain reliableresults
homogeneous layers of tryps in and MALDI matrix solution
were applied using a piezo printer(Chemical Inkjet Printer,
ChIP-1000 ) to the protein-carrying PE-UHMW material
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