Knitted Medical Devices Modified by Low-Temperature Plasma Treatment -Part 2

Abstract The impact of simulated storage conditions (accelerated ageing) developed surface, which promotes the growing through of connective tissue and provides better fastening of the implant, particularly in suture-less surgery. Moreover in the scope of the investigation, low-temperature plasma treatment of the surface of knitted implants was applied, which was expected to reduce the risk of complications related to the adhesion of the implant to internal organs, particularly in low-invasive surgery. The main quality demands that are set before the implant materials are as follows: a fast growing-through of the tissue, quick healing, biocompatibility, and lack of irritation of human tissue. The reaction of the human organism to a foreign body is distinctly reduced whenever ultra-light, non-resorbable polypropylene monofilament meshes (trade name OPTOMESH TM , Optilene ® Mesh LP) are used in hernia repair. The monofilament structure minimises the risk of bacterial infection. The present investigation is aimed at a possible reduction of the irritating action of the polymeric material introduced to the organism by applying ultra-light polypropylene monofilament knitwear of possibly low surface density and very fine spatial structure, which has the effect of introducing a minimised amount of synthetic material to the patient's organism. Chemical purity and its stability in medical devices is a key issue concerning biocompatibility. It is assumed that materials tissue defects in the abdominal and inguinal surgery. It enables a tissue tensionfree operation, thus largely reducing pain and shortening hospitalisation. Knitted fabrics find ever wider application in medicine by improving patients' health, thanks to features like high tenacity, low density, lack of water imbibition and good healing properties. Research made so far in the designing of the medical devices from innovative, polypropylene, ultra-light knitted fabrics for use in urogynaecology (procedures in the treatment of female incontinence and vagina plastic surgery) and general surgery (hernia treatment) has met the expectations of medicine and patients themselves [4 -12]. Non-resorbable meshes made of polypropylene multi -or monofilaments (class IIB) are designed for hernia surgery. They have been available world-wide for many years, for example in Poland under the trade name, OPTOMESH ® PP, DALLOP. Urology polypropylene bands have long since been used in the surgery for female incontinence (market product DALLOP ® NM, class IIB). One of the main disadvantages of the implants is a high surface density reflected in the high mass of the synthetic material implanted, which may be the reason for a local chronic reaction. Therefore lighter implants with a sufficient tenacity are strived for. Another aspect of textile implant structures is the one-side n Introduction Impressive progress can be seen in modern implant surgery. Yet more discriminating quality requirements are set before materials to be used in implants, fostered by the increasing demand. The lowering age of implant users is another factor calling for high quality, which results in novelty materials being sought, made according to the most advanced technologies to provide as high as possible connection between the implant and substituted part of the body and its function. This is why modern medical devices are the most expensive man-made materials There is a wide range of medical applications for resins thanks to their different properties when compared to metallic and ceramic materials. Good forming ability, easy sterilisation, bio-inertness, non-allergenic and non-toxic action, and adequate physical-chemical properties are amongst the beneficial qualities and behaviour of polymers Polypropylene (PP) is an implantable material which has recently found wide use mainly in non-resorbable surgery meshes for the reconstruction of soft tissue defects in hernia repair. The use of polypropylene meshes was a real breakthrough in the treatment of connective 134 which under simulated conditions deliver chemical substances may, though not necessarily, cause a local reaction of the tissue into which they are implanted. Lack of the implant's biocompatibility causes a complex, difficult-to-define reactions which, in the extreme, may lead to the rejection of the implant. This, together with the synergy and complexity of the phenomena proceeding in the course of the migration of substances from the implant, is an obstacle in defining parameters that would quality-and quantity-wise limit the chemical substances. Characteristics of the chemical substance leached from the implant is solely a support in defining the quality base for modified and commercial medical devices, including a stability assessment of the quality and quantity level (assessment in accelerated ageing examination and real ageing). Knowledge of the chemical composition and, what is more important, of the qualitative and quantitative composition of the substance which migrates in the course of simulated clinical use does not permit, particularly in the case of newly developed or much modified medical devices, to relate to biocompatibility. The latter is to be tested in the course of in vivo and in vitro examination as defined in ISO 10993-1. Ultra-light textile implants that come into contact with tissue and body fluids must satisfy specific demands set for biomaterials like material biocompatibility. The last is manifested by the fact that the material is chemically and immunologically neutral and does not reveal any toxic or destructive action in the given environment of the organism. When designing implant materials, complying with demands like mechanical-, useful-and physical-chemical properties is a precondition but does not guarantee success. Really crucial is the response of the cells that approach the implant's surface. Ultra-light textile implants are 3D medical structures with a three-direction orientation of the polypropylene fibers (medical-grade monofilament -class VI according to American Pharmacopoeia). Medical grade PP features the lowest density amongst commercial polymers; it is classified as a neutral polymer, meaning that it contains only a minimal amount of auxiliary substances which are neither delivered to nor degrade in a biological environment. PP is an inert polymer without covalent bonds in the chain, which is why the surface of PP implants requires functionalisation. It was in the prototypes of the medical devices designed that this was accomplished by low-temperature plasma surface modification in the presence of a low-molecular fluoroorganic compound (the fluorocarbon polymer layer deposited). That kind of modification, by the depositing of a thin layer (25 -50 Å), affects the quality of cell response, biocompatibility and hydrophilic properties; moreover it accelerates the adhesion process and proliferation [13 -15]. The active surface layer of the implant not only enables the tailoring of surface properties such as wettability and surface energy but also provides the chance of controlling the material degradation process A very important aspect is the adapting of chemical properties including the quantitative and qualitative profile of leachable substances, and the physical properties of the implant material surface. This results in biocompatibility and the possibility to stimulate tissue regeneration, which increases the chances of the better acceptance of the implant in vivo One unavoidable part of the research is the assessment of the clinical risk of using polymeric materials as implants by analysing the mechanisms of degradation and assessing quantity-wise the degradation products that may be delivered in the course of chemical reactions, migration and depolymerisation. It was also important in the investigation to learn the influence of storage conditions of the final product designed upon the quantitative and qualitative profile of leachable substances determined in the testing of chemical purity. Therefore the method of accelerated ageing is being used more and more for designed medical devices to anticipate risk defined as a profile of potential leachable substances in conditions of simulated storage and clinical handling In this work, as a part of the chemical purity assessment of the implantable medical devices designed, the profile of leachable substances under simulated conditions of use (specific processing conditions, storage, nature and contact time of the product) was estimated on the basis of EN and ISO standards harmonised with European Directives concerning medical devices [19 -23]. The aims of the work were as follows: n determination of the impact of simulated storage conditions on the chemical purity of the prototype knitted implants designed: (a) surface-modified by low-temperature plasma in the presence of a low-molecular fluoroorganic compound and (b) unmodified ones; n the assessment of risk acceptability concerning the impact of storage conditions on the chemical purity of the medical devices designed. Selected prototypes of the medical devices designed were subjected to accelerated ageing according to a research programme based on guidelines of Standard ASTM 1980F:2002. The programme was an extension of research published earlier [6 -9] concerning biomechanical and chemical properties. n Materials Raw-materials Knitted medical devices for hernioplasty and vaginoplasty were designed using polypropylene monofilament fibres with a diameter of 0.08 mm (linear density of 46 dtex) with properties as described in Design of implantable medical devices Estimation of the turbidity of the aqueous extracts A method for turbidity measurements in aqueous extracts was prepared on the basis of a visual method described in Polish Pharmacopoeia, the VII edition. A suspension of formazin was used as a basic turbidity reference equal to 4000 NTU (Nephelometric Turbidity Units), which is a blend of hydrazine sulfate and hexa(methylenetetramine) (urotropin). The visual method was applied in instrumental turbiditymetric measurements with the use of a spectrophotometer -Unicam 5625 UV/VIS, USA. A calibration curve of the basic turbidity pattern was determined from five comparative suspensions: I, II, III, IV & V, representing the turbidity degrees of 3, 6, 18, 30 and 45 NTU. An analytical wave length of λ = 400 nm of the light was adopted. The turbidity degree of the comparative suspensions was estimated by measuring the scattered light compared to purified water as reference. A rectilinear plot was able to be drawn of the turbidity degree determined for the comparative suspensions. A maximal turbidance of 0.171 was seen for comparative suspension No. V. The aqueous extract is considered translucent if its translucence matches that of water measured at the same conditions or if its opalescence does not exceed that of comparative suspension No II, equal to 6 NTU. Determination of heavy metal ions in the aqueous extracts The content of heavy elements: cadmium, chromium (sum of all oxidation states), lead, zinc, and mercury in the aqueous extracts was determined by Atomic Absorption Spectroscopy using a SCAN-1 spectrometer made by Thermo Jawell ASH. Cadmium, chromium, lead and zinc were determined directly in the aqueous extracts by the flame method ASA (FAAS*) at the following parameters: n Cd: wave length λ = 228.8 nm, flame acetylene-air, limit of determination -0.02 mg/dm 3 n Cr: wave length λ = 357.9 nm, flame acetylene-N 2 O, limit of determination -0.2 mg/dm 3 and unmodified (code KO) implants were tested. Exhaustive extraction and three-step extraction The content of leachable substances (profile of leachable substances) was estimated by the methods of exhaustive and three-step extraction. The first was accomplished on Soxhlet apparatus with petroleum ether according to the procedure specified in Standard PN/P-0607:1983. Estimation of chemical purity The three-step extraction was made in accordance with directives given in Standard EN ISO 10993-12:2009, where the following solvents were used in turn: n purified water (water for injection by Baxter Co); n 2-propanol according to the method given in Standard PN/P-04781/06; n petroleum ether. Preparation of aqueous extracts for estimation of chemical purity (analysis of the profile of leachable substances) The aqueous extract was prepared with the following module: 10g of fine cut pieces of the material about 1 cm long on 100 cm 3 of water for injection (Baxter pH estimation in the aqueous extracts The pH reaction of the aqueous extracts was measured in accordance with Standard PN-EN ISO 3071:2007 by a LAB 860 SET pH-meter (Scott, Germany) equipped with a BluLine 14 pH electrode perature plasma (code KO), were finished the same way. Prototypes of the knitted implants (both unmodified and modified by low temperature plasma treatment) were packed in a double medical grade packaging system adaptable for steam sterilisation (OPM/ Poland) as described in Altogether 130 pieces of KO and PF implants were prepared as a prototype batch for testing of accelerated ageing. Samples for testing were taken statistically from the whole knitwear batch, prepared on a semi-industrial scale and resembling typical industrial production. n Methods Accelerated ageing The testing of accelerated ageing was designed on the basis of Standard ASTM F1980:2002: Standard Guide for Accelerated Ageing of Sterile Barrier Systems for Medical Devices. This document specifies guidelines for the accelerated ageing testing of medical packaging. However, it can be easily adapted to the accelerated ageing of medical devices (considering similar potential hazards) to assess the influence of storage conditions upon functional properties and the safety of newly designed medical devices. The medical devices designed placed in typical packaging (direct packaging satisfying quality requirements of Standards PN-EN ISO 11607-1:2011 and PN-EN 868-5:2009) were tested after steam sterilisation in validated industrial conditions. The accelerated ageing was performed in a KBF 240 chamber (Binder GmbH/ Germany), where the elevated temperature was the factor simulating accelerated ageing. The temperature of the chamber was 60 ± 2 ºC and the RH 20 ± 5%, under which conditions the medical devices were kept for 28 days (simulation of 1 year of ageing) and 56 days (simulation of 2 year's ageing). The residence time of the medical devices in the chamber was calculated using the Arrhenius equation (ASTM F 1980(ASTM F :2002, adopting the value of 3.7 as the ageing factor. The testing was performed at the accredited Laboratory of Metrology of the Institute of Security Technologies "MORATEX", Lodz, Poland. Both plasma-modified (code PF) 136 n Pb: wave length λ = 217.0 nm, flame acetylene-air, limit of determination -0.2 mg/dm 3 n Zn: wave length λ = 213.9 nm, flame acetylene-air, limit of determination -0.01 mg/dm 3 . Mercury was determined by the method of cold vapour generation ASA (CVAAS) using a device for the generation of cold vapours -Atomic Vapor Accessory 440, made by Thermo Jawell ASH, at the following parameters: wave length λ = 253.7 nm, reductive solution 5% SnCl 2 in 20% HCl, carrier gas -Ar, and limit of determination -0.01 mg/dm 3 . Determination of the permanganate value (oxidability) of the aqueous extracts The permanganate value was determined according to the directives of Standard PN-P-04896:1984. Water for injection (Baxter Co) served as reference: it was subjected to the same processing conditions as the sample tested. Determination of chloride ion content in the aqueous extracts The content of chloride ions in the aqueous extracts was determined by the visual method described in Standard PN-P-04895:1984. The method employs argentometry titration of the aqueous extracts prepared with a 0.01 mol/dm 3 AgNO 3 solution in the presence of chromium ions. The determination limit of the method is 0.003 mg of [Cl] -ions /1g of the material tested. A 3.0% hydrogen peroxide solution was added at the end of titration to enhance the hue intensity at the colour change point. [Cl] -free injection water (Baxter Co.) was used to prepare the reference and solutions needed for the determination of chloride ions i.e. solutions of potassium chromate, hydrogen peroxide and a standard volume solution of silver nitrate. n Results and discussion For the textile implants designed for use in urogynaecology and general surgery, results are presented of the examination of selected physical-chemical quality parameters which directly affect the operational safety, especially when it comes to biocompatibility. Results of the parameter testing are compared with reference standards (KO and PF) of prototypes taken directly from the manufacturing process, which permitted to estimate the changes in chemical purity of the products proceeding in the course of ageing. pH of the aqueous extracts The pH values estimated for the starting prototype implants designed for hernia and vagina repair, both modified with low-temperature plasma and unmodified, amount to 6.2 (KO) and 6.3 (PF). pH values of aqueous extracts of the prototype medical devices, both modified and unmodified, after 1 and 2 years of accelerated ageing do not differ much from the starting value (reference); the value of pH is close to a neutral reaction and to that of human skin, actually falling into the optimal range for that type of medical device. The results confirm the absence of substances that could affect the change in pH reaction by migration after long storage of the medical devices. It must be stressed that a pH below 4 and above 8 brings about the risk of irritation of the surrounding tissue, which in extreme cases could be the reason for implant rejection or cause the forming of a thick cartilage capsule, leading to a stiffening of the implant locality Turbidity of the aqueous extracts Figure 2 presents changes in the turbidity of aqueous extracts prepared from the test implants proceeding in the course of accelerated ageing