Accelerated in vitro oxidative degradation testing of polypropylene surgical mesh.

Polypropylene (PP) surgical mesh had reasonable success in repair of hernia and treatment of stress urinary incontinence (SUI); however, their use for the repair of pelvic organ prolapse (POP) resulted in highly variable results with lifelong complications in some patients. One of several factors that could be associated with mesh-related POP complications is changes in the properties of the implanted surgical mesh due to oxidative degradation of PP in vivo. Currently, there are no standardized in vitro bench testing methods available for assessing the susceptibility to oxidative degradation and estimating long-term in vivo stability of surgical mesh. In this work, we adapted a previously reported automated reactive accelerated aging (aRAA) system, which uses elevated temperatures and high concentrations of hydrogen peroxide (H2 O2 ), for accelerated bench-top oxidative degradation testing of PP surgical mesh. Since H2 O2 is highly unstable at elevated temperatures and for prolonged periods, the aRAA system involves a feedback loop based on electrochemical detection methods to maintain consistent H2 O2 concentration in test solutions. Four PP mesh samples with varying mesh knit designs, filament diameter, weight, and % porosity, were selected for testing using aRAA up to 4 weeks and characterized using thermal analysis, Fourier-transform infrared spectroscopy-attenuated total reflectance (FTIR-ATR) and scanning electron microscopy (SEM). Additionally, the oxidation index (OI) values were calculated based on the FTIR-ATR spectra to estimate the oxidative degradation and oxidation reaction kinetics of PP surgical mesh. The OI values and surface damage in the form of surface flaking, peeling, and formation of transverse cracks increased with aRAA aging time. The aRAA test method introduced here could be used to standardize the assessment of long-term stability of surgical mesh and may also be adopted for accelerated oxidative degradation testing of other polymer-based medical devices.

Putative mechanobiological impact of surface texture on cell activity around soft-tissue implants undergoing micromotion.

Recent reports of adverse health effects (e.g., capsular contracture, lymphoma) linked to the absence or presence of texture on soft-tissue implants (e.g., breast implants) suggest surface topography may have pathological impact(s). We propose that surface texture influences the transfer of displacements, experienced by an implant undergoing micromotion, to surrounding interfacial extracellular matrix, which in turn impacts the activity of the resident cells and is based on degree of tissue integration. We hypothesize that transfer of displacements due to micromotion promotes interstitial fluid movement that imposes hydrodynamic stresses (pressures, shear stresses) on cells residing in the interfacial tissues and impacts their activity. To address this, we developed a computer simulation to approximate hydrodynamic stresses in the interstitial environment of saturated poroelastic tissues (model soft-tissue implantation sites) generated from oscillatory implant micromotion as a function of the magnitude of translational displacement, direction of motion, degree of tissue integration, and surface roughness of the implant. Highly integrated implants were predicted to generate the highest fluid shear stresses within model tissues, with oscillatory fluid shear stresses up to 80 dyn/cm2 for a 20-µm displacement. Notably, application of oscillatory 80 dyn/cm2 shear stress to cultured human fibroblasts elicited cell death after 20 h compared to cells maintained under static conditions or exposed to 80 dyn/cm2 steady, unidirectional shear. These results indicate that oscillatory interstitial fluid stresses generated by micromotion of an integrated implant may influence the activity of the surrounding cells and play a role in the body's fibrotic response to textured soft-tissue implants.

Chemical Characterization and Non-targeted Analysis of Medical Device Extracts: A Review of Current Approaches, Gaps, and Emerging Practices.

The developers of medical devices evaluate the biocompatibility of their device prior to FDA's review and subsequent introduction to the market. Chemical characterization, described in ISO 10993-18:2020, can generate information for toxicological risk assessment and is an alternative approach for addressing some biocompatibility end points (e.g., systemic toxicity, genotoxicity, carcinogenicity, reproductive/developmental toxicity) that can reduce the time and cost of testing and the need for animal testing. Additionally, chemical characterization can be used to determine whether modifications to the materials and manufacturing processes alter the chemistry of a patient-contacting device to an extent that could impact device safety. Extractables testing is one approach to chemical characterization that employs combinations of non-targeted analysis, non-targeted screening, and/or targeted analysis to establish the identities and quantities of the various chemical constituents that can be released from a device. Due to the difficulty in obtaining a priori information on all the constituents in finished devices, information generation strategies in the form of analytical chemistry testing are often used. Identified and quantified extractables are then assessed using toxicological risk assessment approaches to determine if reported quantities are sufficiently low to overcome the need for further chemical analysis, biological evaluation of select end points, or risk control. For extractables studies to be useful as a screening tool, comprehensive and reliable non-targeted methods are needed. Although non-targeted methods have been adopted by many laboratories, they are laboratory-specific and require expensive analytical instruments and advanced technical expertise to perform. In this Perspective, we describe the elements of extractables studies and provide an overview of the current practices, identified gaps, and emerging practices that may be adopted on a wider scale in the future. This Perspective is outlined according to the steps of an extractables study: information gathering, extraction, extract sample processing, system selection, qualification, quantification, and identification.

Effect of Dexamethasone on Room Temperature Three-Dimensional Printing, Rheology, and Degradation of a Low Modulus Polyester for Soft Tissue Engineering.

Three-dimensional (3D) printing has enabled benchtop fabrication of customized bioengineered constructs with intricate architectures. Various approaches are being explored to enable optimum integration of such constructs into the physiological environment including addition of bioactive fillers. In this work, we incorporated a corticosteroid drug, dexamethasone (Dex), in a low modulus polyester (SC5050) and examined the effect of Dex incorporation on solvent-, initiator-, and monomer-free pneumatic extrusion-based 3D printing of the polymer. Dex-SC5050 interactions were characterized by plotting thermodynamic binary phase diagrams based on the Flory-Huggins theory. The effect of Dex composition on the 3D printability of the SC5050 polyester was examined by rheological characterization and by image analysis of each layer of the 3D printed scaffolds. The drug release and the degradation of the polymer from the 3D printed scaffolds was used to analyze the effect of Dex composition on the performance of the 3D printed scaffolds. We found that Dex was insoluble in SC5050 polyester at relevant 3D printing temperatures and the insoluble drug particles physically reinforced the polymer, increasing the viscosity and the shear modulus of the base polymer. In addition, the reinforcing effect improved the shape fidelity of the printed filaments and the overall quality of the scaffolds. The Dex particles demonstrated a two-phase release, with an initial burst release and a slower sustained release of drug under in vitro conditions. To investigate preliminary host response of the 3D printed SC5050 scaffolds for tissue engineering applications, the printed scaffolds were implanted subcutaneously in Sprague-Dawley rats for 6 weeks and examined for fibrous tissue formation, infiltration of cells, and vascularization into the pores of the scaffolds.

In Vitro Sugar Interference Testing With Amperometric Glucose Oxidase Sensors.

BACKGROUND: Electrochemical enzymatic glucose sensors are intended to measure blood or interstitial fluid glucose concentrations. One class of these glucose sensors are continuous glucose monitors (CGMs), indicated for tracking and trending of glucose concentrations in interstitial fluid and as an adjunct to blood glucose testing. Currently approved CGMs employ a glucose oxidase (GOx) electrochemical detection scheme. Potential interfering agents can impact the accuracy of results obtained by glucose sensors, including CGMs. METHODS: Seven sugars, seven sugar alcohols, and three artificial sweeteners were in vitro screened for interference with amperometric glucose oxidase (GOx) sensors at concentrations greater than physiologic concentrations. Galactose was investigated further at physiologically relevant concentrations using a custom amperometric system. Furthermore, glucose and galactose calibration experiments were conducted to facilitate multiple enzyme kinetic analysis approaches (Michaelis-Menten and Hill equation) to understand the potential source and mechanism of interference from galactose. RESULTS: Under in vitro testing, except for galactose, xylose and mannose, all screened compounds exhibited interference bias, expressed in mean absolute relative difference (MARD), of ⩽ 20% even at concentrations significantly higher than normal physiologic concentrations. Galactose exhibited, CGM-dependent, MARD of 47-72% and was subjected to further testing. The highest recorded mean relative difference (MRD) was 6.9 ± 1.3% when testing physiologically relevant galactose concentrations (0.1-10 mg/dL). Enzyme kinetic analysis provided calculations of maximum reaction rates ( imax ), apparent Michaelis constants ( Kmapp ), and Hill equation h parameters for glucose and galactose substrates for the enzymes in the CGMs. CONCLUSION: Under the conditions of in vitro screening, 14 of the 17 compounds did not exhibit measuarable interference. Galactose exhibited the highest interference during screening, but did not substantially interfere with CGMs under the conditions of in vitro testing at physiologically relevant concentrations. Enzyme kinetic analysis conducted with galactose supported the notion that (1) the reactivity of GOx enzyme toward nonglucose sugars and (2) the presence of enzymatic impurities (such as galactose oxidase) are two potential sources for sugar interference with GOx glucose sensors, and thus, should be considered during device development.

Conservative Exposure Predictions for Rapid Risk Assessment of Phase-Separated Additives in Medical Device Polymers.

A novel approach for rapid risk assessment of targeted leachables in medical device polymers is proposed and validated. Risk evaluation involves understanding the potential of these additives to migrate out of the polymer, and comparing their exposure to a toxicological threshold value. In this study, we propose that a simple diffusive transport model can be used to provide conservative exposure estimates for phase separated color additives in device polymers. This model has been illustrated using a representative phthalocyanine color additive (manganese phthalocyanine, MnPC) and polymer (PEBAX 2533) system. Sorption experiments of MnPC into PEBAX were conducted in order to experimentally determine the diffusion coefficient, D = (1.6 ± 0.5) × 10-11 cm2/s, and matrix solubility limit, C s = 0.089 wt.%, and model predicted exposure values were validated by extraction experiments. Exposure values for the color additive were compared to a toxicological threshold for a sample risk assessment. Results from this study indicate that a diffusion model-based approach to predict exposure has considerable potential for use as a rapid, screening-level tool to assess the risk of color additives and other small molecule additives in medical device polymers.

Interactions of Staphylococcus aureus with ultrasoft hydrogel biomaterials.

Ultrasoft biomaterials-polymers, gels, and human soft tissues with an elastic modulus less than ∼100 kPa-are increasingly used in medical devices. While bacterial interactions (adhesion and biofilm formation) have been extensively studied on stiffer materials, little is known about how bacteria colonize ultrasoft materials as a nidus for infection. The goal of this work was to determine how material properties of ultrasoft hydrogels used for dermal fillers might affect pathogenesis of associated infections. We first synthesized a range of polyacrylamide hydrogels (PAAm) with moduli similar to clinically used dermal fillers and characterized the rheological, morphological and porous properties. We then developed a novel microfabricated insert to contain the PAAm in a flow system for quantification of bacterial adhesion and biofilm formation. The rate of adhesion and numbers of adherent Staphylococcus aureus on the surface of PAAm both decreased as the modulus increased. Adhesion was reduced by 3 logs (from 93 × 10(4)/cm(2) to 0.083 × 10(4)/cm(2)) with increasing modulus (from 17 Pa to 654 Pa). However, the number of bacteria in the bulk was the highest within the stiffest gels. This trend was further amplified in subsequent biofilm studies, where interfacial coverage of biofilm decreased as the modulus increased, while the fraction of biofilm in the bulk was the highest within the stiffest gel. The results show significant differences in bacterial colonization of PAAm based on material properties, and reveal how the injection process may unexpectedly create discontinuities that provide a microenvironmental niche for bacterial colonization.

Rapid evaluation of the durability of cortical neural implants using accelerated aging with reactive oxygen species.

OBJECTIVE: A challenge for implementing high bandwidth cortical brain-machine interface devices in patients is the limited functional lifespan of implanted recording electrodes. Development of implant technology currently requires extensive non-clinical testing to demonstrate device performance. However, testing the durability of the implants in vivo is time-consuming and expensive. Validated in vitro methodologies may reduce the need for extensive testing in animal models. APPROACH: Here we describe an in vitro platform for rapid evaluation of implant stability. We designed a reactive accelerated aging (RAA) protocol that employs elevated temperature and reactive oxygen species (ROS) to create a harsh aging environment. Commercially available microelectrode arrays (MEAs) were placed in a solution of hydrogen peroxide at 87 °C for a period of 7 days. We monitored changes to the implants with scanning electron microscopy and broad spectrum electrochemical impedance spectroscopy (1 Hz-1 MHz) and correlated the physical changes with impedance data to identify markers associated with implant failure. MAIN RESULTS: RAA produced a diverse range of effects on the structural integrity and electrochemical properties of electrodes. Temperature and ROS appeared to have different effects on structural elements, with increased temperature causing insulation loss from the electrode microwires, and ROS concentration correlating with tungsten metal dissolution. All array types experienced impedance declines, consistent with published literature showing chronic (>30 days) declines in array impedance in vivo. Impedance change was greatest at frequencies <10 Hz, and smallest at frequencies 1 kHz and above. Though electrode performance is traditionally characterized by impedance at 1 kHz, our results indicate that an impedance change at 1 kHz is not a reliable predictive marker of implant degradation or failure. SIGNIFICANCE: ROS, which are known to be present in vivo, can create structural damage and change electrical properties of MEAs. Broad-spectrum electrical impedance spectroscopy demonstrates increased sensitivity to electrode damage compared with single-frequency measurements. RAA can be a useful tool to simulate worst-case in vivo damage resulting from chronic electrode implantation, simplifying the device development lifecycle.

Synthesis of "click" alginate hydrogel capsules and comparison of their stability, water swelling, and diffusion properties with that of Ca(+2) crosslinked alginate capsules.

Ionically crosslinked alginate hydrogels have been extensively explored for encapsulation and immunoisolation of living cells/tissues to develop implantable cell therapies, such as islet encapsulation for bioartificial pancreas. Chemical instability of these hydrogels during long-term implantation hinders the development of viable cell therapy. The exchange between divalent crosslinking ions (e.g., Ca(+2) ) with monovalent ions from physiological environment causes alginate hydrogels to degrade, resulting in exposure of the donor tissue to the host's immune system and graft failure. The goal of this study was to improve stability of alginate hydrogels by utilizing covalent "click" crosslinking while preserving other biomedically viable hydrogel properties. Alginate was first functionalized to contain either pendant alkyne or azide functionalities, and subsequently reacted via "click" chemistry to form "click" gel capsules. Alginate functionalization was confirmed by NMR and gel permeation chromatography. When compared with Ca(+2) capsules, "click" capsules exhibited superior stability in ionic media, while showing higher permeability to small size diffusants and similar molecular weight cut-off and water swelling. Physicochemical properties of "click" alginate hydrogels demonstrate their potential utility for therapeutic cell encapsulation and other biomedical applications.

Comparative stability of the bioresorbable ferric crosslinked hyaluronic acid adhesion prevention solutions.

The Intergel® ferric crosslinked hyaluronate (FeHA) adhesion prevention solution (APS) (FDA) is associated with serious post-operative complications (Henley, http://www.lawyersandsettlements.com/features/gynecare-intergel/intergel-timeline.html, 2007; FDA, 2003; Roman et al., Fertil Steril 2005, 83 Suppl 1:1113-1118; Tang et al., Ann Surg 2006;243(4):449-455; Wiseman, Fertil Steril 2006;86(3):771; Wiseman, Fertil Steril 2006;85(4):e7). This prompted us to examine the in situ stability of crosslinked HA materials to hyaluronidase lyase degradation. Variables such as ferric ionic crosslink density, HA concentration, gel geometry, and molecular weight (MW) of HA polymer were studied. Various formulations of the crosslinked "in house" [Isayeva et al., J Biomed Mater Res: Part B - Appl Biomater 2010, 95B (1):9-18] FeHA (0.5%, w/v; 30, 50, 90% crosslinked), the Intergel® FeHA (0.5%, w/v; 90%), and the non-crosslinked HA (0.05-0.5%, w/v) were degraded at a fixed activity of hyaluronidase lyase from Streptomyces hyalurolyticus (Hyase) at 37°C over time according to the method [Payan et al., J Chrom B: Biomed Sci Appl 1991;566(1):9-18]. Under our conditions, the data show that the crosslink density affects degradation the most, followed by HA concentration and then gel geometry. We found that MW has no effect. Our results are one possible explanation of the observations that the Intergel® FeHA APS (0.5%, w/v; 90%) material persisted an order of magnitude longer than expected [t1/2 = 500 hrs vs. t1/2 = 50 hrs (FDA; Johns et al., Fertil Steril 1997;68(1):37-42)]. These data also demonstrate the sensitivity of the in vitro hyaluronidase assay to predict the in situ stability of crosslinked HA medical products as previously reported [Sall et al., Polym Degrad Stabil 2007;92(5):915-919].

Ionically cross-linked hyaluronic acid: wetting, lubrication, and viscoelasticity of a modified adhesion barrier gel.

Hyaluronic acid (HA), in linear or cross-linked form, is a common component of cosmetics, personal care products, combination medical products, and medical devices. In all cases, the ability of the HA solution or gel to wet surfaces and/or disrupt and lubricate interfaces is a limiting feature of its mechanism of action. We synthesized ferric ion-cross-linked networks of HA based on an adhesion barrier, varied the degree of cross-linking, and performed wetting goniometry, viscometry, and dynamic mechanical analysis. As cross-linking increases, so do contact angle, viscosity, storage modulus, and loss modulus; thus, wetting and lubrication are compromised. These findings have implications in medical device materials, such as adhesion barriers and mucosal drug delivery vehicles.

pH effect on the synthesis, shear properties, and homogeneity of iron-crosslinked hyaluronic acid-based gel/adhesion barrier.

Iron-crosslinked hyaluronic acid hydrogel (FeHA) has been used to reduce postsurgical adhesions in patients undergoing open, gynecological surgery. The performance of FeHA gel as an adhesion barrier device is influenced by many factors, including the physicochemical gel properties, which, in turn, depend on the chemistry and conditions of the device manufacturing. In this work, we demonstrate the effect of reaction pH on rheology and homogeneity of FeHA gels formulated in house and also compare the viscoelastic properties of FeHA gels with that of uncrosslinked HA solution of similar HA concentration and ionic strength. Dynamic mechanical analyses provide evidence that the reaction of HA with Fe(III) ions leads to the formation of "weak" gels. The viscoelastic properties and homogeneity of FeHA gels vary depending on the pH at which crosslinking was initiated. When solution pH, at the start of crosslinking, varied between 1.5 and 3, the low-shear rate viscosity of FeHA varied between 10,000 and 40,000 cPoise (10-40 Pa s). The highest steady-state shear viscosity and viscoelasticity were measured when pH was around 2.6, which is similar to the pH-dependent viscoelasticity of pure HA solution. Initiating HA crosslinking at pH ≤ 3 led to relatively homogenous solutions, while crosslinking higher pH > 3 caused instantaneous gel precipitation and inhomogeneities. Sensitivity of FeHA gel properties to small variations in reaction pH clearly supports the need for a tight manufacturing control during medical device fabrication.