Jörg Bossert

Glass matrix composites from coal flyash and waste glass

Abstract Glass matrix composites have been fabricated from waste materials by means of powder technology. Flyash from coal power stations and waste glass, residue of float glass production, were used. Commercial alumina platelets were employed as the reinforcing component. For flyash contents up to 20% by weight nearly fully dense compacts could be fabricated by using relatively low sintering temperatures (650°C). For higher flyash contents the densification was hindered due to the presence of crystalline particles in the as-received flyash, which jeopardized the viscous flow densification mechanism. The addition of alumina platelets resulted in better mechanical properties of the composites than those of the unreinforced matrix, despite a residual porosity present. Youngs modulus, modulus of rupture, hardness and fracture toughness increase with platelet volume fraction. The low brittleness index of the composites ( B ≈ 3 μm −1 2 ) suggests that the materials have good machinability. A qualitative analysis of the wear behaviour showed that the composite containing 20% by volume platelet addition has a higher wear resistance than the unreinforced matrix. Overall, the results indicate that the materials may compete with conventional glasses and glass-ceramics in technical applications.

Enhanced mechanical properties of a novel, injectable, fiber-reinforced brushite cement.

Injectable, brushite-forming calcium phosphate cements (CPCs) have great potential as bone replacement materials due to enhanced degradability and long-term inclusion in bone remodeling. However, the use of such brushite-forming CPCs in load-bearing areas is limited by their low mechanical strength. One approach to overcome this limitation is the use of reinforcing fibers. Thus, an injectable, biodegradable, brushite-forming CPC based on beta-tricalcium phosphate/phosphoric acid with fiber reinforcement was developed for minimally invasive surgery. The fibers (diameter 25 µm; length 0.25, 1 or 2mm) were extruded from poly(l-lactide-co-glycolide) acid (PLGA) and added to the CPC (2.5, 5 or 7.5% (w/w)). Independent of the fiber content, injectability of the CPC was retained up to a fiber length of 1mm. The addition of all PLGA fiber types increased diametral tensile strength, biaxial flexural strength, and flexural strength by up to 25% (p ≤ 0.05 for the diametral tensile strength for the CPC with 5% (w/w) 1mm fibers and the biaxial flexural strength of the CPC with 5% (w/w) 0.25 mm fibers). In contrast, the work of fracture strongly and significantly increased (p<0.01) by up to 12.5-fold. At constant fiber content, the mechanical properties of the fiber-reinforced CPC were mostly augmented with increasing fiber length. Also, the addition of PLGA fibers to the brushite-forming CPC (up to 7.5% (w/w)) only transiently delayed cell growth and did not decrease cell viability. Fiber reinforcement of CPCs thus augments their mechanical strength while preserving the injectability and biocompatibility required for their application in modern surgery.

Acids with an equivalent taste lead to different erosion of human dental enamel

OBJECTIVES The consumption of acidic soft drinks may lead to demineralization and softening of human dental enamel, known as dental erosion. The aims of this in vitro study were to determine: (i) if different acids with a similar sensorial acidic taste lead to different hardness loss of enamel and (ii) if the fruit acids tartaric, malic, lactic or ascorbic acid lead to less hardness loss of enamel than citric or phosphoric acid when their concentration in solution is based on an equivalent sensorial acidic taste. METHODS Enamel samples of non-erupted human third molars were treated with acidic solutions of tartaric (TA), malic (MA), lactic (LA), ascorbic (AA), phosphoric (PA) and citric (CA) acids with a concentration that gave an equivalent sensorial acidic taste. The acidic solutions were characterized by pH value and titratable acidity. Atomic force microscopy (AFM) based nanoindentation was used to study the nano mechanical properties and scanning electron microscopy (SEM) was used to study the morphology of the treated enamel samples and the untreated control areas, respectively. RESULTS The investigated acids fell into two groups. The nano hardnesses of MA, TA and CA treated enamel samples (group I) were statistically significantly greater (p<0.05) than the nano hardnesses of PA, AA and LA treated enamel samples (group II). Within each group the nano hardness was not statistically significantly different (p>0.05). The SEM micrographs showed different etch prism morphologies depending on the acid used. SIGNIFICANCE In vitro, the acids investigated led to different erosion effects on human dental enamel, despite their equivalent sensorial acidic taste. This has not been reported previously.

Reproducible Biofilm Cultivation of Chemostat-Grown Escherichia coli and Investigation of Bacterial Adhesion on Biomaterials Using a Non-Constant-Depth Film Fermenter

Biomaterials-associated infections are primarily initiated by the adhesion of microorganisms on the biomaterial surfaces and subsequent biofilm formation. Understanding the fundamental microbial adhesion mechanisms and biofilm development is crucial for developing strategies to prevent such infections. Suitable in vitro systems for biofilm cultivation and bacterial adhesion at controllable, constant and reproducible conditions are indispensable. This study aimed (i) to modify the previously described constant-depth film fermenter for the reproducible cultivation of biofilms at non-depth-restricted, constant and low shear conditions and (ii) to use this system to elucidate bacterial adhesion kinetics on different biomaterials, focusing on biomaterials surface nanoroughness and hydrophobicity. Chemostat-grown Escherichia coli were used for biofilm cultivation on titanium oxide and investigating bacterial adhesion over time on titanium oxide, poly(styrene), poly(tetrafluoroethylene) and glass. Using chemostat-grown microbial cells (single-species continuous culture) minimized variations between the biofilms cultivated during different experimental runs. Bacterial adhesion on biomaterials comprised an initial lag-phase I followed by a fast adhesion phase II and a phase of saturation III. With increasing biomaterials surface nanoroughness and increasing hydrophobicity, adhesion rates increased during phases I and II. The influence of materials surface hydrophobicity seemed to exceed that of nanoroughness during the lag-phase I, whereas it was vice versa during adhesion phase II. This study introduces the non-constant-depth film fermenter in combination with a chemostat culture to allow for a controlled approach to reproducibly cultivate biofilms and to investigate bacterial adhesion kinetics at constant and low shear conditions. The findings will support developing and adequate testing of biomaterials surface modifications eventually preventing biomaterial-associated infections.

Decreased extrusion of calcium phosphate cement versus high viscosity PMMA cement into spongious bone marrow—an ex vivo and in vivo study in sheep vertebrae

BACKGROUND CONTEXT Vertebroplasty or kyphoplasty of osteoporotic vertebral fractures bears the risk of pulmonary cement embolism (3.5%-23%) caused by leakage of commonly applied acrylic polymethylmethacrylate (PMMA) cement to spongious bone marrow or outside of the vertebrae. Ultraviscous cement and specific augmentation systems have been developed to reduce such adverse effects. Rapidly setting, resorbable, physiological calcium phosphate cement (CPC) may also represent a suitable alternative. PURPOSE This study aimed to compare the intravertebral extrusion of CPC and PMMA cement in an ex vivo and in vivo study in sheep. STUDY DESIGN/SETTING A prospective experimental animal study was carried out. METHODS Defects (diameter 5 mm; 15 mm depth) were created by a ventrolateral percutaneous approach in lumbar vertebrae of female Merino sheep (2-4 years) either ex vivo (n=17) or in vivo (n=6), and injected with: (1) CPC (L3); (2) CPC reinforced with 10% poly(l-lactide-co-glycolide) (PLGA) fibers (L4); or (3) PMMA cement (L5; Kyphon HV-R). Controls were untouched (L1) or empty defects (L2). The effects of the cement injections were assessed in vivo by blood gas analysis and ex vivo by computed tomography (CT), micro-CT (voxel size: 67 µm), histology, and biomechanical testing. RESULTS Following ex vivo injection, micro-CT documented significantly increased extrusion of PMMA cement in comparison to CPC (+/- fibers) starting at a distance of 1 mm from the edge of the defect (confirmed by histology); this was also demonstrated by micro-CT following in vivo cement injection. In addition, blood gas analysis showed consistently significantly lower values for the fraction of oxygenized hemoglobin/total hemoglobin (FO2Hb) in the arterial blood until 25 minutes following injection of the PMMA cement (p ≤ .05 vs. CPC; 7, 15 minutes). Biomechanical testing following ex vivo injection showed significantly lower compressive strength and Young modulus than untouched controls for the empty defect (40% and 34% reduction, respectively) and all three cement-injected defects (21%-27% and 29%-32% reduction, respectively), without significant differences among the cements. CONCLUSIONS Because of comparable compressive strength, but significantly lower cement extrusion into spongious bone marrow than PMMA cement, physiological CPC (+/- PLGA fibers) may represent an attractive alternative to PMMA for vertebroplasty or kyphoplasty of osteoporotic vertebral fractures to reduce the frequency or severity of adverse effects.

Enhanced bone formation in sheep vertebral bodies after minimally invasive treatment with a novel, PLGA fiber-reinforced brushite cement.

BACKGROUND CONTEXT Injectable, brushite-forming calcium phosphate cements (CPC) show potential for bone replacement, but they exhibit low mechanical strength. This study tested a CPC reinforced with poly(l-lactide-co-glycolide) acid (PLGA) fibers in a minimally invasive, sheep lumbar vertebroplasty model. PURPOSE The study aimed to test the in vivo biocompatibility and osteogenic potential of a PLGA fiber-reinforced, brushite-forming CPC in a sheep large animal model. STUDY DESIGN/SETTING This is a prospective experimental animal study. METHODS Bone defects (diameter: 5 mm) were placed in aged, osteopenic female sheep, and left empty (L2) or injected with pure CPC (L3) or PLGA fiber-reinforced CPC (L4; fiber diameter: 25 µm; length: 1 mm; 10% [wt/wt]). Three and 9 months postoperation (n=20 each), the structural and functional CPC effects on bone regeneration were documented ex vivo by osteodensitometry, histomorphometry, micro-computed tomography (micro-CT), and biomechanical testing. RESULTS Addition of PLGA fibers enhanced CPC osteoconductivity and augmented bone formation. This was demonstrated by (1) significantly enhanced structural (bone volume/total volume, shown by micro-CT and histomorphometry; 3 or 9 months) and bone formation parameters (osteoid volume and osteoid surface; 9 months); (2) numerically enhanced bone mineral density (3 and 9 months) and biomechanical compression strength (9 months); and (3) numerically decreased bone erosion (eroded surface; 3 and 9 months). CONCLUSIONS The PLGA fiber-reinforced CPC is highly biocompatible and its PLGA fiber component enhanced bone formation. Also, PLGA fibers improve the mechanical properties of brittle CPC, with potential applicability in load-bearing areas.

First-time systematic postoperative clinical assessment of a minimally invasive approach for lumbar ventrolateral vertebroplasty in the large animal model sheep

BACKGROUND CONTEXT Large animal models are highly recommended for meaningful preclinical studies, including the optimization of cement augmentation for vertebral body defects by vertebroplasty/kyphoplasty. PURPOSE The aim of this study was to perform a systematic characterization of a strictly minimally invasive in vivo large animal model for lumbar ventrolateral vertebroplasty. STUDY DESIGN/ SETTING This is a prospective experimental animal study. METHODS Lumbar defects (diameter 5 mm; depth approximately 14 mm) were created by a ventrolateral percutaneous approach in aged, osteopenic, female sheep (40 Merino sheep; 6-9 years; 68-110 kg). L1 remained untouched, L2 was left with an empty defect, and L3 carried a defect injected with a brushite-forming calcium phosphate cement (CPC). Trauma/functional impairment, surgical techniques (including drill sleeve and working canula with stop), reproducibility, bone defects, cement filling, and functional cement augmentation were documented by intraoperative incision-to-suture time and X-ray, postoperative trauma/impairment scores, and ex vivo osteodensitometry, microcomputed tomography (CT), histology, static/fluorescence histomorphometry, and biomechanical testing. RESULTS Minimally invasive vertebroplasty resulted in short operation times (28±2 minutes; mean±standard error of the mean) and X-ray exposure (1.59±0.12 minutes), very limited local trauma (score 0.00±0.00 at 24 hours), short postoperative recovery (2.95±0.29 hours), and rapid decrease of the postoperative impairment score to 0 (3.28±0.36 hours). Reproducible defect creation and cement filling were documented by intraoperative X-ray and ex vivo conventional/micro-CT. Vertebral cement augmentation and osteoconductivity of the CPC was verified by osteodensitometry (CPC>control), micro-CT (CPC>control and empty defect), histology/static histomorphometry (CPC>control and empty defect), fluorescence histomorphometry (CPC>control; all p<.05 for 3 and 9 months), and compressive strength measurements (CPC numerically higher than control; 102% for 3 months and 110% for 9 months). CONCLUSIONS This first-time systematic clinical assessment of a minimally invasive, ventrolateral, lumbar vertebroplasty model in aged, osteopenic sheep resulted in short operation times, rapid postoperative recovery, and high experimental reproducibility. This model represents an optimal basis for standardized evaluation of future studies on vertebral augmentation with resorbable and osteoconductive CPC.

Effects of oxygen plasma treatment on interfacial shear strength and post-peak residual strength of a PLGA fiber-reinforced brushite cement.

Biodegradable calcium phosphate cements (CPCs) are promising materials for minimally invasive treatment of bone defects. However, CPCs have low mechanical strength and fracture toughness. One approach to overcome these limitations is the modification of the CPC with reinforcing fibers. The matrix-fiber interfacial shear strength (ISS) is pivotal for the biomechanical properties of fiber-reinforced CPCs. The aim of the current study was to control the ISS between a brushite-forming CPC and degradable PLGA fibers by oxygen plasma treatment and to analyze the impact of the ISS alterations on its bulk mechanical properties. The ISS between CPC matrix and PLGA fibers, tested in a single-fiber pull-out test, increased up to 2.3-fold to max. 3.22±0.92MPa after fiber oxygen plasma treatment (100-300W, 1-10min), likely due to altered surface chemistry and morphology of the fibers. This ISS increase led to more efficient crack bridging and a subsequent increase of the post-peak residual strength at biomechanically relevant, moderate strains (up to 1%). At the same time, the work of fracture significantly decreased, possibly due to an increased proportion of fractured fibers unable to further absorb energy by frictional sliding. Flexural strength and flexural modulus were not affected by the oxygen plasma treatment. This study shows for the first time that the matrix-fiber ISS and some of the resulting mechanical properties of fiber-reinforced CPCs can be improved by chemical modifications such as oxygen plasma treatment, generating the possibility of avoiding catastrophic failures at the implant site and thus enhancing the applicability of biodegradable CPCs for the treatment of (load-bearing) bone defects.

Mechanical properties of microwave cured glass fibre epoxy composites prepared by resin transfer moulding

The aim of this study was to significantly reduce the curing time for glass fibre epoxy composites in industrial relevant dimensions without worsening of the mechanical properties. With the combination of microwave heating and resin transfer moulding (RTM), the time between filling the mould and demoulding the samples was reduced to only 1 h and 15 min compared to at least 6 h for conventional curing of the same material. Based on the different dielectric losses of cured and uncured resin a pulsed microwave process was developed. In this way homogenously cured samples were obtained. Tensile strength, flexural strength, flexural modulus of elasticity and Charpy impact strength of microwave cured samples were compared to conventionally cured samples. No statistically significant differences were found. Thus, microwave curing shows a high potential to improve the efficiency of fibre composite production while maintaining the mechanical properties.

Hemodynamic aspects of reduced platelet adhesion on bioinspired microstructured surfaces.

Occlusion by thrombosis due to the absence of the endothelial cell layer is one of the most frequent causes of failure of artificial vascular grafts. Bioinspired surface structures may have a potential to reduce the adhesion of platelets contributing to hemostasis. The aim of this study was to investigate the hemodynamic aspects of platelet adhesion, the main cause of thrombosis, on bioinspired microstructured surfaces mimicking the endothelial cell morphology. We tested the hypothesis that platelet adhesion is statistically significantly reduced on bioinspired microstructured surfaces compared to unstructured surfaces. Platelet adhesion as a function of the microstructure dimensions was investigated under flow conditions on polydimethylsiloxane (PDMS) surfaces by a combined experimental and theoretical approach. Platelet adhesion was statistically significantly reduced (by up to 78%; p≤0.05) on the microstructured PDMS surfaces compared to that on the unstructured control surface. Finite element method (FEM) simulations of blood flow dynamic revealed a micro shear gradient on the microstructure surfaces which plays a pivotal role in reducing platelet adhesion. On the surfaces with the highest differences of the shear stress between the top of the microstructures and the ground areas, platelet adhesion was reduced most. In addition, the microstructures help to reduce the interaction strength between fluid and surfaces, resulting in a larger water contact angle but no higher resistance to flow compared to the unstructured surface. These findings provide new insight into the fundamental mechanisms of reducing platelet adhesion on microstructured bioinspired surfaces and may lay the basis for the development of innovative next generation artificial vascular grafts with reduced risk of thrombosis.