Jani Pelto

Surface properties and interaction forces of biopolymer-doped conductive polypyrrole surfaces by atomic force microscopy

Surface properties and electrical charges are critical factors elucidating cell interactions on biomaterial surfaces. The surface potential distribution and the nanoscopic and microscopic surface elasticity of organic polypyrrole-hyaluronic acid (PPy-HA) were studied by atomic force microscopy (AFM) in a fluid environment in order to explain the observed enhancement in the attachment of human adipose stem cells on positively charged PPy-HA films. The electrostatic force between the AFM tip and a charged PPy-HA surface, the tip-sample adhesion force, and elastic moduli were estimated from the AFM force curves, and the data were fitted to electrostatic double-layer and elastic contact models. The surface potential of the charged and dried PPy-HA films was assessed with Kelvin probe force microscopy (KPFM), and the KPFM data were correlated to the fluid AFM data. The surface charge distribution and elasticity were both found to correlate well with the nodular morphology of PPy-HA and to be sensitive to the electrochemical charging conditions. Furthermore, a significant change in the adhesion was detected when the surface was electrochemically charged positive. The results highlight the potential of positively charged PPy-HA as a coating material to enhance the stem cell response in tissue-engineering scaffolds.

Structural and functional characteristics of chimeric avidins physically adsorbed onto functionalized polythiophene thin films.

Stabilized bioreceptor layers are of great importance in the design of novel biosensors. In earlier work, chimeric avidins enabled immobilization of biotinylated antibodies onto gold surfaces with greater stability compared to more conventional avidins (wild-type avidin and streptavidin). In the present study, the applicability of chimeric avidins as a general binding scaffold for biotinylated antibodies on spin-coated functionalized polythiophene thin films has been studied by surface plasmon resonance and atomic force microscopy. Novel chimeric avidins showed remarkably increased binding characteristics compared with other avidins, such as wild-type avidin, streptavidin, and bacterial avidin when merely physically adsorbed onto the polythiophene surface. They gave the highest binding capacities, the highest affinity constant, and the highest stability for biotinylated probe immobilization. Introduction of carboxylic acid groups to polythiophene layer further enhanced the binding level of the avidins. Polythiophene layers functionalized with chimeric avidins thus offered a promising generic platform for biosensor applications.

Nonlinear electrical properties and mechanical strength of EPDM with polyaniline and carbon black filler

The influence of filler loading of polyaniline (PANI) and carbon black (CB) on the nonlinear electrical properties as well as the mechanical properties (tensile strength and Youngs modulus) of ethylene propylene diene monomer (EPDM) was investigated. It has been found that low field conductivity was most sensitive to filler addition and in particular carbon black. Threshold field for nonlinearity onset was also influenced but the relationship between parameter values and filler content was less clear. For the nonlinearity coefficient, tensile strength and Youngs modulus there was no observable relation to filler content. The study performed here show that the polyaniline/EPDM is a composite with good possibilities for tailoring the low field conductive properties whereas the mechanical strength seems unaffected by the variation of filler content.

Effect of low amount of nanosilica on dielectric properties of polypropylene

This paper presents the results of the dielectric properties measurements conducted on Silica-Polypropylene (PP) nanocomposites. According to prior investigations by the authors silica nanoparticles have improved the performance of the dielectric material considering capacitor applications. Especially breakdown strengths with ac and dc voltages and resistance against surface degradation have increased. The relative permittivity and dielectric losses have also been comparable to reference PP. In this paper the results of the dielectric measurements conducted on composites with 1–2 wt-% silica are compared with reference PP. Silica dispersion in PP was confirmed with transmission electron microscopy (TEM). Weibull analysis was applied to the breakdown strength measurement results with dc voltage. Capacitance and loss factor measurements were conducted as a function of temperature to study thermal behavior. The measurements were conducted at the high voltage laboratories of Tampere University of Technology and Chalmers University of Technology. Statistical analysis was applied to the results to determine the significance of the differences between the materials.

Hydrophobization and smoothing of cellulose nanofibril films by cellulose ester coatings

The Cellulose nanofibrils (CNF), also referred to as nanocellulose, is one of the most studied bio-based material in recent year, which has good potential in the future for packaging applications due to its excellent mechanical strength and oxygen barrier properties. In the future, CNF films may also find new applications for example in printed electronics, if the surface smoothness of CNF films can be improved. One way to improve surface smoothness is to use thin coating solutions with zero porosity, such as molar mass controlled cellulose ester coatings. In this study, we have coated CNF films using molar mass controlled cellulose esters with different side chain lengths forming 3-layer film (ester-CNF-ester). These coatings improved significantly the smoothness of CNF films. The 3-layer films have also good water vapor barrier and mechanical properties and the films are heat-sealable, which enable various new applications in the future.

Polypyrrole coating on poly-(lactide/glycolide)-β-tricalcium phosphate screws enhances new bone formation in rabbits

Polypyrrole (PPy) has gained interest as an implant material due to its multifunctional properties and its high compatibility with several cell and tissue types. For the first time, the biocompatibility and osteointegration of PPy coating, incorporated with chondroitin sulfate (CS), were studied in vivo by implanting PPy-coated bioabsorbable bone fixation composite screws of poly-(lactide/glycolide) copolymer (PLGA) and β-tricalcium phosphate (TCP) into New Zealand white rabbits. Uncoated bioabsorbable polymer composite screws and commercially available stainless steel cortical screws were used as reference implants. The rabbits were euthanized 12 and 26 weeks after the implantation. The systemic effects were evaluated from food and water consumption, body weight, body temperature, clinical signs, blood samples, internal organ weights, and histological examination. Local effects were studied from bone tissue and surrounding soft tissue histology. New bone formation was evaluated by micro-computed tomography, tetracycline labeling and torsion tests. Torsion tests were performed in order to capture the peak value of the torsion force during the course of the screws loosening. The coated screws induced significantly more bone formation than the uncoated screws. In addition, none of the implants induced any systemic or local toxicity. The results suggest that PPy is biocompatible with bone tissue and is a potential coating for enhancing osteointegration in orthopedic implants.

Encapsulation of 3-iodo-2-propynyl N-butylcarbamate (IPBC) in polystyrene-polycaprolactone (PS/PCL) blends

Abstract Polystyrene (PS, 1), polycaprolactone homopolymers (PCL, 2) and 3-Iodo-2-propynyl n-butylcarbamate (IPBC, 3) were physically mixed in dichloromethane (DCM) and processed into solid microspheres by using emulsion solvent evaporation method. Five different compositions with varying PS/PCL ratio were tested. The phase morphology of the microspheres was studied using Phase imaging atomic force microscopy (AFM) of polished cross-sections. Scanning electron microscopy was utilized to assess the distribution of IPBC in the polymer microspheres. The phase separation of the PS and PCL polymers in solvent cast films was assessed using polarized light optical microscopy of 11 polymer blends (0–100 wt-% PCL in PS). The PS/PCL-IPBC microspheres were incubated in water at RT and the release of IPBC was studied using high performance liquid chromatography (HPLC) at time points 1, 7 and 30 days. The microspheres dispersed in water borne outdoor paint matrix were tested for their antifouling activity against moulds in vitro.

Hydrophobization of cellophane and cellulose nano-fibrils films by supercritical state carbon dioxide impregnation with walnut oil

Bio-based films are typically sensitive towards moisture which limits the industrial applicability. In this study the cellulosic films produced from cellulose nanofibrils and cellophane were treated using supercritical state carbon dioxide impregnation with hydrophobic polyunsaturated oil mixture (walnut oil). The effects on surface hydrophobicity with cellophane films were clear and indisputable. Water contact angles of non-treated cellophane, walnut oil impregnated and further 1,6-hexanediol dimethacrylate crosslinked films were 62, 82 and 91°, respectively. Also the moisture absorption and water vapour transmission rate as well as oxygen transmission rate at 80% relative humidity decreased as a result of supercritical state carbon dioxide impregnation. Water vapour transmission rate of walnut oil impregnated cellulose nanofibrils film decreased by 30% as compared to non-treated film. Based on the FTIR analysis and surface roughness measurements the walnut oil penetrated better into the structure of cellophane as compared to CNF film. The developed hydrophobization method can be exploited in strengthening the position of cellulosic films in high performance film applications.

Uniform and electrically conductive biopolymer-doped polypyrrole coating for fibrous PLA

Three-dimensional, fibrous scaffolds can be easily fabricated from polylactide (PLA) using melt spinning and textile techniques. However, the surface properties of PLA scaffolds are not ideal for tissue engineering purposes. Furthermore, electrically conducting scaffolds are required to deliver electrical stimulation to cells. In this study, uniform, electrically conducting polypyrrole (PPy) coatings were fabricated on biodegradable PLA fibers. Biopolymer dopants-hyaluronic acid (HA) and chondroitin sulfate (CS)-were compared, and a PPy/CS composition was analyzed further. The effect of the oxidative polymerization conditions on the PLA fibers and CS counterion was studied. Furthermore, the initial molecular weight of CS and its degree of polymerization were determined. Our experiments showed that the molecular weight of CS decreases under oxidizing conditions but that the decay is not significant with the short polymerization process we used. The coating process was transferred to nonwoven PLA fabrics, and the stability of PPy/CS coating was studied during in vitro incubation in phosphate buffer solution at physiological temperature. The conductivity and surface roughness of the coating decayed during the 20-day incubation. The mechanical strength, however, remained at the initial level. Thus, the fabricated structures are suitable for short-term electrical stimulation adequate to promote cell functions in specific cases.

Influence of processing on the nonlinear conductive properties of PAni/XLPE and PAni/EPDM composites

Materials with nonlinear conductivity is sometimes used as field grading materials in high voltage applications. With respect to this, the electrical conductive properties of composites containing polyaniline (PAni) and carbon black (CB) as electroactive fillers have been studied with respect to the filler/matrix ratio, type of matrix material and how the composites were processed. Flat pads were made pressed from EPDM-PAni-composites, XLPE-PAni-granulates and from extruded XLPE-PAni-tapes, with eight different PAni-CB/matrix ratios in order to represent molded and vulcanized tape applications. It was found that the choice of matrix indicate an influence on the non-linearity of the conductivity while the low field conductivity was in the same range. The amount of polyaniline and carbon black had also an influence on the conductive properties of the composites; most notably the amount of carbon black filler had a significant impact on the low field conductivity levels. The changing of sample preparation procedure affected conductivity by several decades and it is concluded that the processing parameters at preparation of field grading layers has a significant impact on the conductive properties.