The Role of Self-Assembled Monolayers in the Surface Modification and Interfacial Contact of Copper Fillers in Electrically Conductive Adhesives.

Printing of electrical circuits and interconnects using isotropic conductive adhesives (ICAs) is of great interest due to their low-temperature processing and compatibility with substrates for applications in sensors, healthcare, and flexible devices. As a lower cost alternative to silver (Ag), copper (Cu)-filled ICAs are desirable but limited by the formation of high-resistivity Cu surface oxides. To overcome this limitation, self-assembled monolayers (SAMs) of octadecanethiol (ODT) have been demonstrated to reduce the oxidation of micrometer-scale Cu powder particles for use in ICAs. However, the deposition and function of the SAM require further investigation, as described in this paper. As part of this work, the stages of the SAM deposition process, which included etching with hydrochloric acid to remove pre-existing oxides, were studied using X-ray photoelectron spectroscopy (XPS), which showed low levels of subsequent Cu oxidation when ODT coated. The treated Cu powders were combined with one- or two-part epoxy resins to make Cu-ICAs, and the effect of the Cu surface condition and weight loading on electrical conductivity was examined. When thermally cured in an inert argon atmosphere, ICAs filled with Cu protected by ODT achieved electrical conductivity up to 20 × 105 S·m-1, comparable to Ag-ICAs, and were used to make a functional circuit. To understand the function of the SAM in these Cu-ICAs, scanning and transmission electron microscopy were used to examine the internal micro- and nano-structures along with the elemental distribution at the interfaces within sections taken from cured samples. Sulfur (S), indicative of the ODT, was still detected at the internal polymer-metal interface after curing, and particle-to-particle contacts were also examined. XPS also identified S on the surface of cured Cu-ICAs even after thermal treatment. Based on the observations, electrical contact and conduction mechanisms for these Cu-filled ICAs are proposed and discussed.

Sodium Trimetaphosphate Crosslinked Starch Films Reinforced with Montmorillonite.

Synthetic polymers are the main food packaging material, although they are nonbiodegradable and their recycling process is expensive. A biodegradable, eco-friendly material, with high availability and low cost, such as starch, is a promising solution for the production of films for food packaging. To enhance starch film mechanical and barrier properties, nanoclays have been incorporated within the film matrix. Crosslinking is a well-established method to modify starch properties, but it has not been investigated in combination with nanoclay addition. In the present study, films were developed with starch that was crosslinked through the addition of 5, 15, and 40% wt. sodium trimetaphosphate (STMP) based on dry starch weight. To investigate the interaction between crosslinking and nanoclay addition, montmorillonite (MMT) was added at a 10.5% wt. concentration based on dry starch weight. Experimental data revealed a synergistic effect between STMP crosslinking and MMT addition regarding film thickness, elongation at break, color properties, and opacity. Regarding barrier properties, MMT addition negated the effect of STMP crosslinking, while, in the case of moisture content, it did not alter the effect of STMP crosslinking. Finally, in the case of tensile strength, a synergistic effect followed by a negative interaction was observed. In conclusion, the addition of MMT can potentially enhance, alongside crosslinking, some properties of the films, while other properties are not affected any more than just by crosslinking.

Optimizing the Functional Properties of Starch-Based Biodegradable Films.

A definitive screening design was used in order to evaluate the effects of starch, glycerol and montmorillonite (MMT) concentrations, as well as the drying temperature, drying tray type and starch species, on packaging film's functional properties. Optimization showed that in order to obtain films with the minimum possible thickness, the maximum elongation at break, the maximum tensile strength, as well as reduced water vapor permeability and low opacity, a combination of factors should be used as follows: 5.5% wt starch concentration, 30% wt glycerol concentration on a dry starch basis, 10.5% wt MMT concentration on a dry starch basis, 45 °C drying temperature, chickpea as the starch species and plexiglass as the drying tray type. Based on these results, starch films were prepared, and fresh minced meat was stored in them for 3 days. It was shown that the incorporation of MMT at 10.5% wt on a dry starch basis in the packaging films led to a decreased mesophilic and psychrotrophic bacteria growth factor compared to commercial packaging. When assessed for their biodegradability, the starch films disintegrated after 10 days of thermophilic incubation under simulated composting conditions. Finally, to prove their handling capability during industrial production, the starch films were rewound in a paper cylinder using an industrial-scale rewinding machine.

Printability and mechanical performance of biomedical PDMS-PEEK composites developed for material extrusion.

Polydimethylsiloxane (PDMS) materials are widely adopted in the manufacture of facial prostheses, lab-on-chip devices and scaffolds for soft-tissue engineering applications; however, their processing by additive manufacturing (AM) has proved challenging. Liquid silicone rubbers (LSRs) are favoured for their high shape fidelity when cast, but their low viscosity and surface tension often prevent self-support, post-extrusion. Poly(ether) ether ketone (PEEK) particle reinforcement through interfacial bonding has proven to enhance key properties of PDMS, expanding their end-use functionality. Still, the impact of such particles on the printability of LSR-PDMS is not explored. In this study, for the first time, solvent-free biocompatible PDMS-PEEK composites (up to 30 wt% PEEK) were successfully characterised for material extrusion (ME) printing. Rheological analysis confirmed shear-thinning of all PDMS-PEEK composites under applied load (within the tolerances of the printer) and dominant storage moduli at rest (i.e. prints can self-support), considered highly desirable for ME-based printing. Attained rheological datasets were used to guide initial printability studies, which revealed finer track fidelity with rising fractional content of PEEK, at comparable print speed and displacement values. Composites with higher PEEK content demonstrated significant increases in Shore A hardness and stiffness (in tension and compression) in bulk form. Last but not least, enhanced shape fidelity (thanks to PEEK reinforcement) and geometrical autonomy further expanded the manufacturing freedom of PDMS, whereby infill density could be controlled in order to increase the range of mechanical performance, previously unachievable with conventional casting fabrication. Fundamentally, this could lead to the manufacture of bespoke spatially graded multi-material structures and devices that could be used to replicate the heterogenous properties of soft human tissues and in other advanced material applications.

3D conformal bandpass millimeter-wave frequency selective surface with improved fields of view.

Conventional planar frequency selective surfaces (FSSs) are characterized in the far-field region and they are sensitive to the incidence angle of impinging waves. In this paper, a spherical dome FSS is presented, aiming to provide improved angular stable bandpass filtering performance as compared to its planar counterpart when the FSS is placed in the near-field region of an antenna source. A comparison between the conformal FSS and a finite planar FSS is presented through simulations at the frequency range between 26 to 40 GHz in order to demonstrate the advantages of utilizing the conformal FSS in the near-field. The conformal FSS is 3D printed and copper electroplated, which leads to a low-cost and lightweight bandpass filter array. Placing it in the near-field region of a primary antenna can be used as radomes to realize compact high-performance mm-wave systems. The comparison between simulated and measured conformal FSS results is in good agreement. The challenges that arise when designing, manufacturing, and measuring this type of structure are reported and guidelines to overcome these are presented.

Comparative analysis of four beta-galactosidases from Bifidobacterium bifidum NCIMB41171: purification and biochemical characterisation.

Four different beta-galactosidases (previously named BbgI, BbgII, BbgIII and BbgIV) from Bifidobacterium bifidum NCIMB41171 were overexpressed in Escherichia coli, purified to homogeneity and their biochemical properties and substrate preferences comparatively analysed. BbgI was forming a hexameric protein complex of 875 kDa, whereas BbgII, BbgIII and BbgIV were dimers with native molecular masses of 178, 351 and 248 kDa, respectively. BbgII was the only enzyme that preferred acidic conditions for optimal activity (pH 5.4-5.8), whereas the other three exhibited optima in more neutral pH ranges (pH 6.4-6.8). Na(+) and/or K(+) ions were prerequisite for BbgI and BbgIV activity in Bis-Tris-buffered solutions, whereas Mg(++) was strongly activating them in phosphate-buffered solutions. BbgII and BbgIII were slightly influenced from the presence or absence of cations, with Mg(++), Mn(++) and Ca(++) ions exerting the most positive effect. Determination of the specificity constants (k(cat)/K(m)) clearly indicated that BbgI (6.11 x 10(4) s(-1) M(-1)), BbgIII (2.36 x 10(4) s(-1) M(-1)) and especially BbgIV (4.01 x 10(5) s(-1) M(-1)) are highly specialised in the hydrolysis of lactose, whereas BbgII is more specific for beta-D-(1-->6) galactobiose (5.59 x 10(4) s(-1) M(-1)) than lactose (1.48 x 10(3) s(-1) M(-1)). Activity measurements towards other substrates (e.g. beta-D-(1-->6) galactobiose, beta-D-(1-->4) galactobiose, beta-D-(1-->4) galactosyllactose, N-acetyllactosamine, etc.) indicated that the beta-galactosidases were complementary to each other by hydrolysing different substrates and thus contributing in a different way to the bacterial physiology.

A novel alpha-galactosidase from Bifidobacterium bifidum with transgalactosylating properties: gene molecular cloning and heterologous expression.

A genomic library of Bifidobacterium bifidum (NCIMB 41171) DNA was constructed in Escherichia coli RA11r (melA(-)B(+)) and one alpha-galactosidase encoding gene was isolated. Conceptual translation combined with insertional mutagenesis analysis indicated an open reading frame (ORF) of 759 amino acid (aa) residues encoding an alpha-galactosidase (named as MelA) of 82.8 kDa. Partial purification and characterisation showed that the enzyme had an apparent native molecular mass of approximately 243 kDa and a subunit size of approximately 85 kDa. The enzyme belongs to glycosyl hydrolases 36 family with high aa sequence similarities (approximately 73%) to other known alpha-galactosidases of bifidobacterial origin. Under optimum pH conditions for activity (pH 6.0) and high melibiose concentration (40% w/v), the enzyme was able to form oligosaccharides with degree of polymerisation (DP) > or = 3 at higher concentration than DP = 2, with a total yield of 20.5% (w/w).

Expression of four beta-galactosidases from Bifidobacterium bifidum NCIMB41171 and their contribution on the hydrolysis and synthesis of galactooligosaccharides.

This paper deals with two aspects tightly related to the enzymatic characteristics and expression of four beta-galactosidases (BbgI, BbgII, BbgIII and BbgIV) from Bifidobacterium bifidum NCIMB41171. The growth patterns of this strain indicated a preference towards complex (i.e. lactose, galactooligosaccharides (GOSs)) rather than simple carbohydrates (i.e. glucose and galactose) and a collaborative action and synergistic relation of more than one beta-galactosidase isoenzyme for either lactose or GOS hydrolysis and subsequent assimilation. Native polyacrylamide gel electrophoresis analysis of protein extracts from cells growing on different carbohydrates (i.e. glucose, lactose or GOS) indicated that two lactose hydrolysing enzymes (BbgI and BbgIII) and one GOS hydrolysing enzyme (BbgII) were constitutively expressed, whereas a fourth lactose hydrolysing enzyme (BbgIV) was induced in the presence of lactose or different GOS fractions. Furthermore, the beta-galactosidase expression profiles of B. bifidum cells and the transgalactosylating properties of each individual isoenzyme, with lactose as substrate, clearly indicated that mainly three isoenzymes (BbgI, BbgIII and BbgIV) are implicated in GOS synthesis when whole B. bifidum cells are utilised. Two of the isoenzymes (BbgI and BbgIV) proved to have better transgalactosylating properties giving yields ranging from 42% to 47% whereas the rest (BbgI and BbgIII) showed lower yields (15% and 29%, respectively).

Molecular cloning and comparative analysis of four beta-galactosidase genes from Bifidobacterium bifidum NCIMB41171.

Bifidobacterium bifidum NCIMB41171 carries four genes encoding different beta-galactosidases. One of them, named bbgIII, consisted of an open reading frame of 1,935 amino acid (a.a.) residues encoding a protein with a multidomain structure, commonly identified on cell wall bound enzymes, having a signal peptide, a membrane anchor, FIVAR domains, immunoglobulin Ig-like and discoidin-like domains. The other three genes, termed bbgI, bbgII and bbgIV, encoded proteins of 1,291, 689 and 1,052 a.a. residues, respectively, which were most probably intracellularly located. Two cases of protein evolution between strains of the same species were identified when the a.a. sequences of the BbgI and BbgIII were compared with homologous proteins from B. bifidum DSM20215. The homologous proteins were found to be differentiated at the C-terminal a.a. part either due to a single nucleotide insertion or to a whole DNA sequence insertion, respectively. The bbgIV gene was located in a gene organisation surrounded by divergently transcribed genes putatively for sugar transport (galactoside-symporter) and gene regulation (LacI-transcriptional regulator), a structure that was found to be highly conserved in B. longum, B. adolescentis and B. infantis, suggesting optimal organisation shared amongst those species.

Synthesis of prebiotic galactooligosaccharides using whole cells of a novel strain, Bifidobacterium bifidum NCIMB 41171.

A novel strain of Bifidobacterium bifidum NCIMB 41171, isolated from a faecal sample from a healthy human volunteer and able to express beta-galactosidase activity, was used in synthesis reactions for the production of galactooligosaccharide from lactose. The beta-galactosidase activity of whole bifidobacterial cells showed an optimum activity at pH 6.8-7.0 and 40 degrees C. The transgalactosylation activity of the B. bifidum cells from 50% (w/w) lactose resulted in a galactooligosaccharide mixture (20% w/w) comprising (w/w): 25% disaccharides, 35% trisaccharides, 25% tetrasaccharides and 15% pentasaccharides. Using different initial lactose concentrations, the conversion rate to galactooligosaccharides was maximum (35%) when 55% (w/w) lactose was used. In fermentation experiments, B. bifidum showed an increased preference towards the produced galactooligosaccharide mixture, displaying higher growth rate and short-chain fatty acid production when compared with commercially available oligosaccharides.

A novel galactooligosaccharide mixture increases the bifidobacterial population numbers in a continuous in vitro fermentation system and in the proximal colonic contents of pigs in vivo.

Prebiotics are nondigestible food ingredients that encourage proliferation of selected groups of the colonic microflora, thereby altering the composition toward a more beneficial community. In the present study, the prebiotic potential of a novel galactooligosaccharide (GOS) mixture, produced by the activity of galactosyltransferases from Bifidobacterium bifidum 41171 on lactose, was assessed in vitro and in a parallel continuous randomized pig trial. In situ fluorescent hybridization with 16S rRNA-targeted probes was used to investigate changes in total bacteria, bifidobacteria, lactobacilli, bacteroides, and Clostridium histolyticum group in response to supplementing the novel GOS mixture. In a 3-stage continuous culture system, the bifidobacterial numbers for the first 2 vessels, which represented the proximal and traverse colon, increased (P < 0.05) after the addition of the oligosaccharide mixture. In addition, the oligosaccharide mixture strongly inhibited the attachment of enterohepatic Escherichia coli (P < 0.01) and Salmonella enterica serotype Typhimurium (P < 0.01) to HT29 cells. Addition of the novel mixture at 4% (wt:wt) to a commercial diet increased the density of bifidobacteria (P < 0.001) and the acetate concentration (P < 0.001), and decreased the pH (P < 0.001) compared with the control diet and the control diet supplemented with inulin, suggesting a great prebiotic potential for the novel oligosaccharide mixture.

Synthesis of isomaltooligosaccharides and oligodextrans in a recycle membrane bioreactor by the combined use of dextransucrase and dextranase.

A recycle ultrafiltration membrane reactor was used to develop a continuous synthesis process for the production of isomaltooligosaccharides (IMO) from sucrose, using the enzymes dextransucrase and dextranase. A variety of membranes were tested and the parameters affecting reactor stability, productivity, and product molecular weight distribution were investigated. Enzyme inactivation in the reactor was reduced with the use of a non-ionic surfactant but its use had severe adverse effects on the membrane pore size and porosity. During continuous isomaltooligosaccharide synthesis, dextransucrase inactivation was shown to occur as a result of the dextranase activity and it was dependent mainly on the substrate availability in the reactor and the hydrolytic activity of dextranase. Substrate and dextranase concentrations (50-200 mg/mL(-1) and 10-30 U/mL(-1), respectively) affected permeate fluxes, reactor productivity, and product average molecular weight. The oligodextrans and isomaltooligosaccharides formed had molecular weights lower than in batch synthesis reactions but they largely consisted of oligosaccharides with a degree of polymerization (DP) greater than 5, depending on the synthesis conditions. No significant rejection of the sugars formed was shown by the membranes and permeate flux was dependent on tangential flow velocity.