Björn Carlberg

Electrospun polyurethane scaffolds for proliferation and neuronal differentiation of human embryonic stem cells

Adult central nervous system (CNS) tissue has a limited capacity to recover after trauma or disease. Hence, tissue engineering scaffolds intended for CNS repair and rehabilitation have been subject to intense research effort. Electrospun porous scaffolds, mimicking the natural three-dimensional environment of the in vivo extracellular matrix (ECM) and providing physical support, have been identified as promising candidates for CNS tissue engineering. The present study demonstrates in vitro culturing and neuronal differentiation of human embryonic stem cells (hESCs) on electrospun fibrous polyurethane scaffolds. Electrospun scaffolds composed of biocompatible polyurethane resin (Desmopan 9370A, Bayer MaterialScience AG) were prepared with a vertical electrospinning setup. Resulting scaffolds, with a thickness of approximately 150 microm, exhibited high porosity (84%) and a bimodal pore size distribution with peaks at 5-6 and 1 microm. The mean fiber diameter was measured to approximately 360 nm with a standard deviation of 80 nm. The undifferentiated hESC line SA002 (Cellartis AB, Göteborg, Sweden) was seeded and cultured on the produced scaffolds and allowed propagation and then differentiation for up to 47 days. Cultivation of hESC on electrospun fibrous scaffolds proved successful and neuronal differentiation was observed via standard immunocytochemistry. The results indicate that predominantly dopaminergic tyrosine hydroxylase (TH) positive neurons are derived in co-culture with fibrous scaffolds, in comparison to reference cultures under the same differentiation conditions displaying large proportions of GFAP positive cell types. Scanning electron micrographs confirm neurite outgrowth and connection to adjacent cells, as well as cell attachment to individual fibers of the fibrous scaffold. Consequently, electrospun polyurethane scaffolds have been proven feasible as a substrate for hESC propagation and neuronal differentiation. The physical interaction between cells and the fibrous scaffold indicates that these scaffolds provide a three-dimensional physical structure; a potential candidate for neural tissue engineering repair and rehabilitation.

A complete carbon-nanotube-based on-chip cooling solution with very high heat dissipation capacity

Heat dissipation is one of the factors limiting the continuous miniaturization of electronics. In the study presented in this paper, we designed an ultra-thin heat sink using carbon nanotubes (CNTs) as micro cooling fins attached directly onto a chip. A metal-enhanced CNT transfer technique was utilized to improve the interface between the CNTs and the chip surface by minimizing the thermal contact resistance and promoting the mechanical strength of the microfins. In order to optimize the geometrical design of the CNT microfin structure, multi-scale modeling was performed. A molecular dynamics simulation (MDS) was carried out to investigate the interaction between water and CNTs at the nanoscale and a finite element method (FEM) modeling was executed to analyze the fluid field and temperature distribution at the macroscale. Experimental results show that water is much more efficient than air as a cooling medium due to its three orders-of-magnitude higher heat capacity. For a hotspot with a high power density of 5000 W cm(-2), the CNT microfins can cool down its temperature by more than 40 °C. The large heat dissipation capacity could make this cooling solution meet the thermal management requirement of the hottest electronic systems up to date.

Surface-Confined Synthesis of Silver Nanoparticle Composite Coating on Electrospun Polyimide Nanofibers

A methodology for fabricating hierarchical nanostructures by surface-confined synthesis of silver nanoparticles on electrospun polyimide nanofibers is reported. Through surface-confined imide cleavage at the dianhydride domain via immersion in an aqueous KOH solution, potassium polyamate coatings of accurately defined thickness are formed (at a rate of 25 nm h(-1) ). By utilizing the ion-exchange capability of the polyamate resin, silver ions are introduced through immersion in an aqueous AgNO3 solution. Subsequent reduction of the metal ion species leads to the formation of nanoparticles at the fiber surface. Two modes of reduction, chemical and thermal, are investigated in the report, each leading to distinct morphologies of the nanoparticle coatings. Via thermal reduction, a composite surface layer consisting of monodisperse silver nanoparticles (average diameter 5.2 nm) embedded in a re-imidized polyimide matrix is achieved. In the case of chemical reduction, the reduction process occurs preferentially at the surface of the fiber, leading to the formation of silver nanoparticles anchored at the surface, though not embedded, in a polyamic acid matrix. By regulating the modification depth, control of the particle density on the fiber surface is established. In both reduction approaches, the polyimide nanofiber core exhibits maintained integrity.

Templated Growth of Covalently Bonded Three‐Dimensional Carbon Nanotube Networks Originated from Graphene

A template-assisted method that enables the growth of covalently bonded three-dimensional carbon nanotubes (CNTs) originating from graphene at a large scale is demonstrated. Atomic force microscopy-based mechanical tests show that the covalently bonded CNT structure can effectively distribute external loading throughout the network to improve the mechanical strength of the material.

Low temperature transfer and formation of carbon nanotube arrays by imprinted conductive adhesive

This letter demonstrates the transfer and formation of aligned carbon nanotube (CNT) arrays at low temperature by imprinted conductive adhesive. A thermoplastic isotropic conductive adhesive is patterned by an imprint and heat transfer process. The CNTs grown by thermal chemical vapor deposition are then transferred to another substrate by the conductive adhesive, forming predefined patterns. The current-voltage response of the transferred CNT bundles verifies that good electrical connection has been established. This process can enable the integration of CNTs into various temperature-sensitive processeses and materials.

Polymer‐metal nano‐composite films for thermal management

Purpose – The purpose of this paper is to present a novel nanostructured polymer‐metal composite film providing continuous all‐metal thermally conductive pathways, intended to meet future performance requirements on thermal interface materials (TIMs) in microelectronics packaging applications.Design/methodology/approach – Porous polymer structures with a thickness of approximately 100 μm were manufactured using electrospinning technology. Pressure‐assisted infiltration of low‐melting temperature alloy into the porous polymeric carrier resulted in the final composite film. Thermal performance was evaluated using an accurate and improved implementation of the ASTM D5470 standard in combination with an Instron 5548 MicroTester. Finally, a brief comparative study using three current state‐of‐the‐art commercial TIMs were carried out for reference purposes.Findings – Composite films with continuous all‐metal thermally conductive pathways from surface to surface were successfully fabricated. Thermal resistances ...

Nanostructured polymer-metal composite for thermal interface material applications

Continued miniaturization in combination with increased performance of microelectronics has generated an urgent need for improved thermal management techniques in order to maintain reliability of systems and devices. Development of advanced thermal interface materials has been identified as crucial, absorbing a portion of the advancements necessary within packaging technology. In this paper we introduce a novel nanostructured polymer-metal composite for thermal interface material applications together with an introduction to the associated manufacturing technology. The composite provides all-metal high thermal conductivity pathways between surfaces. Results show total thermal resistances as low as 8.5 Kmm2 W-1 at bondline thicknesses of approximately 70 mum, corresponding to an effective thermal conductivity of 8 Wm-1 K-1. In addition, a test fixture for thermal interface material characterization is introduced, combining the basic characteristics of the ASTM D5470 standard with a high precision Instron 5548 MicroTester. The test setup, acting as a subcomponent of the Instron 5548 MicroTester, exhibited excellent precision and repeatability throughout measurements.

A method to integrate patterned electrospun fibers with microfluidic systems to generate complex microenvironments for cell culture applications

The properties of a cells microenvironment are one of the main driving forces in cellular fate processes and phenotype expression invivo. The ability to create controlled cell microenvironments invitro becomes increasingly important for studying or controlling phenotype expression in tissue engineering and drug discovery applications. This includes the capability to modify material surface properties within well-defined liquid environments in cell culture systems. One successful approach to mimic extra cellular matrix is with porous electrospun polymer fiber scaffolds, while microfluidic networks have been shown to efficiently generate spatially and temporally defined liquid microenvironments. Here, a method to integrate electrospun fibers with microfluidic networks was developed in order to form complex cell microenvironments with the capability to vary relevant parameters. Spatially defined regions of electrospun fibers of both aligned and random orientation were patterned on glass substrates that were irreversibly bonded to microfluidic networks produced in poly-dimethyl-siloxane. Concentration gradients obtained in the fiber containing channels were characterized experimentally and compared with values obtained by computational fluid dynamic simulations. Velocity and shear stress profiles, as well as vortex formation, were calculated to evaluate the influence of fiber pads on fluidic properties. The suitability of the system to support cell attachment and growth was demonstrated with a fibroblast cell line. The potential of the platform was further verified by a functional investigation of neural stem cell alignment in response to orientation of electrospun fibers versus a microfluidic generated chemoattractant gradient of stromal cell-derived factor 1 alpha. The described method is a competitive strategy to create complex microenvironments invitro that allow detailed studies on the interplay of topography, substrate surface properties, and soluble microenvironment on cellular fate processes.

Recent progress of thermal interface materials

This paper reviews the status and recent progress achieved in the research of thermal interface materials (TIMs). The focus is on the research work performed in academia. The research and development work carried out in industry is also generally introduced. The existing TIM technologies have been categorized into eight main types and comprehensively analyzed. The state-of-the-art-research is then summarized and discussed with an emphasis on the carbon-filled materials. Other aspects of the TIM-related research, including theoretical study, modeling work, and characterization etc., are also briefly covered.

Electrospun Nano-Fibrous Polymer Films with Barium Titanate Nanoparticles for Embedded Capacitor Applications

Continued miniaturization, increased performance, as well as increased reliability of microelectronics require development of new design and manufacturing methods. Embedding discrete passive components into the substrate has been identified as a solution capable of accommodating a portion of the future demands on microelectronics. As embedded passive components are fundamentally different from discrete passive components, development of new materials is necessary. These new materials must meet requirements on manufacturability, electrical performance, reliability and cost. This paper presents the results of a parametric study where electrospun nano-fibrous polymer films containing barium titanate nanoparticles have been evaluated as possible dielectric materials for embedded decoupling capacitor applications. The study is of experimental character and demonstrates a novel technique for manufacturing of embedded capacitor dielectrics. Samples were produced with a standard electrospinning setup using various processing parameters. The produced samples were electrically characterized by guidance of the ASTM standard, using a parallel-plate test fixture and a HP 4284A precision LCR meter. The properties studied were specific capacitance and dissipation factor. Two different polymers were studied, designated polymer A and polymer B. Successful samples of polymer A and polymer B loaded with barium titanate were electrospun and characterized. Scanning electron microscopy (SEM) was used to characterize the surface morphology of the electrospun films. Polymer A samples showed good mechanical performance with and without barium titanate loading. Polymer B samples demonstrated a contrary behavior having inferior adhesion to the substrate and being brittle. Inclusion of barium titanate nanoparticles into the samples, of both polymer A and polymer B, showed indications of improved adhesion to the substrate and higher Youngs Modulus of samples. It was shown that inclusion of barium titanate into polymer A significantly changed the electrical properties of the films, increasing both specific capacitance and dissipation factor but also drastically reducing the frequency stability. The highest achieved specific capacitance for polymer A loaded with barium titanate was approximately 210 pF/cm2. High dissipation factors, reaching up to 0.6, were observed. Characterization of electrospun pure polymer B revealed a frequency stable dielectric with a low dissipation factor in the order of 0.01. The achieved specific capacitance was approximately 54 pF/cm2. Polymer B with 50.0 weight percent barium titanate nanoparticle loading, reaching values of 175 pF/cm2, had a different dispersion showing less frequency stability. As previously, inclusion of particles led to increased dissipation factors, reaching values of approximately 0.7.