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See detailUltrafast and highly sensitive photodetectors fabricated on high-energy nitrogen-implanted GaAs,
Mikulics, M.; Marso, Michel UL; Kordoš, P. et al

in Applied Physics Letters (2003), 83

We have fabricated and tested metal–semiconductor–metal ~MSM! photodetectors based on nitrogen-ion-implanted GaAs. Nitrogen ions with energy of 700 and 880 keV, respectively, were implanted into epitaxial ... [more ▼]

We have fabricated and tested metal–semiconductor–metal ~MSM! photodetectors based on nitrogen-ion-implanted GaAs. Nitrogen ions with energy of 700 and 880 keV, respectively, were implanted into epitaxial GaAs films at an ion concentration of 331012 cm22. Ti/Au MSM photodetectors with 1-um-wide fingers were fabricated on top of the implanted GaAs. In comparison to low-temperature-grown GaAs photodetectors, produced in parallel in identical MSM geometry, the 880 keV N1-implanted photodetectors exhibited almost two orders of magnitude lower dark current ~10 nA at 1 V bias! and the responsivity more than doubled ~.20 mA/W at 20 V bias!. Illumination with 100-fs-wide, 810 nm wavelength laser pulses, generated ;2.5-ps-wide photoresponse signals with amplitudes as high as 2 V. The 2.5 ps relaxation time was the same for both the ion-implanted and low-temperature-grown devices and was limited by the MSM capacitance time constant. [less ▲]

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See detailConductivity and Hall effect measurements on intentionally undoped and doped AlGaN/GaN heterostructures before and after passivation,
Bernat, J.; Javorka, P.; Marso, Michel UL et al

in Applied Physics Letters (2003), 83((2003)), 5455

Conductivity and Hall effect measurements were performed before and after Si3N4 passivation of intentionally undoped and doped AlGaN/GaN heterostructures on Si and SiC substrates. An increase of the sheet ... [more ▼]

Conductivity and Hall effect measurements were performed before and after Si3N4 passivation of intentionally undoped and doped AlGaN/GaN heterostructures on Si and SiC substrates. An increase of the sheet carrier density ~up to ;30%! and a slight decrease of the electron mobility ~less than 10%! are found in all samples after passivation. The passivation induced sheet carrier density is 1.5– 231012 cm22 in undoped samples and only 0.731012 cm22 in 5–1031018 cm23 doped samples. The decrease of the electron mobility after passivation is slightly lower in highly doped samples. The channel conductivity in both types of unpassivated samples on Si and SiC substrates increases with an increase in doping density. After passivation, a well-resolved increase of channel conductivity is observed in the undoped or lightly doped samples and nearly the same channel conductivity results in the highly doped samples. [less ▲]

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See detailOn the phase sequence of antiferroelectric liquid crystals and its relation to orientational and translational order
Lagerwall, Jan UL; Rudquist, Per; Lagerwall, Sven T. et al

in Liquid Crystals (2003), 30(4), 399-414

The substance MHPOBC is the oldest and still most important reference antiferroelectric liquid crystal (AFLC). There is still considerable controversy concerning the correct phase designations for this ... [more ▼]

The substance MHPOBC is the oldest and still most important reference antiferroelectric liquid crystal (AFLC). There is still considerable controversy concerning the correct phase designations for this material and, in particular, about the presence or absence of SmC* in its phase sequence. By means of dielectric spectroscopy and polarizing microscopy, we show that whereas the pure compound lacks the SmC* phase, this phase rapidly replaces the SmC*b subphase through the reduced purity resulting from temperature-induced chemical degradation which is hard to avoid under standard experimental conditions. X-ray investi- gations furthermore show that this change in phase sequence is coupled to a decrease in translational order. This explains the large variations in the reported phase sequence and electro-optic behaviour of MHPOBC, in particular concerning the SmC*b phase which has been said to exhibit ferro-, ferri- as well as antiferroelectric properties. It is likely that the sensitivity of the AFLC phase sequence to sample purity is a general property of AFLC materials. We discuss the importance of optical and chemical purity as well as tilt and spontaneous polarization for the observed phase sequence and propose that one of the key features determining the existence of the different tilted structures is the antagonism between orientational (nematic) and translational (smectic) order. The decreased smectic order (increased layer interdigitation) imposed by chemical impurities promotes the synclinic SmC* phase at the cost of the AFLC phases SmC*a , SmC*b , SmC*c and SmC*a . We also propose that the SmA* phase in FLC and AFLC materials may actually have a somewhat different character and, depending on its microstructure, some of the tilted phases can be expected to appear or not to appear in the phase sequence. AFLC materials exhibiting a direct SmA*–SmC*a transition are found to be typical ‘de Vries smectics’, with very high orientational disorder in the SmA* phase. Finally, we discuss the fact that SmC*b and SmC*c have two superposed helical superstructures and explain the observation that the handedness of the large scale helix may very well change sign, while the handedness on the unit cell level is preserved. [less ▲]

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See detailPhysics of imaging p-n junctions by scanning tunneling microscopy and spectroscopy
Jäger, N. D.; Marso, Michel UL; Salmeron, M. et al

in Physical Review. B : Condensed Matter (2003), ((2003)), 165307

Combined voltage-dependent scanning tunneling microscopy ~STM! images with atomic resolution, local scanning tunneling spectroscopy, and simulations of the potential distribution in the interface STM tip ... [more ▼]

Combined voltage-dependent scanning tunneling microscopy ~STM! images with atomic resolution, local scanning tunneling spectroscopy, and simulations of the potential distribution in the interface STM tip system are used to extract the physical imaging mechanisms of GaAs p-n interfaces in STM images. It is shown that ~i! the tip-induced changes of the potential near the interface result in the tunneling characteristics of the p-type (n-type! layer being dragged into the interfaces’ depletion region at positive ~negative! sample voltage. ~ii! This leads to a considerable reduction of the apparent width of the image of the depletion zone in STM images. ~iii! At small negative sample voltages, a pronounced depression line appears. The depression is directly correlated with the electronic interface. It arises from the interplay of competing current contributions from the valence and conduction bands. This understanding of the imaging process allows us to develop methods on how to extract accurate physical data about the properties of the electronic interfaces from scanning tunneling microscopy images. [less ▲]

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See detailMaterial and Device Issues of AlGaN/GaN HEMTs on Silicon Substrates
Javorka, P.; Alam, A.; Marso, Michel UL et al

in Microelectronics Journal (2003), 34

Selected material and device issues related to the performance of AlGaN/GaN HEMTs on (111) Si substrates are reported. It is shown that these devices can sustain significantly higher dc power (16 W/mm ... [more ▼]

Selected material and device issues related to the performance of AlGaN/GaN HEMTs on (111) Si substrates are reported. It is shown that these devices can sustain significantly higher dc power (16 W/mm) than those grown on sapphire. Consequently smaller degradation in the device performance at higher temperatures (up to 400 8C) is demonstrated. Photoionisation spectroscopy reveals trap level of 1.85 eV, additional to two another levels found before in GaN-based HEMTs prepared on sapphire. Thus, AlGaN/GaN HEMTs on Si substrates demonstrate the viability of this technology for commercial application of high power rf devices. [less ▲]

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See detailInfluence of doping concentration on DC and RF performance of AlGaN/GaN HEMTs on silicon substrate
Marso, Michel UL; Javorka, P.; Dikme, Y. et al

in Physica Status Solidi A. Applications and Materials Science (2003), (1), 179-182

AlGaN/GaN HEMTs with different carrier supply layers are fabricated on Si substrate, using MOVPE growth technique. The influence of the carrier supply doping concentration on the HEMT properties is ... [more ▼]

AlGaN/GaN HEMTs with different carrier supply layers are fabricated on Si substrate, using MOVPE growth technique. The influence of the carrier supply doping concentration on the HEMT properties is investigated by Hall measurements and by electrical DC and RF characterisation of transistor devices. Hall mobility is found to decrease with increasing sheet concentration, while the gate leakage current increases. The device with the highest carrier supply doping concentration of 1019 cm–3 is provided with a GaN cap layer to reduce gate leakage. The transistors based on modulation doped layer structures show much higher DC performance than the undoped device. Best RF properties are obtained for a doping level of 5 × 1018 cm–3. High frequency measurements exhibit fT and fMAX values of 35 GHz and 37 GHz, respectively, for devices with 300 nm gate length. [less ▲]

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See detailPeculiarities of low frequency noise in GaN-based high electron mobility transistors
Vitusevich, S. A.; Petrychuk, M. V.; Klein, N. et al

in Proc. 17th Internat. Conf. Noise and Fluctuations ICNF (2003)

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See detailInfluence of SiO/sub 2/ and Si/sub 3/N/sub 4/ passivation on AlGaN/GaN/Si HEMT performance,
Javorka, P.; Bernát, J.; Fox, A. et al

in Electronics Letters (2003), 39((2003)), 1155-1157

Different influence of SiO2 and Si3N4 passivation on performance of AlGaN=GaN=Si HEMTs is reported. DC characteristics are less enhanced by using SiO2 than Si3N4. This is in agreement with carrier ... [more ▼]

Different influence of SiO2 and Si3N4 passivation on performance of AlGaN=GaN=Si HEMTs is reported. DC characteristics are less enhanced by using SiO2 than Si3N4. This is in agreement with carrier concentration changes after passivation, as follows from Hall data. Small signal RF performance is degraded after applying SiO2 and enhanced after Si3N4 passivation, e.g. for unpassivated devices fTffi17 GHz which decreased to 9 GHz and increased to 28 GHz for SiO2 and Si3N4 respectively. The fmax=fT ratio has not changed after passivation. [less ▲]

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See detailWet low-temperature gate oxidation for nanoscale vertical field-effect transistors,
Goryll, M.; Moers, J.; Trellenkamp, S. et al

in Physica E (2003), 19(2003), 18-22

In this work, we present an approach towards improving a vertical eld-e ect transistor based on a narrow mesa that is capable of showing complete channel inversion. Contrary to similar concepts it does ... [more ▼]

In this work, we present an approach towards improving a vertical eld-e ect transistor based on a narrow mesa that is capable of showing complete channel inversion. Contrary to similar concepts it does not necessarily require the use of an SOI substrate due to the chosen vertical layer sequence. An important issue during process flow is the limited thermal budget in order to preserve the desired channel length. Here a low-temperature wet oxidation process is investigated to prevent dopant di ffusion in early process steps. Results on the thickness homogeneity and electrical properties of this gate oxide will be presented and discussed. [less ▲]

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See detailInvestigation of traps in AlGaN/GaN HEMTs on silicon substrate
Wolter, M.; Marso, Michel UL; P.Javorka, J. et al

in Physica Status Solidi C. Current Topics in Solid State Physics (2003), (7), 2360-2363

Deep levels in AlGaN/GaN heterostructures are known to be responsible for trapping processes like current collapse or dispersion. In order to investigate these processes we performed different ... [more ▼]

Deep levels in AlGaN/GaN heterostructures are known to be responsible for trapping processes like current collapse or dispersion. In order to investigate these processes we performed different measurements on HEMTs fabricated with heterostructures grown on silicon substrate. First by photoionization spectroscopy we found three different traps with activation energies of about 2.1 eV, 2.9 eV and 3.2 eV. Secondly, the temperature dependent relaxation of the drain current was investigated by the backgating current deep level transient spectroscopy (DLTS) method. Hereby we detected majority and minority carrier traps in the GaN buffer at the energies EV + 0.41 eV and EC – 0.55 eV, respectively. The latter energy can be attributed to the well known “E2” level in GaN. [less ▲]

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See detailMaterial and Device Issues of AlGaN/GaN HEMTs on Silicon Substrates
Javorka, P.; Alam, A.; Marso, Michel UL et al

in Proceedings of the MRS Fall Meeting, Boston 2002 (2003)

Results on the preparation and properties of AlGaN/GaN HEMTs on silicon substrates are presented and selected issues related to the material structure and device performance devices are discussed ... [more ▼]

Results on the preparation and properties of AlGaN/GaN HEMTs on silicon substrates are presented and selected issues related to the material structure and device performance devices are discussed. Virtually crack-free AlGaN/GaN heterostructures (xAlN ≅ 0.25), with low surface roughness (rms of 0.64 nm), ns ≅ 1×1013 cm-2 and μ ≅ 1100 cm2/V s at 300 K, were grown by LP-MOVPE on 2-inch (111)Si substrates. HEMT devices with Lg = 0.3−0.7 μm were prepared by conventional device processing steps. Photoionization spectroscopy measurements have shown that a trap level of 1.85 eV, additional to two levels of 2.9 and 3.2 eV found before on GaN-based HEMTs on sapphire, is present in the structures investigated. Self-heating effects were studied by means of temperature dependent dc measurements. The channel temperature of a HEMT on Si increases with dissipated power much slower than for similar devices on sapphire substrate (e.g. reaches 95 and 320 °C on Si and sapphire, respectively, for 6 W/mm power). Prepared AlGaN/GaN/Si HEMTs exhibit saturation currents up to 0.91 A/mm, a good pinch-off, peak extrinsic transconductances up to 150 mS/mm and static heat dissipation capability up to ~16 W/mm. Unity current gain frequencies fT up to 21 and 32 GHz were obtained on devices with gate length of 0.7 and 0.5 μm, respectively. The saturation current and fT values are comparable to those known for similar devices using sapphire and SiC substrates. Properties of AlGaN/GaN/Si HEMTs investigated show that this technology brings a prospect for commercial application of high power rf devices. [less ▲]

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See detailInvestigation of buffer traps in an AlGaN/GaN/Si high electron mobility transistor by backgating current deep level transient spectroscopy
Marso, Michel UL; Wolter, M.; Javorka, P. et al

in Applied Physics Letters (2003), 82

The influence of a substrate voltage on the dc characteristics of an AlGaN/GaN high electron mobility transistor ~HEMT! on silicon ~111! substrate is profited to investigate traps that are located between ... [more ▼]

The influence of a substrate voltage on the dc characteristics of an AlGaN/GaN high electron mobility transistor ~HEMT! on silicon ~111! substrate is profited to investigate traps that are located between the substrate and the two-dimensional electron gas channel. The transient of the drain current after applying a negative substrate voltage is evaluated in the temperature range from 30 to 100 °C. With this method, known as backgating current deep level transient spectroscopy, majority carrier traps with activation energy of 200 meV as well as minority carrier traps at 370 meV are identified. The experiments are performed on completed HEMTs, allowing the investigation of the influence of device fabrication technology. [less ▲]

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See detailBackgating high-current and breakdown characterisation of AlGaN/GaN HEMTs on silicon substrates
Kuzmik, J.; Blaho, M.; Pogany, D. et al

in Proceedings of the ESSDERC 2003. Estoril, Portugal (2003)

Backgating effect as well as breakdown and high-current performance of AlGaN/GaN HEMTs on silicon substrates are studied. The material structure of investigated devices differ in the thickness of ... [more ▼]

Backgating effect as well as breakdown and high-current performance of AlGaN/GaN HEMTs on silicon substrates are studied. The material structure of investigated devices differ in the thickness of stressrelaxing intermediate layer sequence (~1 μm and ~2.5 μm thick). It is shown that the transistor backgating effect is reduced for the thicker sequence. Similarly, the reverse gate current is two orders of the magnitude lower and the gate-drain breakdown voltage increases substantially in devices with the thicker sequence. Increase from ~40 V to ~160 V of the HEMT blocking capability measured under electrostatic discharge-like conditions is also observed. [less ▲]

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See detailTyrosine phosphorylation disrupts elongin interaction and accelerates SOCS3 degradation.
Haan, Serge UL; Ferguson, Paul; Sommer, Ulrike et al

in Journal of Biological Chemistry (2003), 278(34), 31972-9

The suppressors of cytokine signaling (SOCS) are negative feedback inhibitors of cytokine and growth factor-induced signal transduction. The C-terminal SOCS box region is thought to regulate SOCS protein ... [more ▼]

The suppressors of cytokine signaling (SOCS) are negative feedback inhibitors of cytokine and growth factor-induced signal transduction. The C-terminal SOCS box region is thought to regulate SOCS protein stability most likely via an elongin C interaction. In the present study, we have found that phosphorylation of SOCS3 at two tyrosine residues in the conserved SOCS box, Tyr204 and Tyr221, can inhibit the SOCS3-elongin C interaction and activate proteasome-mediated SOCS3 degradation. Jak-mediated phosphorylation of SOCS3 decreased SOCS3 protein half-life, and phosphorylation of both Tyr204 and Tyr221 was required to fully destabilize SOCS3. In contrast, a phosphorylation-deficient mutant of SOCS3, Y204F,Y221F, remained stable in the presence of activated Jak2 and receptor tyrosine kinases. SOCS3 stability correlated with the relative amount that bound elongin C, because in vitro phosphorylation of a SOCS3-glutathione S-transferase fusion protein abolished its ability to interact with elongin C. In addition, a SOCS3/SOCS1 chimera that co-precipitates with markedly increased elongin C, was significantly more stable than wild-type SOCS3. The data suggest that interaction with elongin C stabilizes SOCS3 protein expression and that phosphorylation of SOCS box tyrosine residues disrupts the complex and enhances proteasome-mediated degradation of SOCS3. [less ▲]

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See detailA fusion protein of the gp130 and interleukin-6Ralpha ligand-binding domains acts as a potent interleukin-6 inhibitor.
Ancey, Cecile; Kuster, Andrea; Haan, Serge UL et al

in Journal of Biological Chemistry (2003), 278(19), 16968-72

Interleukin (IL)-6 is involved in the maintenance and progression of several diseases such as multiple myeloma, rheumatoid arthritis, or osteoporosis. The present work aims at the development of an IL-6 ... [more ▼]

Interleukin (IL)-6 is involved in the maintenance and progression of several diseases such as multiple myeloma, rheumatoid arthritis, or osteoporosis. The present work aims at the development of an IL-6 inhibitor for the use in anti-cytokine therapies. The IL-6 receptor is composed of two different subunits, an alpha-subunit (IL-6Ralpha) that binds IL-6 with low affinity and a beta-subunit (gp130) that binds the IL-6.IL-6Ralpha complex with high affinity and as a result triggers intracellular signaling. In its soluble form, gp130 is a natural antagonist that neutralizes IL-6.soluble IL-6Ralpha complexes. It was our strategy to appropriately fuse the two receptor subunit fragments involved in IL-6 receptor complex formation to bind IL-6 with high affinity and to antagonize its effects. The ligand-binding domains of gp130 (D1-D2-D3) and IL-6Ralpha (D2-D3) were connected using three different linkers. The resulting constructs were expressed in stably transfected insect cells and tested for their ability to inhibit IL-6 activity in several in vitro systems. All fusion proteins were strong inhibitors of IL-6 signaling and abrogated IL-6-induced phosphorylation of STAT3, proliferation of transfected Ba/F3 cells, and induction of acute-phase protein synthesis. As intended, the fused receptors were much more effective than the separately expressed soluble receptor proteins. The fusion protein strategy presented here can also be applied to other cytokines that signal via receptors composed of two different subunits to design new potent inhibitors for anti-cytokine therapies. [less ▲]

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See detailActivation of STAT3 by IL-6 and IL-10 in primary human macrophages is differentially modulated by suppressor of cytokine signaling 3.
Niemand, Claudia; Nimmesgern, Ariane; Haan, Serge UL et al

in Journal of immunology (Baltimore, Md. : 1950) (2003), 170(6), 3263-72

On human macrophages IL-10 acts as a more potent anti-inflammatory cytokine than IL-6, although both cytokines signal mainly via activation of the transcription factor STAT3. In this study we compare IL ... [more ▼]

On human macrophages IL-10 acts as a more potent anti-inflammatory cytokine than IL-6, although both cytokines signal mainly via activation of the transcription factor STAT3. In this study we compare IL-10 and IL-6 signaling in primary human macrophages derived from blood monocytes. Pretreatment of macrophages with PMA or the proinflammatory mediators LPS and TNF-alpha blocks IL-6-induced STAT3 activation, whereas IL-10-induced activation of STAT3 remains largely unaffected. Although LPS induces the feedback inhibitor suppressor of cytokine signaling 3 (SOCS3) in macrophages, inhibition of IL-6 signal transduction by LPS occurs rapidly and does not depend on gene transcription. We also found that pretreatment of macrophages with IL-10 inhibits subsequent STAT3 activation by IL-6, whereas IL-10-induced STAT3 activation is not affected by preincubation with IL-6. This cross-inhibition is dependent on active transcription and might therefore be explained by different sensitivities of IL-10 and IL-6 signaling toward the feedback inhibitor SOCS3, which is induced by both cytokines. In contrast to the IL-6 signal transducer gp130, which has been previously shown to recruit SOCS3 to one of its phosphotyrosine residues (Y759), peptide precipitation experiments suggest that SOCS3 does not interact with phosphorylated tyrosine motifs of the IL-10R. Taken together, different sensitivities of IL-10 and IL-6 signaling toward mechanisms that inhibit the Janus kinase/STAT pathway define an important mechanism that contributes to the different anti-inflammatory potencies of these two cytokines. [less ▲]

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See detailPrinciples of interleukin (IL)-6-type cytokine signalling and its regulation
Heinrich, P. C.; Behrmann, Iris UL; Haan, Serge UL et al

in Biochemical Journal (2003), 374(Pt 1), 1-20

The IL (interleukin)-6-type cytokines IL-6, IL-11, LIF (leukaemia inhibitory factor), OSM (oncostatin M), ciliary neurotrophic factor, cardiotrophin-1 and cardiotrophin-like cytokine are an important ... [more ▼]

The IL (interleukin)-6-type cytokines IL-6, IL-11, LIF (leukaemia inhibitory factor), OSM (oncostatin M), ciliary neurotrophic factor, cardiotrophin-1 and cardiotrophin-like cytokine are an important family of mediators involved in the regulation of the acute-phase response to injury and infection. Besides their functions in inflammation and the immune response, these cytokines play also a crucial role in haematopoiesis, liver and neuronal regeneration, embryonal development and fertility. Dysregulation of IL-6-type cytokine signalling contributes to the onset and maintenance of several diseases, such as rheumatoid arthritis, inflammatory bowel disease, osteoporosis, multiple sclerosis and various types of cancer (e.g. multiple myeloma and prostate cancer). IL-6-type cytokines exert their action via the signal transducers gp (glycoprotein) 130, LIF receptor and OSM receptor leading to the activation of the JAK/STAT (Janus kinase/signal transducer and activator of transcription) and MAPK (mitogen-activated protein kinase) cascades. This review focuses on recent progress in the understanding of the molecular mechanisms of IL-6-type cytokine signal transduction. Emphasis is put on the termination and modulation of the JAK/STAT signalling pathway mediated by tyrosine phosphatases, the SOCS (suppressor of cytokine signalling) feedback inhibitors and PIAS (protein inhibitor of activated STAT) proteins. Also the cross-talk between the JAK/STAT pathway with other signalling cascades is discussed. [less ▲]

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See detailCompte rendu des premières rencontres de l’UMR de droit comparé
Poillot, Elise UL

in Zeitschrift für Europäisches Privatrecht (2003)

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See detailGabapentin reduces the mechanosensitivity of fine afferent nerve fibres in normal and inflamed rat knee joints.
Hanesch, Ulrike UL; Pawlak, Matthias; McDougall, Jason

in Pain (2003), 104

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