Nanotechnology and Dental Implants

The long-term clinical success of dental implants is related to their early osseointegration. This paper reviews the different steps of the interactions between biological fluids, cells, tissues, and surfaces of implants. Immediately following implantation, implants are in contact with proteins and platelets from blood. The differentiation of mesenchymal stem cells will then condition the peri-implant tissue healing. Direct bone-to-implant contact is desired for a biomechanical anchoring of implants to bone rather than fibrous tissue encapsulation. Surfaces properties such as chemistry and roughness play a determinant role in these biological interactions. Physicochemical features in the nanometer range may ultimately control the adsorption of proteins as well as the adhesion and differentiation of cells. Nanotechnologies are increasingly used for surface modifications of dental implants. Another approach to enhance osseointegration is the application of thin calcium phosphate (CaP) coatings. Bioactive CaP nanocrystals deposited on titanium implants are resorbable and stimulate bone apposition and healing. Future nanometer-controlled surfaces may ultimately direct the nature of peri-implant tissues and improve their clinical success rate

Propriétés optoélectroniques de LEDs à nanofils coeur-coquille InGaN/GaN

III-nitrides i.e. GaN, InN, AlN and their alloys are semiconductors of choice to fabricate optoelectronic devices such as Light Emitting Diodes (LEDs). One of their most interesting features relies in their direct band gap that covers a very wide spectral range, from infrared (0.65 eV for InN) to UV (6.2 eV for AlN). However, the lack of lattice-matched substrate has been responsible for strong crystalline quality issues in heteroepitaxial thin films. Thanks to stress relaxation at their surface, nanowires provide a smart solution to this problem. Besides, they have a few more assets. In comparison to thin films, nanowires can improve the internal quantum efficiency of LEDs because of their higher effective surface that leads to lowered current densities and thus mitigated Auger effect. The internal quantum efficiency also benefits from the possibility to grow the active region on non polar facets, thus getting rid of the detrimental high internal polarization-induced electric field in quantum wells. Furthermore, the photon extraction efficiency is enhanced by the guiding effect of nanowires. However, despite all this promising advantages, one of the main challenges remains the control of structural homogeneity from wire to wire but also inside single wires.In this context, my work has consisted into studying from an experimental and theoretical point of view the consequences of these structural inhomogeneities on the optoelectronic properties of nanowire based LED devices. I have shown that the current tends to gather into indium-rich regions for moderate bias. At higher bias, the dominant current path though the junction is generally located under the p-contact on the nanowire shell. I have theoretically demonstrated that the unipolar and ambipolar diffusion of carriers as well as their drift induced by a composition gradient inside the quantum wells is not significant in the devices I have studied. Moreover, I took also an interest in the detailed analyze of I-V curves. Thanks to a simple model, I have identified the presence of leakage current related to defect- and phonon-assisted tunneling effect. In the second part of my work, I have focused onto the characterization of core-shell wires using the Electron Beam Induced Current technique. The bias-dependant and acceleration voltage-dependant EBIC maps has been explained with a theoretical model based on equivalent circuits. This study leads me to suggest a new experimental method that can be used to map the nanowire core and shell resistivity.Les nitrures d’éléments III, à savoir GaN, InN, AlN et leurs alliages, forment une famille de matériaux semi-conducteurs dont les propriétés sont particulièrement intéressantes pour la réalisation de diodes électroluminescentes (LEDs). Leur intérêt réside en particulier dans leur bande interdite qui est directe et qui couvre une large bande spectrale de l’infrarouge (0,65eV pour InN) à l’ultraviolet (6,2eV pour AlN). En raison de l’absence de substrats accordés en maille avec ces matériaux, les couches minces hétéroépitaxiées de nitrure sont généralement touchées par des problèmes de qualité cristalline. Grâce au phénomène de relaxation des contraintes en surface, les nanofils offrent une solution prometteuse pour résoudre ce problème. Ils combinent de nombreux autres avantages : en comparaison des couches minces, l’efficacité quantique interne des LEDs peut être améliorée (surface effective plus importante permettant de diminuer l’effet Auger à courant injecté identique, absence de champ de polarisation en utilisant les facettes non polaires des nanofils) et l’extraction des photons est facilitée par l’effet guide d’onde des nanofils. Cependant, une des difficultés est de parvenir à contrôler la synthèse de ces nano-objets pour garantir une homogénéité des propriétés structurales d’un fil à l’autre et au sein d’un même fil. Dans ce contexte, mon travail de thèse a consisté à étudier d’un point de vue expérimental et théorique l’impact des inhomogénéités structurales sur les propriétés optoélectroniques de dispositifs à nanofils de type LED. J’ai pu mettre en évidence et modéliser un effet de concentration du courant dans les régions riches en indium lorsque les courants injectés sont modérés. Pour de forts courants, le courant se concentre à proximité du contact sur la coquille dopée p. Théoriquement, j’ai montré que la dérive des porteurs de charge dans les puits quantiques et leur diffusion unipolaire et ambipolaire en présence d’un gradient de compositions des puits étaient négligeables. Par ailleurs, je me suis également intéressé à l’interprétation des caractéristiques courant-tension. A l’aide d’un modèle simple, j’ai également identifié la présence de courant de fuite par effet tunnel dans des structures présentant une densité importante de défaut. Dans une seconde partie de ma thèse, je me suis également intéressé à la caractérisation de nanofils à structure coeur-coquille par la technique de courant induit par faisceau d’électrons (Electron Beam Induced Current). La dépendance des cartographies EBIC en fonction de la tension appliquée et de l’énergie du faisceau incident a été modélisée. Ce travail m’a notamment amené à proposer une nouvelle méthode de caractérisation permettant de cartographier les résistivités du coeur et de la coquille des nanofils

Résonateurs à haut Q en mode d'extension 2D pour possibles applications temps-fréquence

International audienceThis paper presents recent advances on two dimensional length-extension mode (2D-LEM) quartz resonators providing high quality (Q) factor on resonances at a few MHz. The resonators have been collectively manufactured using one or two steps quartz deep reactive-ion etching (DRIE) processes. These resonators combine the intrinsic qualities of quartz in comparison to silicon (i.e. high Q factor, low temperature sensitivity and piezoelectricity) and the advantages of microelectromechanical systems (MEMS) resonators: small dimensions, low power consumption and collective processes. Samples vibrating at frequencies f of 2.2, 3 and 4.5 MHz have shown promising results with very high Q factor. Q factor as high as 180,000 for fundamental mode vibrating at 2.2 MHz and 89,000 for harmonic mode at 8.9 MHz were measured which lead to quality factor and resonance frequency products (Q·f) figure of merit near 10^12 Hz at the state of the art for 2D-LEM quartz resonators and the higher Q factor measured for DRIE made quartz resonators. Two designs, several dimensions and two processes have been investigated. Two main limiting damping mechanisms were identified and one of them is strongly linked to the technological limits of the etching process.Ce document présente les avancées récentes sur les résonateurs à quartz en mode d'extension bidimensionnelle de la longueur (2D-LEM) qui fournissent facteur de haute qualité (Q) sur les résonances à quelques MHz. Les résonateurs ont été fabriqués collectivement en utilisant un ou deux étapes des processus de gravure profonde par ions réactifs du quartz (DRIE). Ces résonateurs combinent les qualités intrinsèques du quartz dans par rapport au silicium (facteur Q élevé, faible sensibilité à la température et piézoélectricité) et les avantages de la microélectromécanique les résonateurs des systèmes MEMS : petites dimensions, faible consommation d'énergie et processus collectifs.Des échantillons vibrant à des fréquences f de 2,2, 3 et 4,5 MHz ont donné des résultats prometteurs avec un facteur Q très élevé. Le facteur Q comme jusqu'à 180 000 pour le mode fondamental vibrant à 2,2 MHz et 89 000 pour le mode harmonique à 8,9 MHz ont été mesurés qui conduisent à un facteur de qualité et à des produits de fréquence de résonance (Qxf) d'un mérite proche de 10^12 Hz à l'état de l'art pourLes résonateurs à quartz 2D-LEM et le facteur Q plus élevé mesuré pour le MEIR ont fait des résonateurs à quartz. Deux modèles, plusieurs et deux processus ont été étudiés. Deux principaux mécanismes d'amortissement limitatifs ont été identifiés et l'un des est fortement liée aux limites technologiques du processus de gravure

InGaN/GaN core/shell nanowires for visible to ultraviolet range photodetection

International audienceWe report on the fabrication and characterization of single nitride nanowire visible-to-ultraviolet p-n photodetec-tors. Nitride nanowires containing 30 InGaN/GaN radial quantum wells with 18% indium fraction were grown by catalyst-free metal-organic vapour phase epitaxy. Single nanowires were contacted using optical lithography. As expected for a radial p-n junction, the current-voltage (I-V) curves of single wire detectors show a rectifying behavior in the dark and a photocurrent under illumination. The detectors present a response in the visible to UV spectral range starting from 2.8 eV. The peak responsivity is 0.17 A/W at 3.36 eV. The on-off switching time under square light pulses is found to be below 0.1 sec

Computational model combined with in vitro experiments to analyse mechanotransduction during mesenchymal stem cell adhesion.

The shape that stem cells reach at the end of adhesion process influences their differentiation. Rearrangement of cytoskeleton and modification of intracellular tension may activate mechanotransduction pathways controlling cell commitment. In the present study, the mechanical signals involved in cell adhesion were computed in in vitro stem cells of different shapes using a single cell model, the so-called Cytoskeleton Divided Medium (CDM) model. In the CDM model, the filamentous cytoskeleton and nucleoskeleton networks were represented as a mechanical system of multiple tensile and compressive interactions between the nodes of a divided medium. The results showed that intracellular tonus, focal adhesion forces as well as nuclear deformation increased with cell spreading. The cell model was also implemented to simulate the adhesion process of a cell that spreads on protein-coated substrate by emitting filopodia and creating new distant focal adhesion points. As a result, the cell model predicted cytoskeleton reorganisation and reinforcement during cell spreading. The present model quantitatively computed the evolution of certain elements of mechanotransduction and may be a powerful tool for understanding cell mechanobiology and designing biomaterials with specific surface properties to control cell adhesion and differentiation

Lateral root emergence in Arabidopsisis dependent on transcription factor LBD29 regulation of auxin influx carrier LAX3

Lateral root primordia (LRP) originate from pericycle stem cells located deep within parental root tissues. LRP emerge through overlying root tissues by inducing auxin-dependent cell separation and hydraulic changes in adjacent cells. The auxin-inducible auxin influx carrier LAX3 plays a key role concentrating this signal in cells overlying LRP. Delimiting LAX3 expression to two adjacent cell files overlying new LRP is crucial to ensure that auxin-regulated cell separation occurs solely along their shared walls. Multiscale modeling has predicted that this highly focused pattern of expression requires auxin to sequentially induce auxin efflux and influx carriers PIN3 and LAX3, respectively. Consistent with model predictions, we report that auxin-inducible LAX3 expression is regulated indirectly by AUXIN RESPONSE FACTOR 7 (ARF7). Yeast one-hybrid screens revealed that the LAX3 promoter is bound by the transcription factor LBD29, which is a direct target for regulation by ARF7. Disrupting auxin-inducible LBD29 expression or expressing an LBD29-SRDX transcriptional repressor phenocopied the lax3 mutant, resulting in delayed lateral root emergence. We conclude that sequential LBD29 and LAX3 induction by auxin is required to coordinate cell separation and organ emergence

Quiescent center initiation in the Arabidopsislateral root primordia is dependent on the SCARECROW transcription factor

Lateral root (LR) formation is an important determinant of root system architecture. In Arabidopsis, LRs originate from pericycle cells, which undergo a programme of morphogenesis to generate a new LR meristem. Despite its importance for root meristem organisation, the onset of organizing center (termed quiescent center; QC) formation during LR morphogenesis remains unclear. Here, we used live 3D confocal imaging to monitor cell organization and identity acquisition during LR development. Our dynamic observations revealed an early morphogenesis phase and a late meristem formation phase as proposed in the bi-phasic growth model described by Sussex and co-workers. LR QC establishment coincided with this developmental phase transition. QC precursor cells originated from the outer layer of stage II LR primordia, within which the SCARECROW (SCR) transcription factor was specifically expressed. Disrupting SCR function abolished periclinal divisions in this LR primordia cell layer and perturbed the formation of QC precursor cells. We conclude that de novo QC establishment in LR primordia operates via SCR-mediated formative cell division and coincides with the developmental phase transition