La plupart des équipements électroniques et électriques sont enrobés ou encapsulés par de la résine epoxy, choisie pour ses qualités physiques, chimiques et surtout diélectriques. Cependant, ce matériau présente un inconvénient majeur : sa faible conductivité thermique (0.2 W/mK). Dans ce contexte, nous avons élaboré et caractérisé des composites epoxy / carbone dans le but d’améliorer la conductivité thermique de ce type de résine tout en conservant ses propriétés diélectriques. Nous avons ainsi testé le potentiel d’une large gamme de charges carbonées, de structures, formes et tailles variées (sphères, tubes et plaquettes), telles que des micro-sphères de carbone et des nanotubes multi-parois synthétisées par CVD et PECVD, mais aussi des charges industrielles : nano-plaquettes de graphite (graphite exfolié), du coke de pétrole, du graphite synthétique et naturel. Des échantillons de matériaux composites massifs (50 x 50 x 4 mm) ont été préparés à partir d’une résine industrielle DGEBA de viscosité élevée 8.5-15 Pa.s, en faisant varier le taux de charge. Les propriétés thermiques des composites ont été mesurées à partir de la technique du hot disk (source plane instationnaire). Les meilleurs résultats ont été obtenus à partir des nano-plaquettes de graphite : les conductivités thermiques des composites ont atteint (0.55 W/mK) pour une charge admissible maximale de 2.67 vol.%. L’accroissement relatif de conductivité thermique a été de 66 % pour 1 vol.%. Cet accroissement est particulièrement élevé dans la mesure où les meilleurs résultats reportés sont de 20 % / vol.% dans le cas de résines à viscosité plus faible de type DGEBF (2.5 - 4.5 Pa.s). La concentration de charge admissible (1.3 vol.%) pour conserver une résistivité électrique suffisamment élevée (> 105 ohm.m) nous a permis d’atteindre une conductivité thermique de 0.37 W/mK (soit une augmentation de 100% par rapport à la résine initiale). Ces résultats sont interprétés en termes de transport des phonons acoustiques dans un système composite bi-phasique. Les nano-plaquettes de graphite sont caractérisées par une morpholigie anisotrope, d’ une surface d’environ 26 x 26 microns dont l’épaisseur est de l’ordre de 6 nm. Elles combinent une structure lamellaire périodique bien ordonnée dans les plans de graphène (caractérisation par XPS, EDX et DRX), et des rapports d’acicularité élevés ( 4300), estimés à partir de différentes techniques : TEM, SEM et BET. Nous montrons qu’accroitre l’acicularité des nano-plaquettes de graphite par exfoliation, en préservant une grande surface des plans de graphène, et sans générer de défauts de structure, constitue un défi. Cette morphologie 2D particulière permet d’une part de conserver voire augmenter la conductivité intrinsèque des charges, favorisée dans les plans de graphène, et d’autre part, en raison de leur grande surface spécifique, de garantir après leur dispersion dans la résine, un meilleur transport des phonons acoustiques dans le composite. / Most electronic and electrical equipment are coated or encapsulated by epoxy resin due to its physical, chemical and dielectric properties. However, this material has a major drawback: its low thermal conductivity ( 0.2 W / mK). In this context, we have developed and characterized epoxy / carbon composites in order to improve the thermal conductivity of this type of resin while maintaining its dielectric properties. We have tested the potential of a wide range of carbonaceous fillers, structures, shapes and sizes (spheres, tubes and plates), such as carbon micro-spheres and multi-walled carbon nanotubes synthesized by CVD and PECVD, but also industrial fillers: graphite nano-platelets (exfoliated graphite), petroleum coke, synthetic and natural graphite. Large composite samples (50 x 50 x 4 mm) were prepared from a DGEBA engineering resin of high viscosity 8.5-15 Pa.s, by varying the charge vol%. The thermal properties of the composites were measured from the transient plane source technique (hot disk). The best results are obtained from graphite nano-platelets: the thermal conductivity reach (0.55 W / mK) for a maximum load of 2.67 vol%.. The relative increase of thermal conductivity is 66% to 1 vol.%. This increase is particularly high to the extent that the best results reported so far is 20% / vol% for resins with lower viscosity, type DGEBF (2.5 - 4.5 Pa.s). The allowable concentration (1.3 vol.%) to maintain a sufficiently high electrical resistivity (> 105 ohm.m) permits to increase of the thermal conductivity to 100% (0.37 W / mK) compared to the initial resin. These results are interpreted in terms of transport of acoustic phonons in the composite two-phase system. Graphite nano-platelets are characterized by anisotropic shapes with a surface of about 26 x 26 microns whose thickness is of the order of 6 nm. They combine an ordered periodic structure in graphene planes (characterization by XPS, EDX and XRD), and a high aspect ratio ( 4300), estimated using various techniques: TEM, SEM and BET. We show that graphite exfoliation permit to increase the aspect ratio of graphite nanoplatelets, maintaining large micronic graphene surface, and without generating structural defects is a challenge. This peculiar 2D morphology allows on one hand, to retain or even increase the intrinsic filler conductivity, favored in the graphene planes, and on another hand, due to their high surface area, to ensure after their dispersion in the resin, a better transport of acoustic phonons through the composite.

No description available.

A regeneração óssea é um processo fisiológico que promove a neoformação de tecido ósseo saudável e funcional com características idênticas antes da lesão. Entretanto, frente a defeitos críticos, o osso é incapaz de se regenerar espontaneamente. Diante destas deficiências, a bioengenharia de tecidos ósseos (BTO) é uma opção promissora para a regeneração deste tipo de defeito. A maioria das abordagens de BTO utiliza as células-tronco mesenquimais da medula óssea (BMSC), porém, a coleta de BMSC dos pacientes é um processo bastante invasivo e doloroso. Por estas desvantagens, a busca por abordagens acessíveis e menos invasivas de novas fontes de células-tronco (CT) se tornou necessária. Neste contexto, as células-tronco de polpa de dentes decíduos (SHED) foram identificadas e sua aplicação na BTO, desde então, vem sendo amplamente estudada devido ao seu potencial osteogênico e por se tratar de uma fonte não invasiva. A obtenção de células-tronco do músculo orbicular do lábio (OOMDSC) também não causa dor adicional aos indivíduos, pois os fragmentos deste tecido são rotineiramente descartados durante as cirurgias de reconstrução do lábio. No presente trabalho investigamos o potencial de diferenciação osteoblástico in vitro e in vivo das OOMDSC e comparamos com as SHED, além disto, associamos estas células a biomateriais de HA/&beta;-TCP e investigamos a sua contribuição na neoformação óssea in vivo. O imunofenótipo de cada amostra de SHED e OOMDSC foi verificado para certificar a identidade de CT mesenquimais. Em seguida, as células em cultura foram submetidas à diferenciação osteoblástica in vitro. Em 9 e 14 dias de diferenciação as OOMDSC apresentaram menor atividade de fosfatase alcalina (p<0,0001) e menor marcação de matriz extracelular mineralizada, comparado às SHED (p<0,001), enquanto que em 21 dias estas diferenças não foram mais observadas. Quando associadas a biomateriais e implantadas em defeitos críticos calvariais bilaterais em ratos Wistar, tanto OOMDSC e SHED foram capazes de induzir neoformação óssea após 50 dias de cirurgia, conforme evidenciado pela análise morfológica e por micro-CT. Todavia, as células ósseas encontradas nos sítios da neoformação óssea não eram de origem humana. A avaliação da neoformação óssea in vivo induzida por SHED assim como a sua distribuição no enxerto foi verificada também em 07, 15 e 30 dias pós-cirúrgicos. Nestes períodos não há evidência de neoformação óssea, entretanto, as SHED estão localizadas no tecido conjuntivo que se forma e preenche o enxerto. Além disto, os dados sugerem que estas células estão relacionadas à modificações na microarquitetura do biomaterial e ainda à modulação dos números dos osteoclastos, também verificada nestas amostras. Portanto, podemos concluir que as OOMDSC são tão capazes de se diferenciar em osteoblastos quanto às SHED in vitro, porém esta diferenciação é mais lenta. Os experimentos in vivo indicam que as SHED possuem maior capacidade de indução à neoformação óssea quando comparadas às OOMDSC e que, em nosso modelo, as CT humanas não se diferenciam em osteoblastos in vivo. De qualquer forma a adição das CT ao biomaterial favorece a neoformação óssea, variações de microarquitetura e modulação dos osteoclastos. O fato de as ilhas ósseas não serem de origem humana indica que as células-tronco possam estar secretando fatores de indução à osteogênese, estimulando a neoformação óssea a partir das células do hospedeiro. / Bone regeneration is a physiological process, which promotes the growth of tissue at the site of injury, with the same characteristics of the original bone. However, when faced with critical defects the bone is unable to regenerate spontaneously. Bone tissue engineering (BTE) is a promising option for regenerating this type of defect. The majority of the approaches in BTE use Bone Marrow derived Mesenchymal Stem Cells (BMSC); however, the aspiration of bone marrow is a very invasive and painful procedure. Due to these disadvantages, the search for new, affordable and less invasive sources of stem cells (SC) has become necessary. In this context, stem cells from exfoliated deciduous teeth (SHED) have been identified and their application in BTE, since then, has been widely studied because they can be obtained non-invasively and due to their osteogenic potential. Stem cells from the orbicularis oris muscle (OOMDSC) are also obtained non-invasively and do not cause additional pain to individuals, because the fragments of this tissue are routinely discarded during lip reconstruction surgeries. In the present work we investigated, in vitro and in vivo, the osteoblastic differentiation potential of OOMDSC and compared with SHED; furthermore, we associated these cells with HA/&beta;-TCP scaffolds and investigate its contribution in the bone formation in vivo. The immunophenotype of each OOMDSC and SHED sample was verified to attest their mesenchymal stem cell identity. Then, cell cultures were submitted to osteoblastic differentiation in vitro. In 9 and 14 days of differentiation, OOMDSC exhibited lower alkaline phosphatase activity (p <0.0001) and lower mineralized extracellular matrix staining compared to SHED (p <0.001), whereas at 21 days, these differences were no longer observed. When associated with scaffolds and implanted into bilateral critical-sized calvarial defects in Wistar rats, both OOMDSC and SHED were able to induce bone formation after 50 days of surgery, as evidenced by morphological analysis and micro-CT. However, bone cells found at sites of bone formation were not of human origin. The evaluation of new bone formation in vivo induced by SHED as well as its distribution in the graft was performed at 07, 15 and 30 days after surgery. During these periods there was no evidence of new bone formation, however, SHED were located in the connective tissue that formed and filled the graft. Furthermore, our results suggest that these cells are related to changes in the microarchitecture of the scaffold and also to the modulation of the number of osteoclasts observed in these samples. In summary, our results suggest that OOMDSC are as capable to differentiate into osteoblasts as SHED in vitro, but this differentiation is slower. In vivo experiments indicate that SHED has a greater ability to induce bone formation when compared with OOMDSC, and that in our model, the human stem cells do not differentiate into osteoblasts in vivo. Nonetheless, the addition of SC to the scaffolds promotes bone formation, as well as variations in microarchitecture and modulation of osteoclasts. The fact that the bone islands are not of human origin indicates that the stem cells may be secreting osteogenesis-inducing factors, stimulating the host\'s cells to regenerate the defects.

The aim of this dissertation study was to optimize the isolation and long term cultivation protocols for human dental pulp stem cells. The protocols which showed best results were used for cultivation of dental pulp stem cell isolated from exfoliated teeth (SHED). Additional aims were to characterize DPSC and SHED and prove their ability to proliferate over Hayflick's limit and differentiate into mature cell lines (osteoblasts, chondroblasts and adipocytes). In order to find optimal protocols for isolation of dental pulp from tooth, we tried three different approaches. During optimization of cultivation protocol we focused on decreasing amount of fetal calf serum (FCS) from 10 % FCS in cultivation media (most often used in literature) into 2 % and thus get closer to cultivation conditions suitable for clinical usage. We compared DPSC cultivated in three different media (medium with 10 % FCS, 2 % FCS supplemented with growth factors and media with 2 % FCS supplemented with ITS and growth factors). For characterization of DPSC and SHED we used basic biological properties (proliferation activity, viability, morphology), their phenotype and karyotype. The study demonstrated that the best protocol for isolation of dental pulp from tooth was to break the roots and extract the pulp throw this aperture. We...

No description available.

Previous studies have reported that combination of electroacupuncture (EA) and mesenchymal stem/stromal cells (MSC) promoted survival, differentiation and functional recovery in spinal cord-transected rats. In this study, it was examined the therapeutic effects of stem cells from canine exfoliated dental pulp (SCED) combined with EA treatment in dogs with chronic naturally occurred spinal cord injury due to intervertebral disc herniation (IVDH). Dogs were randomly assigned to four experimental groups (n=4 for each group; total of 16 animals): SCED, EA, SCED + EA) and control. Mild increase in the neurological scoring was found in one animal from SCED group (1/4; 2 points gained), one from EA group (1/4; 8 points gained), three from SCED+EA group (3/4; 16 points gained) and one from control group (1/4; 2 points gained). Functional outcome improvements were observed two animals from SCED group (2/4; 3 points gained), two from EA group (2/4; 4 points gained), one from SCED+EA group (1/4; 1 point gained) and two were from control group (2/4; 6 points gained). However no statistical differences were observed. Magnetic resonance imaging (MRI) findings did not suggest improvement comparing pre- and post-treatment within groups, excepted from one animal from SCED group (1/4), and 10 animals from all groups (10/16) presented signs of injury progression in the SCI in post-treatment exam, which could not be associated to the procedures from study, but could be related to the natural evolution of the disease. Limitation such as number of transplanted stem cells, delivery route, injury chronicity and intrinsic variation among naturally spinal cord injured dogs could have influence outcomes negatively. Moreover, canine deciduous exfoliated teeth were easily obtained and SCED were simply isolated, and no mortality followed up 7 month from procedure were observed. / Estudos anteriores demonstraram que a associação da eletroacupuntura e células-tronco mesenquimais/estromais (CTMs) pode promover a sobrevivência e diferenciação das CTMs, assim como recuperação funcional em ratos com transecção da medula espinal. Neste estudo, foram avaliados os efeitos terapêuticos da associação de células-tronco derivadas de polpa de dente decíduo esfoliado de cães (CPDEc) e eletroacupuntura (EAP) em cães com lesão de medula espinhal crônica causada de forma natural por herniação do disco interververtebral. Os cães foram divididos aleatoriamente em quatro grupos experimentais (n=4 para cada grupo; total de 16 animais): CPDEc, EAP, CPDEc+EAP e grupo controle. Foram encontradas pequenas melhoras na pontuação do exame neurológico em um animal do grupo CPDEc (1/4; 2 pontos ganhos), um do grupo EAP (1/4; 8 pontos ganhos), três do grupo CPDEc+EAP (3/4; 16 pontos ganhos) e um do grupo controle (1/4; 2 pontos ganhos). Na avaliação funcional, pequenas melhoras também foram observadas em dois animais do grupo CPDEc (2/4; 3 pontos ganhos), dois do grupo EAP (2/3; 4 pontos ganhos), um do grupo CPDEc+EAP (1/4; 1 ponto ganho) e dois do grupo controle (2/4; 6 pontos ganhos). No entanto, não foram encontradas diferenças estatísticas entre os grupos. Os achados ressonância magnética não sugeriram melhoras comparando os exames pré e pós tratamento entre os grupos, com exceção de um animal do grupo CPDEc (1/4), e 10 animais dentre todos os grupos (10/16) apresentaram sinais de progressão na lesão da medula espinhal, que não puderam ser associados com os procedimentos do estudo, mas podem estar relacionados à progressão natural da doença. Além disso, os dentes decíduos esfoliados foram obtidos facilmente e as CPDEc foram isoladas de forma simples, ademais, não foi observada nenhuma mortalidade foi observada até 7 meses após o procedimento.

The introduction of a nanoscopic reinforcing phase to a polymer matrix offers great possibilities of obtaining improved properties, enabling applications outside the boundaries of traditional composites. The majority of the work in this thesis has been devoted to polymer/clay nanocomposites in coating applications, using the hydroxyl-functional hyperbranched polyester Boltorn® as matrix and montmorillonite clay as nanofiller. Nanocomposites with a high degree of exfoliation were readily prepared using the straightforward solution-intercalation method with water as solvent. Hard and scratch-resistant coatings with preserved flexibility and transparency were obtained, and acrylate functionalization of Boltorn® rendered a UV-curable system with similar property improvements. In order to elucidate the effect of the dendritic architecture on the exfoliation process, a comparative study on the hyperbranched polyester Boltorn® and a linear analogue of this polymer was performed. X-ray diffraction and transmission electron microscopy confirmed the superior efficiency of the hyperbranched polymer in the preparation of this type of nanocomposites. Additionally, an objective of this thesis was to investigate how cellulose nanofibers can be utilized in high performance polymer nanocomposites. A reactive cellulose “nanopaper” template was combined with a hydrophilic hyperbranched thermoset matrix, resulting in a unique nanocomposite with significantly enhanced properties. Moreover, in order to fully utilize the great potential of cellulose nanofibers as reinforcement in hydrophobic polymer matrices, the hydrophilic surface of cellulose needs to be modified in order to improve the compatibility. For this, a grafting-from approach was explored, using ring-opening polymerization of ε-caprolactone (CL) from microfibrillated cellulose (MFC), resulting in PCL-modified MFC. It was found that the hydrophobicity of the cellulose surfaces increased with longer graft lengths, and that polymer grafting rendered a smoother surface morphology. Subsequently, PCL-grafted MFC film/PCL film bilayer laminates were prepared in order to investigate the interfacial adhesion. Peel tests demonstrated a gradual increase in the interfacial adhesion with increasing graft lengths. / QC20100621

Στην παρούσα εργασία αναπτύχθηκαν και μελετήθηκαν πειραματικά σειρές σύνθετων υλικών πολυμερικής μήτρας, με παράμετρο τον τύπο και την περιεκτικότητα σε ενισχυτική φάση. Ως μήτρα χρησιμοποιήθηκε εποξειδική ρητίνη υψηλών προδιαγραφών. Ως ενισχυτική φάση χρησιμοποιηθήκαν μικροσωματίδια, νανοσωματίδια τιτανικού βαρίου και αποφλοιωμένα γραφιτικά νανοεπίπεδα (exfoliated graphite nanoplatelets). Η επιλογή των υλικών είχε ως στόχο να εκμεταλλευτούν σε κοινό σύνθετο σύστημα οι «θετικές» ιδιότητες των συστατικών του, όπως η θερμο-μηχανική σταθερότητα της μήτρας, η υψηλή διαπερατότητα και η σιδηροηλεκτρική συμπεριφορά του τιτανικού βαρίου και οι καλές μηχανικές ιδιότητες μαζί με την υψηλή ειδική αγωγιμότητα των αποφλοιωμένων γραφιτικών νανοεπιπέδων. Παρασκευάστηκαν και μελετήθηκαν τα παρακάτω συστήματα σύνθετων υλικών, για διάφορες περιεκτικότητες σε ενισχυτική φάση: (α) σύστημα μικροσωματιδίων τιτανικού βαρίου/εποξειδικής ρητίνης, (β) σύστημα νανοσωματιδίων τιτανικού βαρίου/εποξειδικής ρητίνης, (γ) σύστημα αποφλοιωμένων γραφιτικών νανοεπιπέδων/εποξειδικής ρητίνης, (δ) υβριδικό σύστημα μικροσωματιδίων τιτανικού βαρίου/νανοσωματιδίων τιτανικού βαρίου/εποξειδικής ρητίνης, (ε) υβριδικό σύστημα αποφλοιωμένων γραφιτικών νανοεπιπέδων/ νανοσωματιδίων τιτανικού βαρίου/εποξειδικής ρητίνης. Την παρασκευή των δοκιμίων ακολούθησε πολύπλευρος χαρακτηρισμός τους. Για λόγους αναφοράς παρασκευάστηκε και μελετήθηκε και δοκίμιο μη ενισχυμένης ρητίνης. Η μορφολογία τους διερευνήθηκε με την τεχνική της ηλεκτρονικής μικροσκοπίας σάρωσης (scanning electron microscopy) και την τεχνική σκέδασης ακτίνων-Χ (x-ray diffraction scattering). Διαπιστώθηκε η επιτυχής διασπορά των νανο-εγκλεισμάτων αλλά και η ύπαρξη μικρών συσσωματωμάτων. Τα φάσματα σκέδασης ακτίνων-Χ πιστοποίησαν την παρουσία των πληρωτικών μέσων που χρησιμοποιήθηκαν σε κάθε κατηγορία σύνθετου συστήματος. Ακολούθησε θερμικός χαρακτηρισμός των σύνθετων υλικών, με στόχο τον προσδιορισμό της θερμοκρασίας υαλώδους μετάπτωσής τους. Η μελέτη της μηχανικής συμπεριφοράς των συνθέτων έγινε υπό στατικές και δυναμικές συνθήκες. Η στατική συμπεριφορά εξετάστηκε με την τεχνική κάμψης τριών σημείων σε θερμοκρασία περιβάλλοντος. Διαπιστώθηκε αύξηση του μέτρου ελαστικότητας με την περιεκτικότητα σε ενισχυτική φάση, σε όλες τις κατηγορίες σύνθετων συστημάτων. Παράλληλα, διαπιστώθηκε μείωση της μηχανικής αντοχής με τη συγκέντρωση πληρωτικού μέσου σε όλες τις κατηγορίες σύνθετων υλικών που μελετήθηκαν. Η δυναμική μηχανική απόκριση μελετήθηκε με την τεχνική της δυναμικής θερμικής ανάλυσης (dynamic mechanical thermal analysis) σε ευρύ φάσμα θερμοκρασιών. Τα ενισχυμένα συστήματα παρουσιάζουν αυξημένες τιμές του μέτρου αποθήκευσης, ενώ οι κορυφές της εφαπτομένης απωλειών επιτρέπουν τον προσδιορισμό της θερμοκρασίας υαλώδους μετάπτωσης (Tg). Η Tg φαίνεται να διαφοροποιείται ελαφρά με την περιεκτικότητα σε ενισχυτική φάση, άλλοτε προς μεγαλύτερες και άλλοτε προς μικρότερες τιμές. Οι διαφοροποιήσεις αυτές εκφράζουν τις αλληλεπιδράσεις μεταξύ των φάσεων και ίσως την πλήρη ή μη διαβροχή των εγκλεισμάτων από τη μήτρα. Η ηλεκτρική απόκριση των σύνθετων συστημάτων εξετάστηκε με τη μέθοδο της διηλεκτρικής φασματοσκοπίας ευρέως φάσματος, σε μεγάλο εύρος συχνοτήτων και θερμοκρασιών. Η ανάλυση των πειραματικών δεδομένων έγινε μέσω των φορμαλισμών της ηλεκτρικής διαπερατότητας, του ηλεκτρικού μέτρου και της ειδικής αγωγιμότητας εναλλασσομένου. Η χρήση και των τριών φορμαλισμών προσφέρει τη δυνατότητα εξαγωγής περισσότερων πληροφοριών για τις φυσικές διεργασίες που λαμβάνουν χώρα στο εσωτερικό των συνθέτων. Διαπιστώθηκε η παρουσία δύο διηλεκτρικών χαλαρώσεων που σχετίζονται με την πολυμερική μήτρα. Αυτές αποδίδονται, στη μετάπτωση από την υαλώδη στην ελαστομερική φάση της εποξειδικής ρητίνης (α-χαλάρωση) και στην επαναδιευθέτηση πλευρικών πολικών ομάδων (β-χαλάρωση). Η παρουσία των εγκλεισμάτων στο εσωτερικό της μήτρας εισάγει ηλεκτρική ετερογένεια με αποτέλεσμα την εμφάνιση του φαινομένου διεπιφανειακής πόλωσης (interfacial polarization). Μη δέσμια φορτία συσσωρεύονται στη διεπιφάνεια των φάσεων, όπου σχηματίζουν μεγάλα δίπολα που παρουσιάζουν αδράνεια ως προς τον προσανατολισμό τους, παράλληλα του εφαρμοζόμενου πεδίου. Η διεπιφανειακή πόλωση είναι η πλέον αργή διεργασία και παρατηρείται σε χαμηλές συχνότητες και υψηλές θερμοκρασίες. Το πραγματικό μέρος της ηλεκτρικής διαπερατότητας, όπως και η ειδική αγωγιμότητα παρουσίασαν αύξηση με την περιεκτικότητα σε ενισχυτική φάση, ιδιαίτερα στην περίπτωση των συστημάτων με γραφιτικά νανοεπίπεδα. Η δυνατότητα αποθήκευσης ενέργειας στα συστήματα διερευνήθηκε με χρήση της πυκνότητας ενέργειας υπό σταθερό ηλεκτρικό πεδίο. Διαπιστώθηκε αύξηση της αποθηκευόμενης ενέργειας με αύξηση της περιεκτικότητας σε ενισχυτική φάση. Τη βέλτιστη συμπεριφορά επέδειξε το σύστημα με τη μέγιστη περιεκτικότητα σε γραφιτικά νανοεπίπεδα. Η δυναμική των χαλαρώσεων μελετήθηκε μέσω διαγραμμάτων Arrhenius, από τα οποία προέκυψαν και οι τιμές της ενέργειας ενεργοποίησης. Η θερμοκρασιακή γειτνίαση των διεργασιών της α-χαλάρωσης και της διεπιφανειακής πόλωσης οδήγησε σε αλληλοεπικάλυψη των διεργασιών. Από τις ενέργειες ενεργοποίησης που υπολογίστηκαν φαίνεται πως στο δοκίμια της μη ενισχυμένης ρητίνης επικρατεί η συνεισφορά της α-χαλάρωσης, ενώ στα σύνθετα συστήματα επικρατεί η συνεισφορά της διεπιφανειακής πόλωσης. Τα σωματίδια του τιτανικού βαρίου υφίστανται δομικό μετασχηματισμό από την πολική τετραγωνική δομή (σιδηροηλεκτρική φάση) στην μη-πολική κυβική δομή (παραηλεκτρική φάση) σε μία κρίσιμη θερμοκρασία, πλησίον των 130οC. Η μετάβαση αποδείχθηκε μέσω των φασμάτων ακτίνων-Χ και είναι περισσότερο έντονη στην περίπτωση των μικροσωματιδίων. Η λειτουργική συμπεριφορά των συστημάτων σχετίζεται με τη θερμικά διεγειρόμενη δομική μετάβαση από τη σιδηροηλεκτρική στην παραηλεκτρική φάση των εγκλεισμάτων τιτανικού βαρίου, τη μεταβολή του προσήμου του θερμοκρασιακού συντελεστή ειδικής αγωγιμότητας και τη δυνατότητα αποθήκευσης ενέργειας. Η συνύπαρξη σε κοντινές θερμοκρασίες των διεργασιών α-χαλάρωσης και διεπιφανειακής πόλωσης μαζί με την κρίσιμη θερμοκρασία μετάβασης των σιδηροηλεκτρικών εγκλεισμάτων, δυσχεραίνει πολύ την διάκρισή τους. Με την εισαγωγή της διηλεκτρικής συνάρτησης ενίσχυσης (dielectric reinforcing function) έγινε δυνατός ο διαχωρισμός των φαινομένων. Επιπλέον, η συνάρτηση διηλεκτρικής ενίσχυσης προσφέρει τη δυνατότητα εξέτασης της λειτουργικής συμπεριφοράς και της δυνατότητας αποθήκευσης ενέργειας, ανεξάρτητα των γεωμετρικών διαστάσεων του υλικού. Τέλος, το σύνολο των αποτελεσμάτων έγινε αντικείμενο συγκρίσεων και συζήτησης. / In this study, series of polymer matrix composite materials were developed and experimentally studied, varying the reinforcing phase content. The employed matrix was a high tech epoxy resin, while reinforcing phase was micro- and/or nano-barium titanate particles, as well as exfoliated graphite nanoplatelets. The choice of the materials was targeting to take advantage in a common composite system of the thermo-mechanical stability of the matrix, the high dielectric permittivity and the ferroelectric behaviour of barium titanate and the enhanced mechanical properties in tandem with the high conductivity of the exfoliated graphite nanoplatelets. The following composite materials systems were fabricated and studied, for various filler contents: (a) barium titante micro-particles/epoxy resin composite system, (b) barium titante nano-particles/epoxy resin composite system, (c) exfoliated graphite nanoplatelets/epoxy resin composite system, (d) barium titante micro-particles/barium titante nano-particles /epoxy resin hybrid composite system, (e) exfoliated graphite nanoplatelets /barium titante nano-partcles /epoxy resin hybrid composite system. The fabrication of the composites was followed by a multiple characterization of the produced specimens. For reference reasons pure resin was also prepared and studied. Systems’ morphology was investigated by means of scanning electronic microscopy and x-ray diffraction scattering. It was ascertained the existence of fine nanodispersions, as well as of small clusters, within the composites. XRD spectra verified the presence of filler in each category of composite systems. Thermal characterization was conducted via differential scanning calorimetry aiming to determine the glass to rubber transition temperature of all studied systems. Mechanical behaviour was investigated under static and dynamic conditions. Static behaviour was determined via three point bending tests at ambient temperature. It was found that modulus of elasticity increases with filler content in all composite systems categories. On the other hand, mechanical strength decreases with filler content. Dynamic response was studied by means of dynamic mechanical thermal analysis in a wide temperature range. Reinforced systems exhibit higher values of storage modulus, while the loss tangent peaks allow the determination of the glass transition temperature Tg. Tg slightly varies with reinforcing phase content, to higher or lower values depending on the type and the amount of filler concentration. These variations express the interactions between the phases of the composites and possibly the uncompleted wetting of the inclusions in some cases. The electrical response of the composite systems was examined by means of broadband dielectric spectroscopy in a wide frequency and temperature range. The analysis of the experimental data was carried out via the dielectric permittivity, electric modulus, and ac conductivity formalisms. The usage of all three formalisms provides the opportunity to extract more information concerning the physical mechanisms occurring within the composites. It was found that two dielectric processes are related to the polymer matrix. These are attributed to the glass to rubber transition of epoxy resin (α-relaxation) and to the re-arrangement of polar side groups of the main polymer chain (β-relaxation). The presence of inclusions within the matrix introduces electrical heterogeneity resulting in the occurrence of interfacial polarization. Unbounded charges accumulate at the interface of the phases, forming large dipoles, which exhibit inertia in orienting themselves parallel to the applied field. Interfacial polarization is the slowest process in the systems and thus it is observed at low frequencies and high temperatures. The real part of dielectric permittivity, as well as, the conductivity increase with reinforcing phase content, especially in the case of the systems with graphite nanoplatelets. The energy storage efficiency was investigated via the density of energy, at constant electric field. It was found that the energy storage capability increases with filler content. Optimum behaviour is displayed by the system with maximum content in graphite nanoplatelets. The dynamics of the relaxations was studied via Arrhenius graphs, from which the values of activation energy were calculated. Interfacial polarization and α-relaxation appear in adjacent temperature ranges, leading in a superposition of both processes. From the calculated values of activation energy it is concluded that in the pure resin specimen the dominating contribution is related to the α-relaxation, while in the composite systems the contribution of interfacial polarization seems to prevail. Barium titanate particles undergo a structural transition from the polar tetragonal structure (ferroelectric phase) to the non-polar cubic structure (paraelectric phase) at a critical temperature closed to 130oC. This transition was proved via XRD spectra and is more intense in the case of barium titanate microparticles. Systems’ functional behaviour is related to the thermally stimulated structural transition from the ferroelectric to the paraelectric phase of barium titanate inclusions, to the change of sign of the temperature coefficient of conductivity, and their ability for energy storage. The coexistence at adjacent temperatures ranges of α-relaxation and interfacial polarization, as well as the critical transition temperature of ferroelectric inclusions, hampers the discrimination of the effects. By introducing the dielectric reinforcing function the discrimination of the processes became possible. Furthermore, the dielectric reinforcing function provides the possibility to examine the functional behaviour and the energy storage efficiency of the systems, neglecting the materials’ geometrical characteristics influence. Finally, experimental results and analysis are compared and discussed.

Nanocomposites have been receiving a lot the attention in the last decade from both industry and academia, since a small amount of nanofiller can significantly improve the materials properties. In the field of thermoplastics, polypropylene (PP) is one of the most used materials , due its easy processability, good balance of mechanical properties, and low cost. However, PP has certain shortcomings such as poor gas barrier and low thermal stability which limit its application. In order to be classified as nanocomposite the material needs to have at least one phase with one dimension less than 100nm. The properties achieved by nanocomposites will depend on the type of polymer, type of dispersed phase (filler), surface interaction between filler and polymer, and the production method. Nanofillers present many shapes and sizes, but they can be grouped in nanoparticles, nanotubes and nanoplates. Montmorillonite (MMT) is a clay that has been extensively studied to produce PP nanocomposites, due to its availability, high aspect ratio, high modulus and high cation exchange capacity, characteristics that result in composite with improved properties. Three different morphologies can be observed in PP/MMT nanocomposites: agglomerates (similar to the conventional composites); intercalated; or exfoliated. Among these morphologies, exfoliation is the most desirable and the hardest to be achieved in PP/MMT nanocomposites. Several methods have been used to produce PP nanocomposites. They can be grouped in three main groups: solution blending; melt processing; and in situ polymerization. In order to produce an exfoliated nanocomposite, some methods have assisted the exfoliation using supercritical fluids. Supercritical carbon dioxide is by far the most explored one. Polypropylene is a semi-crystalline polymer and its properties rely on amount of its crystallinity, which is related to its stereochemical configurations. Isotactic PP and syndiotactic PP result in a semi-crystalline polymer while atactic results in an amorphous polymer. Two catalyst systems can be used to produce isotactic PP: Metallocene and Ziegler-Natta (ZN). This research study was carried out in order to develop an appropriated process to produce PP/MMT nanocomposites with a high level of exfoliation using in situ polymerization assisted by supercritical propylene. The main idea is to use supercritical propylene to treat the montmorillonite before polymerization. In this process, the small molecules of propylene diffuse inside the clay galleries under supercritical conditions (high pressure and temperature) until reaching complete saturation. Once this saturation is reached the mixture of polypropylene and clay is catastrophically decompressed and fed into an autoclave reactor. The propylene polymerization reaction is them catalyzed by ZN catalyst. The pressure of the mixture of propylene-montmorillonite from the supercritical condition to the reactor autoclave decreased significantly, allowing propylene to expand and exfoliate the clay as it was fed in the reactor. Propylene in supercritical conditions was used in this works because it is the monomer for the subsequently polymerization and because its good properties at supercritical conditions. In order to evaluate the results the following methods were used: transmission electron microscopy (TEM) to investigate the nanoscale sample morphology and evaluate the clay exfoliation, X-ray diffraction (XRD) to determine interlamellar distance, d001, of the clay, differential scanning calorimetry (DSC) to determine the amount of crystallization of polymer and composite, thermogravimetric analysis (TGA) to determine composite clay content, scanning electron microscopy (SEM) to evaluate the morphology, and clay swelling test to evaluate the compatibility among various pairs clays-solvent. The first part of this work evaluated the interaction and swelling effects of different pairs of clay-solvent with or without sonication. This was necessary in order to choose the best clay to carry out the study. Four solvents with different polarity (chlorobenzene, toluene, cyclohexane and hexane) and eight clays (seven organically modified and one unmodified) were evaluated with or without sonication. Closite 15A and 93A presented the best results with different solvents and they were selected for further experiments. The experiments also showed that sonication improves the swelling of the clay. Initial screening of the polymerization reaction was carried out using two conditions: feeding supercritical propylene without clay and adding clay without the addition of supercritical fluid. The addition of supercritical propylene did not modify the morphology and properties of PP in comparison to the normal polymerization. The addition of Cloisite 15A or Cloisite 93A (pre-treated with toluene, not with supercritical propylene) produced nanocomposites. Although Cloisite 15A showed better results on the swelling tests, Cloisite 93A presented much better polymerization yield, therefore it was selected for further investigation using treatment with supercritical propylene. Cloisite93A was submitted to a treatment under four different supercritical propylene conditions (temperature and pressure) for thirty minutes. Each mixture was subsequently fed to the reactor through a catastrophic expansion inside an autoclave reactor running a propylene polymerization reaction. The results from XRD and TEM show a significant improvement on the exfoliation when treating the clay under supercritical propylene conditions followed by in situ polymerization, as compared to the in situ polymerization without treating the clay with supercritical propylene. In conclusion, the utilization of supercritical propylene has improved the dispersion of the clay at the nanoscale during the preparation of these nanocomposites by in situ polymerization.

Previous studies have reported that combination of electroacupuncture (EA) and mesenchymal stem/stromal cells (MSC) promoted survival, differentiation and functional recovery in spinal cord-transected rats. In this study, it was examined the therapeutic effects of stem cells from canine exfoliated dental pulp (SCED) combined with EA treatment in dogs with chronic naturally occurred spinal cord injury due to intervertebral disc herniation (IVDH). Dogs were randomly assigned to four experimental groups (n=4 for each group; total of 16 animals): SCED, EA, SCED + EA) and control. Mild increase in the neurological scoring was found in one animal from SCED group (1/4; 2 points gained), one from EA group (1/4; 8 points gained), three from SCED+EA group (3/4; 16 points gained) and one from control group (1/4; 2 points gained). Functional outcome improvements were observed two animals from SCED group (2/4; 3 points gained), two from EA group (2/4; 4 points gained), one from SCED+EA group (1/4; 1 point gained) and two were from control group (2/4; 6 points gained). However no statistical differences were observed. Magnetic resonance imaging (MRI) findings did not suggest improvement comparing pre- and post-treatment within groups, excepted from one animal from SCED group (1/4), and 10 animals from all groups (10/16) presented signs of injury progression in the SCI in post-treatment exam, which could not be associated to the procedures from study, but could be related to the natural evolution of the disease. Limitation such as number of transplanted stem cells, delivery route, injury chronicity and intrinsic variation among naturally spinal cord injured dogs could have influence outcomes negatively. Moreover, canine deciduous exfoliated teeth were easily obtained and SCED were simply isolated, and no mortality followed up 7 month from procedure were observed. / Estudos anteriores demonstraram que a associação da eletroacupuntura e células-tronco mesenquimais/estromais (CTMs) pode promover a sobrevivência e diferenciação das CTMs, assim como recuperação funcional em ratos com transecção da medula espinal. Neste estudo, foram avaliados os efeitos terapêuticos da associação de células-tronco derivadas de polpa de dente decíduo esfoliado de cães (CPDEc) e eletroacupuntura (EAP) em cães com lesão de medula espinhal crônica causada de forma natural por herniação do disco interververtebral. Os cães foram divididos aleatoriamente em quatro grupos experimentais (n=4 para cada grupo; total de 16 animais): CPDEc, EAP, CPDEc+EAP e grupo controle. Foram encontradas pequenas melhoras na pontuação do exame neurológico em um animal do grupo CPDEc (1/4; 2 pontos ganhos), um do grupo EAP (1/4; 8 pontos ganhos), três do grupo CPDEc+EAP (3/4; 16 pontos ganhos) e um do grupo controle (1/4; 2 pontos ganhos). Na avaliação funcional, pequenas melhoras também foram observadas em dois animais do grupo CPDEc (2/4; 3 pontos ganhos), dois do grupo EAP (2/3; 4 pontos ganhos), um do grupo CPDEc+EAP (1/4; 1 ponto ganho) e dois do grupo controle (2/4; 6 pontos ganhos). No entanto, não foram encontradas diferenças estatísticas entre os grupos. Os achados ressonância magnética não sugeriram melhoras comparando os exames pré e pós tratamento entre os grupos, com exceção de um animal do grupo CPDEc (1/4), e 10 animais dentre todos os grupos (10/16) apresentaram sinais de progressão na lesão da medula espinhal, que não puderam ser associados com os procedimentos do estudo, mas podem estar relacionados à progressão natural da doença. Além disso, os dentes decíduos esfoliados foram obtidos facilmente e as CPDEc foram isoladas de forma simples, ademais, não foi observada nenhuma mortalidade foi observada até 7 meses após o procedimento.