Hydrothermal Aging of an Epoxy Resin Filled with Carbon Nanofillers

The effects of temperature and moisture on flexural and thermomechanical properties of neat and filled epoxy with both multiwall carbon nanotubes (CNT), carbon nanofibers (CNF), and their hybrid components were investigated. Two regimes of environmental aging were applied: Water absorption at 70 °C until equilibrium moisture content and thermal heating at 70 °C for the same time period. Three-point bending and dynamic mechanical tests were carried out for all samples before and after conditioning. The property prediction model (PPM) was successfully applied for the prediction of the modulus of elasticity in bending of manufactured specimens subjected to both water absorption and thermal aging. It was experimentally confirmed that, due to addition of carbon nanofillers to the epoxy resin, the sorption, flexural, and thermomechanical characteristics were slightly improved compared to the neat system. Considering experimental and theoretical results, most of the epoxy composites filled with hybrid carbon nanofiller revealed the lowest effect of temperature and moisture on material properties, along with the lowest sorption characteristics

Κατασκευή και χαρακτηρισμός νανοσωλήνων διοξειδίου του τιτανίου καθώς και μοντελοποίηση της διεπιφάνειας μεταξύ υποστρώματος και νανοσωλήνων σε βιολογικά, φυσικά και μηχανολογικά συστήματα

Science is nowadays directed towards application; however, the path from basic to applied research has been proven sometimes difficult. This is the case of the titanium dioxide nanotube layers (TNTs), discovered more than a decade ago; immediately after being reported, it has been predicted that they would give miraculous characteristics to materials in electronic, biomedical and photovoltaic applications. The synthesis of the titanium dioxide nanotube layers through the electrochemical anodization was considered a very convenient method, due to its simplicity and low cost. Although this method is simple to apply, the dependency on a great number of manufacturing parameters requires deep understanding and control of the whole fabrication process. Until the moment, a great difficulty has been encountered in applying this method on titanium for large scale applications (e.g. in mechanical systems and implantology). Different research groups focused on synthesizing these self-organized nanostructures on pure titanium plates, provided at high cost, from well-known suppliers. However, the thin, well polished, high-quality titanium plates are not appropriate for application to an industrial level. Mechanical structures in aeronautics and naval industries involve thick titanium plates with various internal and external characteristics. Further on, in biomedical applications, titanium implants present complex geometries and porosity. These technical aspects represented a strong impediment in processing the titanium surface and synthesizing the titanium dioxide nanotube layers on various titanium grades with complex geometry, such as for example a titanium dental screw type implant. In practice, a great difference exists between processing a plate surface and an implant surface. Suitable anodizing conditions result in titanium dioxide nanotube layers on pure titanium plates, while the same conditions can only enable the formation of pores on a titanium implant surface. A great need exists for a deeper understanding of the formation of highly organized microstructures on titanium surfaces through the electrochemical anodizing method. The present manuscript describes a set of experiments applied for the surface processing of different titanium plates and implants, through the electrochemical anodizing method and the manufacturing of TNTs based multilayered nanocomposites. The final target of this investigation is the application of the titanium dioxide nanotubes in three main directions: mechanical systems, implantology, and solar cells. The first part of this study consists in a bibliographic investigation. An introduction in the self-organization of materials at nano-level is presented in Chapter 1. Self-organization of materials is a complex scientific area, widely used nowadays in materials’ surface processing. Titanium surface may be processed through self-organizing phenomena during an electrochemical anodization, which leads to titanium dioxide nanotubes formation. The second chapter (Chapter 2) describes several already reported case studies involving carbon and titanium dioxide nanotubes. Considering that composite materials are gaining ground in all kind of application, nanostructured phases such as nanotubes are of particular interest in composites manufacturing. Combining two nanostructured phases is an almost impossible target, except when it comes about multilayered composites. In multilayered composites, two nanostructured phases may come in direct contact. Interactions at the nano-level result in improved structural, mechanical and electrical properties of the composite. Such an example is that one of titanium dioxide nanotubes as a substrate for CNTs in a multilayered architecture. The main issue when studying multilayered hybrid nanocomposites is the interphase effect. Thus, Chapter 3 is about a semi-empirical model, namely the Viscoelastic Hybrid Interphase Model, applied for the prediction of the mechanical properties of a composite, based on an interphase concept. Modeling the interphase properties is an extremely useful tool in advanced composites design and application. Finally, Chapter 4 mentions a number of application areas of multilayered hybrid nanocomposites. Chapter 5 mentions all technical aspects related to the manufacturing of TNTs based multilayered hybrid nanocomposites and the methods used in their investigation. The experimental and theoretical investigations are presented in Chapters 6-8. Chapter 6 describes a number of anodization protocols involving several electrochemical parameters used for processing the titanium surface. Chapter 7 presents two case studies: (i) an investigation of the nanomechanical properties of TNTs based multilayered hybrid nanocomposites and (ii) an investigation of the mechanical properties of TNTs reinforced adhesive single lap joints. Chapter 8 is about the application of the TNTs in implantology, with emphasis on the ‘material-human cell’ interphase effect on implant integration in the host body; this chapter also includes an application of the Viscoelastic Hybrid Interphase Model, already presented in Chapter 3, to predict the properties of the interphase between living and nonliving materials when in contact, as well as an accurate prediction of human cells adhesion quality to various substrates, including TNTs and CNTs, through the model. Chapter 9 is an investigation of the reflectivity of TNTs based multilayered hybrid nanocomposites with possible application in solar cells. Finally, the global conclusions of the entire work and future scientific avenues on this subject are presented in Chapter 9.Η νανοτεχνολογία αποτελεί μια νέα προσέγγιση για την κατανόηση και την άρτια γνώση των ιδιοτήτων της ύλης σε νανοκλίμακα: ένα νανόμετρο (ένα δισεκατομμυριοστό του μέτρου) είναι το μήκος ενός μικρού μορίου. Στο επίπεδο αυτό αποκαλύπτονται διαφορετικές και συχνά καταπληκτικές ιδιότητες της ύλης και είναι δυσδιάκριτα τα όρια μεταξύ των καθιερωμένων επιστημών και τεχνικών κλάδων. Ως εκ τούτου, ο χαρακτήρας της νανοτεχνολογίας είναι άκρως διεπιστημονικός. Είναι συχνές οι αναφορές στο «ρηξικέλευθο» ή «επαναστατικό» δυναμικό της νανοτεχνολογίας, δηλαδή στις δυνατότητες να έχει επιπτώσεις στις μεθόδους βιομηχανικής παραγωγής. Τα μικρότερα, ελαφρύτερα, ταχύτερα και αποδοτικότερα υλικά, κατασκευαστικά στοιχεία και συστήματα που προσφέρει η νανοτεχνολογία είναι δυνατόν να δώσουν λύσεις σε πολλά τρέχοντα προβλήματα. Ανοίγονται έτσι νέες ευκαιρίες για την δημιουργία πλούτου και απασχόλησης. Εξάλλου, αναμένεται ότι η νανοτεχνολογία θα συμβάλει σημαντικά στην αντιμετώπιση παγκόσμιων και περιβαλλοντικών προκλήσεων, επειδή θα καταστήσει δυνατή την κατασκευή προϊόντων και την ανάπτυξη διαδικασιών προσαρμοσμένων σε συγκεκριμένες χρήσεις, στην εξοικονόμηση πόρων και στη μείωση των αποβλήτων και των εκπομπών ρύπων. Υπάρχουν πολλά παραδείγματα πιθανών εφαρμογών της νανοτεχνολογίας. Σε αυτές, περιλαμβάνονται νέα υλικά, νέες ιατρικές , φαρμακευτικές, γεωργικές και περιβαλλοντικές διαδικασίες καθώς και νέες ηλεκτρονικές συσκευές, αισθητήρες κλπ. Η δυνατότητα να χρησιμοποιηθούν οι ατομικές και μοριακές ιδιότητες των υλικών , επιτρέπει στην ανάπτυξη ποικίλων νέων χρήσεων για τα τρέχοντα προϊόντα. Αξιοποίηση της νανοτεχνολογίας στην επιστήµη των υλικών µε εφαρµογές µεγάλου εύρους αναµένεται να επηρεάσουν ουσιαστικά όλους τους τοµείς. Νανοσωµατίδια χρησιµοποιούνται ήδη για την ενίσχυση υλικών και για την βελτίωση της ποιότητας των καλλυντικών. Με τη βοήθεια της νανοτεχνολογίας είναι δυνατό να τροποποιηθούν επιφάνειες έτσι ώστε να µην χαράσσονται, να καθίστανται αδιάβροχες, καθαρές ή αποστειρωµένες. Η επιλεκτική μεταμόσχευση οργανικών µορίων µέσω νανοδοµηµένων επιφανειών αναµένεται ότι θα επηρεάσει την παραγωγή βιοαισθητήρων και µοριακών ηλεκτρονικών συσκευών. Οι επιδόσεις των υλικών σε ακραίες συνθήκες µπορούν να βελτιωθούν σε σηµαντικό βαθµό προς όφελος π.χ. της αεροναυτικής και της διαστημικής βιοµηχανίας. Σκοπός της διατριβής είναι η ανάπτυξη νανοσωλήνων ΤιΟ2 σε υπόστρωμα καθαρού Τιτανίου με την βοήθεια ηλεκτροχημικής μεθόδου, και η μελέτη των πιθανών εφαρμογών των νέων υλικών στην εμβιομηχανική και την τεχνολογία των ηλιακών κυψελίδων στα φωτοβολταικά. Για τον σκοπό αυτό, θα κατασκευαστηκαν, ανάλογα με την περίπτωση, υβριδικά νανοσύνθετα υλικά αποτελούμενα από νανοσωλήνες ΤiO2 και νανοσωλήνες άνθρακα (CNTs) σε μήτρα βιο-αποικοδομήσιμης ρητίνης που θα βρίσκουν εφαρμογές στην ορθοπεδική και την οδοντιατρική, ενώ αντίστοιχα υβριδικά νανοσύνθετα κατασκευάστηκαν με σκοπό την αντικατάσταση της σιλικόνης στα ηλιακά κελιά με λεπτό στρώμα υβριδικού νανοσυνθέτου ΤiO2 με στόχο την μείωση του κόστους και την παράλληλη αύξηση της ενεργειακής απόδοσης των φωτοβολταικών. Στην παρούσα διδακτορική διατριβή θα κατασκευαστηκαν νανοσωλήνες ΤιΟ2 σε υπόστρωμα καθαρού Τιτανίου με την βοήθεια ηλεκτροχημικής μεθόδου. Μελετήθηκαν πειραματικά οι κύριες παράμετροι που επηρεάζουν την γεωμετρία των νανοσωλήνων. Πιο συγκεκριμένα, μελετήθηκαν η επίδραση του PH του ηλεκτρολύτη, της χρονικής διάρκειας και της εφαρμοζόμενης ηλεκτρικής τάσης κατά την ηλεκτρόλυση. Σε κάθε περίπτωση, η γεωμετρία των νανοσωλήνων μελετήθηκε με την παράλληλη χρήση ηλεκτρονικής μικροσκοπίας (SEM) και η χημική σύνθεση με την μέθοδο (EDX). Στην συνέχεια κατασκευάστηκαν δοκίμια νανοσυνθέτων με πολυμερική μήτρα ενισχυμένα με παράλληλα διατεταγμένους νανοσωλήνες TiO2 καθώς και άλλα δοκίμια με παραλληλη ενίσχυση με CNT’s. Το πολυμερές ήταν σε μορφή λεπτού φιλμ, ενώ αναπτυχθεί ειδική μεθοδολογία για την πρόσφυση των νανοσωλήνων άνθρακα τόσο με το πολυμερες όσο και με τους νανοσωλήνες TiO2. Το πολυμερές ανάλογα με την εφαρμογή του Νανοσυνθέτου για την οποία αυτό προορίζεται, ήταν βιο αποικοδομήσιμο (εφαρμογές στην εμβιομηχανική) ή όχι (εφαρμογές στα φωτοβολταικά). Ο πειραματικός χαρακτηρισμός της διεπιφανειακής πρόσφυσης μεταξύ των νανοσωλήνων και του υποστρώματος έγινει με την βοήθεια της νανο-διείσδυσης (nano-identation). Τα πειράματα επιβεβαιώθηκαν και ερμηνεύθηκαν θεωρητικά με την ανάπτυξη και εφαρμογή αναλυτικού θεωρητικού μοντέλου της υβριδικής ιξωδοελαστικής ενδιάμεσης φάσης που αναπτύσσεται στην περιοχή επαφής των νανοσωλήνων με το υπόστρωμα. Τέλος, οι αναλυτικές προβλέψεις καθώς και τα πειραματικά αποτελέσματα συγκρίθηκαν. Για τον έλεγχο των πιθανών εμβιομηχανικών εφαρμογών των ανωτέρω νανοσυνθέτων, μελετήθηκε η δυνατότητα ανάπτυξης ανθρώπινων κυττάρων στην επιφάνειά τους και θα έγινει έλεγχος της βιωσιμότητας, βιοσυμβατότητας καθώς και της αποκόλλησης των κυττάρων από το υπόστρωμα. Για τον έλεγχο των πιθανών εφαρμογών στα φωτοβολταικά μελετήθηκαν οι οπτικές ιδιότητες (reflectivity and photoluminescence), των επιφανειών των υβριδικών νανοσυνθέτων

Influence of electrical parameters on morphology of nanostructured TiO2 layers developed by electrochemical anodization

Ti6Al4V alloy micro rough surfaces with TiO2 self-organized nanostructured layers were synthesized using electrochemical anodization in phosphate/fluoride electrolyte, at different end potentials (5V, 10V, 15V, and 20 V). The current – time characteristics were recorded, and the link between current evolution and the morphology of developing oxide layers was investigated. On flat surfaces of Ti6Al4V alloy we developed TiO2 layers with different morphologies (random pores, nanopores of 25…50 nm, and highly organized nanotubes of 50…100 nm in diameter) depending on electrical parameters of anodization process. In our anodization cell, in optimized conditions, we are able to superimpose nanostructured oxide layers (nanotubular or nanoporous) over micro structured surfaces of titanium based materials used for biomedical implants

The Role of Substrate Topography and Stiffness on MSC Cells Functions: Key Material Properties for Biomimetic Bone Tissue Engineering

The hypothesis of the present research is that by altering the substrate topography and/or stiffness to make it biomimetic, we can modulate cells behavior. Substrates with similar surface chemistry and varying stiffnesses and topographies were prepared. Bulk PCL and CNTs-reinforced PCL composites were manufactured by solvent casting method and electrospinning and further processed to obtain tunable moduli of elasticity in the range of few MPa. To ensure the same chemical profile for the substrates, a protein coating was added. Substrate topography and properties were investigated. Further on, the feedback of Wharton’s Jelly Umbilical Cord Mesenchymal Stem Cells to substrates characteristics was investigated. Solvent casting scaffolds displayed superior mechanical properties compared to the corresponding electrospun films. However, the biomimetic fibrous texture of the electrospun substrates induced improved feedback of the cells with respect to their viability and proliferation. Cells’ adhesion and differentiation was remarkably pronounced on solvent casting substrates compared to the electrospun substrates. Soft substates improved cells multiplication and migration, while stiff substrates induced differentiation into bone cells. Aspects related to the key factors and the ideal properties of substrates and microenvironments were clarified, aiming towards the deep understanding of the required optimum biomimetic features of biomaterials

Combined Optimized Effect of a Highly Self-Organized Nanosubstrate and an Electric Field on Osteoblast Bone Cells Activity

The effect of an electric field within specific intensity limits on the activity of human cells has been previously investigated. However, there are a considerable number of factors that influence the in vitro development of cell populations. In biocompatibility studies, the nature of the substrate and its topography are decisive in osteoblasts bone cells development. Further on, electrical field stimulation may activate biochemical paths that contribute to a faster, more effective self-adjustment and proliferation of specific cell types on various nanosubstrates. Within the present research, an electrical stimulation device has been manufactured and optimum values of parameters that led to enhanced osteoblasts activity, with respect to the alkaline phosphatase and total protein levels, have been found. Homogeneous electric field distribution induced by a highly organized titanium dioxide nanotubes substrate had an optimum effect on cell response. Specific substrate topography in combination with appropriate electrical stimulation enhanced osteoblasts bone cells capacity to self-adjust the levels of their specific biomarkers. The findings are of importance in the future design and development of new advanced orthopaedic materials for hard tissue replacement

Mechanical Performance Enhancement of Aluminum Single-Lap Adhesive Joints Due to Organized Alumina Nanotubes Layer Formation on the Aluminum Adherends

The present investigation aims to take a step forward for the transfer of a simple laboratory electrochemical method of surface nano-treatment of aluminum to industrial applications. The electrochemical method has been applied to process 1050A aluminum. Surface nano-structuring has been achieved and resulted in the formation of an organized alumina nanotubes layer on commercial aluminum plates used as adherends for the manufacturing of aluminum single-lap adhesive joints. The mechanical properties of single-lap aluminum adhesive joints constructed with both non-anodized and anodized adherends were investigated and compared. Two types of epoxy resins were used to prove that the anodization of the adherends is equally effective, independently of the adhesives’ type. Furthermore, three overlap lengths were used (7, 10, and 25 mm) to study the effect of the overlap length on the overall joint mechanical response. Results of both three-point bending and tensile–shear testing showed that there is a considerable improvement of the joints’ mechanical performance with the addition of the nanostructures, for all the overlap lengths. It was found that the anodization method greatly contributes to the strengthening of the joints, leading to a strength increase of up to 176% and 148% for the shear and three-point bending strength, respectively

Gradient 3D Printed PLA Scaffolds on Biomedical Titanium: Mechanical Evaluation and Biocompatibility

The goal of the present investigation was to find a solution to crucial engineering aspects related to the elaboration of multi-layered tissue-biomimicking composites. 3D printing technology was used to manufacture single-layered and gradient multi-layered 3D porous scaffolds made of poly-lactic acid (PLA). The scaffolds manufacturing process was optimized after adjusting key printing parameters. The scaffolds with 60 μm side length (square-shaped pores) showed increased stiffness values comparing to the other specimens. A silicone adhesive has been further used to join biomedical titanium plates, and the PLA scaffolds; in addition, titania nanotubes (TNTs were produced on the titanium for improved adhesion. The titanium-PLA scaffold single lap joints were evaluated in micro-tensile testing. The electrochemical processing of the titanium surface resulted in a 248% increase of the ultimate strength in the overlap area for dry specimens and 40% increase for specimens immersed in simulated body fluid. Finally, the biocompatibility of the produced scaffolds was evaluated with primary cell populations obtained after isolation from bone residual tissue. The manufactured scaffolds present promising features for applications in orthopedic implantology and are worth further

Computational and Experimental Investigation of the Combined Effect of Various 3D Scaffolds and Bioreactor Stimulation on Human Cells’ Feedback

Computational methods were combined with an experimental setup in order to investigate the response of human umbilical cord stem cells to 3D electrospun and printed scaffolds, when dynamically stimulated in a bioreactor. Key parameters associated to bioreactor working conditions were computationally investigated using Comsol software to use the output for the planned experimental setup. Based on the theoretical observations, the influence of the inlet velocity, cell number, and exposure time in the bioreactor were analyzed and the in vitro parameters were adjusted accordingly. MSCs were seeded in different numbers in the 3D porous scaffolds and stimulated in the bioreactor (0.5 and 2 h duration, 3 and 6 mm/s inlet velocity). Polycaprolactone 3D electrospun, and polyurethane and polylactic acid 3D-printed scaffolds were fabricated and fibronectin-coated. The computational study predicted initial events in the process of cells deposition and attachment. Total protein, osteopontin, and osteocalcin levels in cells deposited in scaffolds were investigated; SEM and confocal imaging confirmed the biomarker analysis. MSCs proliferated well in PCL. Polyurethane enabled extremely rapid proliferation followed by differentiation, while PLA induced a moderate proliferation and parallel mineralization. The scaffolds stiffness has been found as the key enabling parameter decisive for cells feedback