Barbieri Thesis - BioMedical Materials program (BMM)

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Chapter 6 – Fluid uptake as instructive factor ADDENDUM 6a.1. Introduction When polymers have comparable molecular weights, their monomer composition, besides influencing on their biological properties as seen previously, also affects their mechanical properties because it determines the molecular structure. As discussed previously, semi–crystalline polymers present ordered and compact molecular regions in their disordered entangled structure. These regions hinder the chain sliding movement when the polymer is under stress, rendering the whole structure stiffer. Polylactide with high content of L–lactide monomer, for example above 70%mol. content, is semi–crystalline, whereas if it has high content of D,L–lactide monomer it will have an amorphous structure. As observed previously, the polymer phase in the three composites has comparable molecular weight thus we may evaluate the effect of the monomer on their elastic and viscoelastic properties. We expect that the composites containing polymers with high L–lactide contents, i.e. MLD and MLDL, will have higher storage modulus and lower damping characteristics as compared to MDL. To test this, we performed dynamic mechanical analyses in dry conditions. 6a.2. Method Bars (50×15×2.5 mm) of the three composites were prepared by hot pressing (150±1°C, 3±0.5 min, 250±10 kN) the granules into steel moulds. The surface of all samples was then etched with 1M sodium hydroxide (NaOH, Merck, Darmstadt, Germany) at room temperature for 5±0.5 minutes to obtain samples with similar surfaces. They were then sterilized with ethylene oxide (IsoTron Nederland BV, Ede, the Netherlands) and used for all the experiments described in this study. Afterwards, the bars were analysed with a dynamic mechanical analyzer (DMA 2980, TA Instruments, New Castle, DE, USA) in three–point bending, over a sweep of physiological frequencies (1–10 Hz, 0.3 Hz/min) at 37±0.1°C in dry air with static force 0.53±0.01 N and dynamic displacement 10±0.01 μm. The samples were kept at 37±0.01°C for 15±1 minutes to stabilize the material prior the measurements. The data measured were storage modulus and loss tangent. 6a.3. Results and discussion The composites that contained polymers with L–lactide monomer, i.e. MLD and MLDL, had storage modulus significantly higher than the material with poly(D,L– lactide) (Figure 16a). In general, the modulus in MLD and MLDL has negligible changes during the analysis showing these materials are stable over a sweep of 146
Chapter 6 – Flu uid uptake as insstructive factor physiologgical frequenccies. On the contrary, thee stiffness in n MDL had a slightly increasingg trend with tthe frequency y. Since the aanalysis has been b done by applying cyclic strresses, it maay be that th he polymer eexhibited stra ain–stiffening behavior, presumabbly, because oof the inability y of the moleccular chains to t relax to the eir neutral conformaation betweenn loading cyc cles, thereby debilitating smooth s interm molecular motion annd requiring mmore stress fo or the same sttrain. Consiste ently, the loss s tangent increasedd with the diminution of L–lactide conntent (Figure 16b), indica ating that amorphouus polymers have better damping d propperties. Intere estingly, such damping propertiess significantlyy decrease ov ver the frequeency for MDL as a conseq quence of increasingg stiffness as indicated by the correspondding storage modulus. m Figure 16. Measured (a) stoorage modulus an nd (b) loss tangeent of the compos sites in dry condit tions. 6a.4. Connclusion The resullts reported inn this Addendu um demonstraate that poly(D D,L–lactide) m a suitablee polymer forr composites to be used in load–bearing sites, whe copolymeers gave rise tto composite materials withh higher dynamic stiffness a stable daamping properrties over the considered sstressing frequ uency range. supplemeental informaation for dev veloping instrructive composites with mechaniccal properties. . From this, it appears that the composit te comprising L–lactide/ /30%mol. D,LL–lactide may y be used in bone tissue regeneration bearing ssites as it is innstructive and has dry dynaamic mechani ical properties those meeasured on dryy human cort tical bone exttracted from fe emora 4). Howevver, as already mentioned also a in Chapteer 4, improvem guaranteee suitable meechanical prop perties in moisstened conditi this compposite). [338] ay not be ereas the and more This is a improved 70%mol. in load– s close to (see e Chapter ments are nec cessary to ions (not mea asured for 147
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INSTRUCTIVE COMPOSITES FOR BONE REG
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INSTRUCTIVE COMPOSITES FOR BONE REG
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献给潘臻 A mamma e papà A Stef
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Summary Summary Developing new biom
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Summary containing 96%mol. L-lactid
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Samenvatting De behoefte aan een de
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Samenvatting structuur, bespoedigt
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Riassunto velocità di dissoluzione
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Riassunto generale, ha migliorato i
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Chapter 1 - Introduction 1.1. Eukar
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Chap pter 1 - Introducction Interst
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Chapter 1 - Introduction bone marro
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Chap pter 1 - Introducction Figure
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Chapter 1 - Introduction regenerati
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Chapter 1 - Introduction Key concep
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Chapter 1 - Introduction grown in c
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Chapter 1 - Introduction Mainly, th
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Chapter 1 - Introduction All these
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Chap pter 1 - Introducction materia
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Chapter 1 - Introduction protein, m
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CHAPTER 2 The role of gels in bone
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Chapter 2 - The role of gels in ins
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CChapter 2 - The role r of gels in
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CHAPTER 3 Instructive composites: e
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Chapter 3 - Instructive composites:
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Chapter 3 - Instructive compos site
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CChapter 3 - Instructive composit t
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CHAPTER 4 Controlling dynamic mecha
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Chapter 4 - Conntrol of mechanical
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Chapter 4 - Control of mechanical a
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CHAPTER 5 Effects of alkali surface
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Chapter 5 - Alkali surface treatmen
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Chapter 5 - Alkali surface treatmen
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Page 215 and 216: References [1] Waggoner B. Eukaryot
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References [72] Kokovay E, Wang Y,
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References [102] McBeath R, Pirone
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References molecule-mediated TGF- t
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References [159] Autumn K, Sitti M,
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References [189] Teixeira AI, McKie
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References [215] Tang L, Jennings T
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References [242] Wei G, Ma PX. Stru
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References [270] Atkinson BL, Hanks
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References [297] Sato I, Akizuki T,
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References hematopoietic stem cell
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References [352] Cullinane DM, Sali
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References [382] Norde W, Giacomell
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References differentiation of rat B
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Acknowledgments thank you for all t
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Acknowledgments desidero tutto il m
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Barbieri Thesis - BioMedical Materials program (BMM)

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