Molly M. Gentleman

The role of surface free energy in osteoblast–biomaterial interactions

Abstract The clinical success of many orthopaedic implants relies on good integration between the implant and adjacent bone. As stabilising bone grows not only to the implant, but from it, the quick adhesion of bone forming cells called osteoblasts, their appropriate differentiation and ability to form mineralised bone are vital to achieve a good clinical outcome. Surface free energy can be thought of as a measure of the ‘unsatisfied bond energy’ resulting from ‘dangling bonds’ exposed at a materials surface. This unsatisfied bond energy affects protein adsorption and cell attachment, and thus controls the early stages of cell–biomaterial interactions and ultimately implant fixation. When water, proteins, or cells approach a surface, their surface domains align to minimise the overall surface free energy of the interface. Determining these interactions, however, is not simple. While contact angle measurements on flat surfaces can predict some surface free energy-related interactions, this is not the case when surface topography is modified. Here, the authors review how surface free energy can be altered on self-assembled monolayers, polymers, metals and ceramics and clarify the differences between measurements of surface free energy and wettability. The authors also review how surface free energy affects protein interactions and osteoblast behaviour. The result is a clearer understanding of the effect of surface free energy on cell behaviour and an unambiguous need for further studies that isolate such effects.

Characterization of Porcine Aortic Valvular Interstitial Cell 'Calcified' Nodules

Valve interstitial cells populate aortic valve cusps and have been implicated in aortic valve calcification. Here we investigate a common in vitro model for aortic valve calcification by characterizing nodule formation in porcine aortic valve interstitial cells (PAVICs) cultured in osteogenic (OST) medium supplemented with transforming growth factor beta 1 (TGF-β1). Using a combination of materials science and biological techniques, we investigate the relevance of PAVICs nodules in modeling the mineralised material produced in calcified aortic valve disease. PAVICs were grown in OST medium supplemented with TGF-β1 (OST+TGF-β1) or basal (CTL) medium for up to 21 days. Murine calvarial osteoblasts (MOBs) were grown in OST medium for 28 days as a known mineralizing model for comparison. PAVICs grown in OST+TGF-β1 produced nodular structures staining positive for calcium content; however, micro-Raman spectroscopy allowed live, noninvasive imaging that showed an absence of mineralized material, which was readily identified in nodules formed by MOBs and has been identified in human valves. Gene expression analysis, immunostaining, and transmission electron microscopy imaging revealed that PAVICs grown in OST+TGF-β1 medium produced abundant extracellular matrix via the upregulation of the gene for Type I Collagen. PAVICs, nevertheless, did not appear to further transdifferentiate to osteoblasts. Our results demonstrate that ‘calcified’ nodules formed from PAVICs grown in OST+TGF-β1 medium do not mineralize after 21 days in culture, but rather they express a myofibroblast-like phenotype and produce a collagen-rich extracellular matrix. This study clarifies further the role of PAVICs as a model of calcification of the human aortic valve.

Perivascular Stem Cells at the Tip of Mouse Incisors Regulate Tissue Regeneration

Cells with in vitro properties similar to those of bone marrow stromal stem cells are present in tooth pulp as quiescent cells that are mobilized by damage. These dental pulp stem cells (DPSCs) respond to damage by stimulating proliferation and differentiation into odontoblast-like cells that form dentine to repair the damage. In continuously growing mouse incisors, tissue at the incisor tips is continuously being damaged by the shearing action between the upper and lower teeth acting to self-sharpen the tips. We investigated mouse incisor tips as a model for the role of DPSCs in a continuous natural repair/regeneration process. We show that the pulp at the incisor tip is composed of a disorganized mass of mineralized tissue produced by odontoblast-like cells. These cells become embedded into the mineralized tissue that is rapidly formed and then lost during feeding. Tetracycline labeling not only revealed the expected incorporation into newly synthesized dentine formation of the incisor but also a zone covering the pulp cavity at the tips of the incisors that is mineralized very rapidly. This tissue was dentine-like but had a significantly lower mineral content than dentine as determined by Raman spectroscopy. The mineral was more crystalline than dentine, indicative of small, defect-free mineral particles. To identify the origin of cells responsible for deposition of this mineralized tissue, we genetically labeled perivascular cells by crossing NG2(ERT2) Cre and Nestin Cre mice with reporter mice. A large number of pericyte-derived cells were visible in the pulp of incisor tips with some having elongated, odontoblast-like shapes. These results show that in mouse incisors, rapid, continuous mineralization occurs at the tip to seal off the pulp tissue from the external environment. The mineral is formed by perivascular-derived cells that differentiate into cells expressing dentin sialo-phosphoprotein (DSPP) and produce a dentine-like material in a process that functions as continuous natural tissue regeneration.

The thermo-mechanical response of PP nanocomposites at high graphene loading

Abstract Authors have successfully fabricated polypropylene/graphene nanoplatelets (PP/GNPs), nanocomposites that are thermally conductive, processable, and flame resistant. Thermal conductivity measurements indicated that the thermal coefficient scaled linearly with GNP loading, where a value of 2.0 W m− 1 K− 1 was achieved at 40 wt-% loading. Tensile measurements indicated that the modulus increased linearly with GNP loading, while the Izod impact, after an initial decrease, remained constant for loadings up to 50 wt-%. Small angle X-ray scattering (SAXS) showed a large decrease in the amount of lamellar structure relative to the neat PP, while wide angle X-ray scattering (WAXS) showed a high degree of crystallinity. These results are consistent with formation of a new type of layered nanocomposite, composed of crystalline PP chains oriented onto layered GNPs.

Probing differential optical and coverage behavior in nanotube–nanocrystal heterostructures synthesized by covalent versus non-covalent approaches

Double-walled carbon nanotube (DWNT)-CdSe heterostructures with the individual nanoscale building blocks linked together by 4-aminothiophenol (4-ATP) have been successfully synthesized using two different and complementary routes, i.e. covalent attachment and non-covalent π-π stacking. Specifically, using a number of characterization methods, we have probed the effects of these differential synthetic coupling approaches on the resulting CdSe quantum dot (QD) coverage on the underlying nanotube template as well as the degree of charge transfer between the CdSe QDs and the DWNTs. In general, based on microscopy and spectroscopy data collectively, we noted that heterostructures generated by non-covalent π-π stacking interactions evinced not only higher QD coverage density but also possibly more efficient charge transfer behavior as compared with their counterparts produced using covalent linker-mediated protocols.

Direct observations of erosion-induced ferroelasticity in EB-PVD thermal barrier coatings

Imaging of the presence and location of erosion-induced ferroelastic toughening was completed on 8-weight percent yttria-stabilized zirconia EB-PVD thermal barrier coatings using confocal polarized Raman spectroscopy. A combination of measurements made using ultraviolet and visible laser radiation was conducted to determine the location and extent of ferroelastic twinning present at and below the eroded surfaces as well as along fracture surfaces of the coatings. Ferroelastic twinning events were identified at three major locations within and on the coating: the erosion surface, just below the surface, and in the bulk of the coating. The results shown here reveal that not all fracture events result in a ferroelastic response. This suggests there may be an opportunity to increase the toughness of thermal barrier coatings by increasing the possibility that a crack can produce a ferroelastic twin in the coating.