Gerald G. Fuller

Vascular anastomosis using controlled phase transitions in poloxamer gels

Vascular anastomosis is the cornerstone of vascular, cardiovascular and transplant surgery. Most anastomoses are performed with sutures, which are technically challenging and can lead to failure from intimal hyperplasia and foreign body reaction. Numerous alternatives to sutures have been proposed, but none has proven superior, particularly in small or atherosclerotic vessels. We have developed a new method of sutureless and atraumatic vascular anastomosis that uses US Food and Drug Administration (FDA)-approved thermoreversible tri-block polymers to temporarily maintain an open lumen for precise approximation with commercially available glues. We performed end-to-end anastomoses five times more rapidly than we performed hand-sewn controls, and vessels that were too small (<1.0 mm) to sew were successfully reconstructed with this sutureless approach. Imaging of reconstructed rat aorta confirmed equivalent patency, flow and burst strength, and histological analysis demonstrated decreased inflammation and fibrosis at up to 2 years after the procedure. This new technology has potential for improving efficiency and outcomes in the surgical treatment of cardiovascular disease.

Preparation of Mineralized Nanofibers: Collagen Fibrils Containing Calcium Phosphate

We report a straightforward, bottom-up, scalable process for preparing mineralized nanofibers. Our procedure is based on flowing feed solution, containing both inorganic cations and polymeric molecules, through a nanoporous membrane into a receiver solution with anions, which leads to the formation of mineralized nanofibers at the exit of the pores. With this strategy, we were able to achieve size control of the nanofiber diameters. We illustrate this approach by producing collagen fibrils with calcium phosphate incorporated inside the fibrils. This structure, which resembles the basic constituent of bones, assembles itself without the addition of noncollagenous proteins or their polymeric substitutes. Rheological experiments demonstrated that the stiffness of gels derived from these fibrils is enhanced by mineralization. Growth experiments of human adipose derived stem cells on these gels showed the compatibility of the fibrils in a tissue-regeneration context.

The interfacial viscoelastic properties and structures of human and animal Meibomian lipids.

As the interface between the aqueous layer of the tear film and air, the lipid layer plays a large role in maintaining tear film stability. Meibomian lipids are the primary component of the lipid layer; therefore the physical properties of these materials may be particularly crucial to the functionality of the tear film. Surface pressure versus area isotherms, interfacial shear and extensional rheology, and Brewster angle microscopy (BAM) were used to characterize the Meibomian lipids from different species known to have different lipid compositions. The isotherms of humans, bovinae, wallabies, rabbits and kultarrs (a small desert marsupial) were qualitatively similar with little hysteresis between compression and expansion cycles. In contrast, several isocycles were necessary to achieve equilibrium behavior in the koala lipids. With the exception of kultarr lipids, the interfacial complex viscosity of all samples increased by one or two orders of magnitude between surface pressures of 5 mN/m and 20 mN/m and exhibited classic gel behavior at higher surface pressures. In contrast, the kultarr lipids were very fluid up to 22 mN/m; the behavior did not depend on surface pressure. Human lipids were very deformable in extensional flow and the BAM images revealed that the film became more homogeneous with compression as the elasticity of the film increased. The morphology of the kultarr lipids did not change with compression indicating a strong correlation between film structure and behavior. These results suggest that the lipid layer of the tear film forms a gel in vivo, which may aid in mechanically stabilization of the tear film.

Liquid crystalline collagen: a self-assembled morphology for the orientation of mammalian cells.

We report the creation of collagen films having a cholesteric banding structure with an orientation that can be systematically controlled. The action of hydrodynamic flow and rapid desiccation was used to influence the orientation of collagen fibrils, producing a film with a twisted plywood architecture. Adult human fibroblasts cultured on these substrates orient in the direction of the flow deposition, and filopodia are extended onto individual bands. Atomic force microscopy reveals the assembly of 30 nm collagen fibrils into the uniform cholesteric collagen films with a periodic surface relief. The generation of collagen with a reticular, basket-weave morphology when using lower concentrations is also discussed.

Designing a tubular matrix of oriented collagen fibrils for tissue engineering

A scaffold composed entirely of an extracellular matrix component, such as collagen, with cellular level control would be highly desirable for applications in tissue engineering. In this article we introduce a novel, straightforward flow processing technique that fabricates a small diameter tubular matrix constructed of anisotropic collagen fibrils. Scanning electron microscopy confirmed the uniform alignment of the collagen fibrils and subsequent matrix-induced alignment of human fibroblasts. The uniform alignment of the fibroblasts along the collagen fibrils demonstrated the ability of the aligned fibrils to successfully dictate the directional growth of human fibroblasts through contact guidance. Various non-cytotoxic cross-linking techniques were also applied to the collagen conduit to enhance the mechanical properties. Tensile testing and burst pressure were the two measurements performed to characterize the mechanical integrity of the conduit. Mechanical characterization of the cross-linked collagen conduits identified 1-ethyl-3-(3-dimethyl aminopropyl) carbodiimide hydrochloride cross-linking as the most promising technique to reinforce the mechanical properties of native collagen. An oriented conduit of biocompatible material has been fabricated with decent mechanical strength and at a small diameter scale, which is especially applicable in engineering cardiovascular tissues and nerve grafts.

Effect of Lysozyme Adsorption on the Interfacial Rheology of DPPC and Cholesteryl Myristate Films

A model tear film lipid layer composed of a binary mixture of cholesteryl myristate (CM) and 1,2-dipalmitoyl- sn-glycero-3-phosphocholine (DPPC) was characterized using surface tension measurements, Brewster angle microscopy (BAM) and interfacial stress rheology (ISR). Isotherms showed that films containing >or=90 mol % CM have a 17-fold greater % area loss between the first and second compressions than the films with less CM. BAM images clearly showed that CM films did not expand after compression, and solid-like regions extending 1-2 mm were observed at low pressures (1 mN/m). Lipid films with <or=70 mol % CM expanded after compression in BAM indicating that the films were primarily fluid. ISR data confirmed these results, as DPPC and 30:70 CM/DPPC films were fluid at all pressures and more compliant than films with higher CM content. Films with >or=50 mol % CM became elastic at higher surface pressures. Increasing CM content reduced the surface pressure at which the mixed film became elastic. Lysozyme adsorption into a CM film increased the compressibility and resulted in a more expanded film. Lysozyme increased the ductility of the CM/DPPC films with no film breakdown occurring up to the highest pressure measured (40 mN/m). In summary, CM increased the elasticity of the lipid films, but also caused them to become brittle and incapable of expansion following compression. Lysozyme adsorption increased the ductility and decreased the isotherm hysteresis for CM/DPPC films.

Structure and dynamics of particle monolayers at a liquid–liquid interface subjected to shear flow

The effect of shear flow on the structure and dynamics of monodisperse spherical polystyrene particles suspended at the interface between decane and water was observed. While undisturbed, the particles arrange themselves on a hexagonal lattice due to strong dipole-dipole repulsion resulting from ionizable sulfate groups on their surfaces. As the interface is subjected to shear flow, however, the lattice adopts a new semi-ordered, anisotropic state for which two distinct regimes are observed. At low particle concentrations or high shear rates, nearest neighbors in the lattice align in the flow direction and create strings of particles that slip past each other fairly readily. This results in a stretching of the overall structure and achievement of a steady state orientation in the system. In contrast, at high concentrations or low shear rates, the interparticle forces gain importance and tend to keep the particles more strongly in their lattice positions. As a result, domains within the lattice are forced to rotate, thus giving rise to movement of particles perpendicular to the flow direction. Thus a rotation, in addition to stretching, of the structure is apparent in this case.

Insertion mechanism of a poly(ethylene oxide)-poly(butylene oxide) block copolymer into a DPPC monolayer.

Interactions between amphiphilic block copolymers and lipids are of medical interest for applications such as drug delivery and the restoration of damaged cell membranes. A series of monodisperse poly(ethylene oxide)-poly(butylene oxide) (EOBO) block copolymers were obtained with two ratios of hydrophilic/hydrophobic block lengths. We have explored the surface activity of EOBO at a clean interface and under 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) monolayers as a simple cell membrane model. At the same subphase concentration, EOBO achieved higher equilibrium surface pressures under DPPC compared to a bare interface, and the surface activity was improved with longer poly(butylene oxide) blocks. Further investigation of the DPPC/EOBO monolayers showed that combined films exhibited similar surface rheology compared to pure DPPC at the same surface pressures. DPPC/EOBO phase separation was observed in fluorescently doped monolayers, and within the liquid-expanded liquid-condensed coexistence region for DPPC, EOBO did not drastically alter the liquid-condensed domain shapes. Grazing incidence X-ray diffraction (GIXD) and X-ray reflectivity (XRR) quantitatively confirmed that the lattice spacings and tilt of DPPC in lipid-rich regions of the monolayer were nearly equivalent to those of a pure DPPC monolayer at the same surface pressures.

Thin film formation of silica nanoparticle/lipid composite films at the fluid-fluid interface.

We report a new and simple method for the formation of thin films at the interface between aqueous silica Ludox dispersions and lipid solutions in decane. The lipids used are stearic acid, stearyl amine, and stearyl alcohol alongside silica Ludox nanoparticle dispersions of varying pH. At basic pH thin films consisting of a mixture of stearic acid and silica nanoparticles precipitate at the interface. At acidic and neutral pH we were able to produce thin films consisting of stearyl amine and silica particles. The film growth was studied in situ with interfacial shear rheology. In addition to that, surface pressure isotherm and dynamic light scattering experiments were performed. The films all exhibit strong dynamic rheological moduli, rendering them an interesting material for applications such as capsule formation, surface coating, or as functional membranes.

Surface Rheology of Hydrophobically Modified PEG Polymers Associating with a Phospholipid Monolayer at the Air-Water Interface

Surface rheology of irreversibly bound hydrophobically modified poly(ethylene glycol) (PEG) polymers (HMPEG) on a dipalmitoylphosphatidylcholine (DPPC) monolayer is investigated to determine attributes that may contribute to immune recognition. Previously, three comb-graft polymers (HMPEG136-DP3, HMPEG273-DP2.5, and HMPEG273-DP5) adsorbed on liposomes were examined for their strength of adsorption and protection from complement binding. The data supported an optimal ratio between the hydrophilicity of the PEG polymer and the number of hydrophobic anchors. The HMPEG polymers have different polymer brush thicknesses (4.2-5.9 nm) and levels of cooperativity (2.5-5 hydrophobes). The results indicate that an increased viscous force (above 0.25 mN s/m) at the surface may enable the polymers to shield liposomes from protein interactions. Similar rheological behavior is shown for all polymer architectures at low polymer surface coverage (0.5 mg/m2, in the mushroom regime), whereas at high surface coverage (>0.5 mg/m2, in the brush regime), we observe a structural dependence of the surface viscous forces at 40 mN/m. This threshold correlates with a 92% decrease in complement protein binding for liposomes coated with 1 mg/m2 HMPEG273-DP5. This may suggest that surface viscous forces play a role in reducing complement protein binding.