Active targeting of cancer cells by CD44 binding peptide-functionalized oil core-based nanocapsules

Selectivity in tumor targeting is one of the major issues in cancer treatment. Therefore, surface functionalization of drug delivery systems with active moieties, able to selectively target tumors, has become a worldwide-recognized strategy. The CD44 receptor is largely used as a biomarker, being overexpressed in several tumors, and consequently as a target thanks to the identification of the CD44 binding peptide. Here we implemented the CD44 binding peptide logic onto an oil core–polymer multilayer shell, taking into account and optimizing all relevant features of drug delivery systems, such as small size (down to 100 nm), narrow size distribution, drug loading capability, antifouling and biodegradability. Besides promoting active targeting, the oil core-based system enables the delivery of natural and synthetic therapeutic compounds. Biological tests, using curcumin as a bioactive compound and fluorescent tag, demonstrated that CD44 binding peptide-functionalized nanocapsules selectively accumulate and internalize in cancer cells, compared to the control, thanks to ligand–receptor binding.

CD44 binding peptide was implemented onto an oil core–polymer multilayer shell of 100 nm size and completely biodegradable. Biological tests, demonstrated that the proposed nanocarrier selectively accumulates and internalizes in cancer cells.

Effects of pulsating heat source on interstitial fluid transport in tumour tissues

Macromolecules and drug delivery to solid tumours is strongly influenced by fluid flow through interstitium, and pressure-induced tissue deformations can have a role in this. Recently, it has been shown that temperature-induced tissue deformation can influence interstitial fluid velocity and pressure fields, too. In this paper, the effect of modulating-heat strategies to influence interstitial fluid transport in tissues is analysed. The whole tumour tissue is modelled as a deformable porous material, where the solid phase is made up of the extracellular matrix and cells, while the fluid phase is the interstitial fluid that moves through the solid matrix driven by the fluid pressure gradient and vascular capillaries that are modelled as a uniformly interspersed fluid point-source. Pulsating-heat generation is modelled with a time-variable cosine function starting from a direct current approach to solve the voltage equation, for different pulsations. From the steady-state solution, a step-variation of vascular pressure included in the model equation as a mass source term via the Starling equation is simulated. Dimensionless 1D radial equations are numerically solved with a finite-element scheme. Results are presented in terms of temperature, volumetric strain, pressure and velocity profiles under different conditions. It is shown that a modulating-heat procedure influences velocity fields, that might have a consequence in terms of mass transport for macromolecules or drug delivery.

A BIOPHYSICAL ANALYSIS TO ASSESS X-RAY SENSITIVITY OF HEALTHY AND TUMOUR CELLS

The mechanobiology is providing novel perspectives in the study of cancer and is contributing to evaluate the cancer responses, from a biophysical point of view, to classical therapeutic approaches- radiotherapy and chemotherapy. Here we have explored the effects of two doses (4 and 8 Gy) of 6 MeV photons on spreading, focal adhesions, migration and mechanical properties of BALB/c 3T3 and their SV40 transformed equivalent, SVT2. Cell biophysical responses to 4 and 8 Gy were analysed and compared with those reported in previous published work when lower doses (1 and 2 Gy) were administered Panzetta et al. (Effects of high energy X-rays on cell morphology and functions. Proc. Book 2017;16:116). We observed that the range of sensitivity to ionising radiations profoundly changes depending on the patho-physiological state of cells. In particular, we found that X-rays induce morphological and functional variations in both cell lines (decreased motility, increased adhesion and increased cytoskeleton stiffness). These changes were slightly dependent on doses in the case of SVT2 cells and may indicate a possible mechanical normalisation in their phenotype. Nevertheless, the responses of BALB/c 3T3 were negligible only for the low dose of 1 Gy and increased significantly in a dose-dependent manner with higher doses. We believe that the characterisation of X-rays effects on the cell mechanobiology could shed new light in the design and customisation of radiotherapy treatments.

Silk-ELR co-recombinamer covered stents obtained by electrospinning

In the field of tissue engineering the choice of materials is of great importance given the possibility to use biocompatible polymers produced by means of biotechnology. A large number of synthetic and natural materials have been used to this purpose and processed into scaffolds using Electrospinning technique. Among materials that could be used for the fabrication of scaffold and degradable membranes, natural polymers such as collagen, elastin or fibroin offer the possibility to design structures strictly similar to the extracellular matrix (ECM). Biotechnology and genetic engineering made possible the advent of a new class of biopolymers called protein-based polymers. One example is represented by the silk-elastin-proteins that combine the elasticity and resilience of elastin with the high tensile strength of silk-fibroin and display engineered bioactive sequences. In this work, we use electrospinning technique to produce a fibrous scaffold made of the co-recombinamer Silk-ELR. Obtained fibres have been characterized from the morphological point of view. Homogeneity and morphology have been explored using Scanning Electron Microscopy. A thorough study regarding the influence of Voltage, flow rate and distance have been carried out to determine the appropriate parameters to obtain the fibrous mats without defects and with a good distribution of diameters. Cytocompatibility has also been in vitro tested. For the first time we use the co-recombinamer Silk-ELR for the fabrication of a 2.5 angioplasty balloon coating. This structure could be useful as a coated scaffold for the regeneration of intima layer of vessels.

Elastin-like-recombinamers multilayered nanofibrous scaffolds for cardiovascular applications

Coronary angioplasty is the most widely used technique for removing atherosclerotic plaques in blood vessels. The regeneration of the damaged intima layer after this treatment is still one of the major challenges in the field of cardiovascular tissue engineering. Different polymers have been used in scaffold manufacturing in order to improve tissue regeneration. Elastin-mimetic polymers are a new class of molecules that have been synthesized and used to obtain small diameter fibers with specific morphological characteristics. Elastin-like polymers produced by recombinant techniques and called elastin-like recombinamers (ELRs) are particularly promising due to their high degree of functionalization. Generally speaking, ELRs can show more complex molecular designs and a tighter control of their sequence than other chemically synthetized polymers Rodriguez Cabello et al (2009 Polymer 50 5159-69, 2011 Nanomedicine 6 111-22). For the fabrication of small diameter fibers, different ELRs were dissolved in 2,2,2-fluoroethanol (TFE). Dynamic light scattering was used to identify the transition temperature and get a deep characterization of the transition behavior of the recombinamers. In this work, we describe the use of electrospinning technique for the manufacturing of an elastic fibrous scaffold; the obtained fibers were characterized and their cytocompatibility was tested in vitro. A thorough study of the influence of voltage, flow rate and distance was carried out in order to determine the appropriate parameters to obtain fibrous mats without beads and defects. Moreover, using a rotating mandrel, we fabricated a tubular scaffold in which ELRs containing different cell adhesion sequences (mainly REDV and RGD) were collected. The stability of the scaffold was improved by using genipin as a crosslinking agent. Genipin-ELRs crosslinked scaffolds  show a good stability and fiber morphology. Human umbilical vein endothelial cells  were used to assess the in vitro bioactivity of the cell adhesion domains within the backbone of the ELRs.

Shedding light on azopolymer brush dynamics by fluorescence correlation spectroscopy

Understanding the response to illumination at a molecular level as well as characterising polymer brush dynamics are key features that guide the engineering of new light-stimuli responsive materials. Here, we report on the use of a confocal microscopy technique that was exploited to discern how a single molecular event such as the photoinduced isomerisation of azobenzene can affect an entire polymeric material at a macroscopic level leading to photodriven mass-migration. For this reason, a set of polymer brushes, containing azobenzene (Disperse Red 1, DR) on the side chains of poly(methacrylic acid), was synthesised and the influence of DR on the polymer brush dynamics was investigated for the first time by Fluorescence Correlation Spectroscopy (FCS). Briefly, two dynamics were observed, a short one coming from the isomerisation of DR and a long one related to the brush main chain. Interestingly, photoinduced polymer aggregation in the confocal volume was observed.

Vasculogenic potential evaluation of bottom-up, PCL scaffolds guiding early angiogenesis in tissue regeneration

Vascularization is a key factor in the successful integration of tissue engineered (TE) grafts inside the host body. Biological functions of the newly formed tissue depend, in fact, on a reliable and fast spread of the vascular network inside the scaffold. In this study, we propose a technique for evaluating vascularization in TE constructs assembled by a bottom-up approach. The rational, ordered assembly of building blocks (BBs) into a 3D scaffold can improve vessel penetration, and-unlike most current technologies-is compatible with the insertion of different elements that can be designed independently (e.g. structural units, growth factor depots etc.). Poly(ε-caprolactone) scaffolds composed of orderly and randomly assembled sintered microspheres were used to assess the degree of vascularization in a pilot in vivo study. Scaffolds were implanted in a rat subcutaneous pocket model, and retrieved after 7 days. We introduce three quantitative factors as a measure of vascularization: the total percentage of vascularization, the vessels diameter distribution and the vascular penetration depth. These parameters were derived by image analysis of microcomputed tomographic scans of biological specimens perfused with a radiopaque polymer. The outcome of this study suggests that the rational assembly of BBs helps the onset and organization of a fully functional vascular network.

Engineered cardiac micromodules for the in vitro fabrication of 3D endogenous macro-tissues

The in vitro fabrication of an endogenous cardiac muscle would have a high impact for both in vitro studies concerning cardiac tissue physiology and pathology, as well as in vivo application to potentially repair infarcted myocardium. To reach this aim, we engineered a new class of cardiac tissue precursor (CTP), specifically conceived in order to promote the synthesis and the assembly of a cardiac extracellular matrix (ECM). The CTPs were obtained by culturing a mixed cardiac cell population, composed of myocyte and non-myocyte cells, into porous gelatin microspheres in a dynamic bioreactor. By engineering the culture conditions, the CTP developed both beating properties and an endogenous immature cardiac ECM. By following a bottom-up approach, a macrotissue was fabricated by molding and packing the engineered tissue precursor in a maturation chamber. During the macrotissue formation, the tissue precursors acted as cardiac tissue depots by promoting the formation of an endogenous and interconnected cardiac network embedding the cells and the microbeads. The myocytes cell fraction pulled on ECM network and induced its compaction against the internal posts represented by the initial porous microbeads. This reciprocal interplay induced ECM consolidation without the use of external biophysical stimuli by leading to the formation of a beating and endogenous macrotissue. We have thus engineered a new class of cardiac micromodules and show its potential for the fabrication of endogenous cardiac tissue models useful for in vitro studies that involve the cardiac tissue remodeling.

A micro-perfusion bioreactor for on line investigation of ECM remodeling under hydrodynamic and biochemical stimulation

Tissue-on-chip (TOC) systems aim at replicating complex biological dynamics in vitro with the potential either to improve the understanding of human biology or to develop more accurate therapeutic strategies. To replicate faithfully the intricate interrelationships between cells and their surrounding microenvironment, the three-dimensional (3D) tissue model must possess a responsive extracellular matrix (ECM). ECM remodeling plays a pivotal role in guiding cells and tissues functions and such aspect is somewhat denied during in vitro studies. For this purpose, we fabricated a micro-perfusion bioreactor capable to sustain the viability of 3D engineered tissue models recapitulating the process of the native ECM deposition and assembly. Engineered human dermis micro-tissue precursors (HD-μTP) were used as building blocks to generate a final tissue. HD-μTP were loaded in the perfusion space of the micro-perfusion bioreactor and, under the superimposition of different fluid dynamic regimes and biochemical stimulation, they synthesized new collagen proteins that were, then, assembled in the perfusion space forming a continuum of cells embedded in their own ECM. The micro-perfusion bioreactor was fabricated to allow the on-line monitoring of the oxygen consumption and the assembly of the newly formed collagen network via real time acquisition of the second harmonic generation (SHG) signal. The possibility to detect the collagen reorganization due to both fluid dynamic and biochemical stimulation, let us to define the optimal perfusion configuration in order to obtain a TOC system based on an endogenous and responsive ECM.

Biophysical properties of dermal building-blocks affects extra cellular matrix assembly in 3D endogenous macrotissue

The fabrication of functional tissue units is one of the major challenges in tissue engineering due to their in vitro use in tissue-on-chip systems, as well as in modular tissue engineering for the construction of macrotissue analogs. In this work, we aim to engineer dermal tissue micromodules obtained by culturing human dermal fibroblasts into porous gelatine microscaffold. We proved that such stromal cells coupled with gelatine microscaffolds are able to synthesize and to assemble an endogenous extracellular matrix (ECM) resulting in tissue micromodules, which evolve their biophysical features over the time. In particular, we found a time-dependent variation of oxygen consumption kinetic parameters, of newly formed ECM stiffness and of micromodules self-aggregation properties. As consequence when used as building blocks to fabricate larger tissues, the initial tissue micromodules state strongly affects the ECM organization and maturation in the final macrotissue. Such results highlight the role of the micromodules properties in controlling the formation of three-dimensional macrotissue in vitro, defining an innovative design criterion for selecting tissue-building blocks for modular tissue engineering.

Surface decoration with gH625-membranotropic peptides as a method to escape the endo-lysosomal compartment and reduce nanoparticle toxicity

The membranotropic peptide gH625 is able to transport different cargos (i.e., liposomes, quantum dots, polymeric nanoparticles) within and across cells in a very efficient manner. However, a clear understanding of the detailed uptake mechanism remains elusive. In this work, we investigate the journey of gH625-functionalized polystyrene nanoparticles in mouse-brain endothelial cells from their interaction with the cell membrane to their intracellular final destination. The aim is to elucidate how gH625 affects the behavior of the nanoparticles and their cytotoxic effect. The results indicate that the mechanism of translocation of gH625 dictates the fate of the nanoparticles, with a relevant impact on the nanotoxicological profile of positively charged nanoparticles.

Optical signature of erythrocytes by light scattering in microfluidic flows

A camera-based light scattering approach coupled with a viscoelasticity-induced cell migration technique has been used to characterize the morphological properties of erythrocytes in microfluidic flows. We have obtained the light scattering profiles (LSPs) of individual living cells in microfluidic flows over a wide angular range and matched them with scattering simulations to characterize their morphological properties. The viscoelasticity-induced 3D cell alignment in microfluidic flows has been investigated by bright-field and holographic microscopy tracking, where the latter technique has been used to obtain precise cell alignment profiles in-flow. Such information allows variable cell probability control in microfluidic flows at very low viscoelastic polymer concentrations, obtaining cell measurements that are almost physiological. Our results confirm the possibility of precise, label-free analysis of individual living erythrocytes in microfluidic flows.

Tunable stability of monodisperse secondary O/W nano-emulsions

Stable and biodegradable oil in water (O/W) nano-emulsions can have a huge impact on a wide range of bio-applications, from food to cosmetics and pharmaceuticals. Emulsions, however, are immiscible systems unstable over time; polymer coatings are known to be helpful, but an effective procedure to stabilize monodisperse and biodegradable O/W nano-emulsions is yet to be designed. Here, we coat biodegradable O/W nano-emulsions with a molecular layer of biodegradable polyelectrolytes such as polysaccharides--like chitosan--and polypeptides--like polylysine--and effectively re-disperse and densify the polymer coating at high pressure, thus obtaining monodisperse and stable systems. In particular, focusing on chitosan, our tests show that it is possible to obtain unprecedented ultra-stable O/W secondary nano-emulsions (diameter sizes tunable from ∼ 80 to 160 nm and polydispersion indices below 0.1) by combining this process with high concentrations of polymers. Depending on the polymer concentration, it is possible to control the level of coating that results in a tunable stability ranging from a few weeks to several months. The above range of concentrations has been investigated using a fluorescence-based approach with new insights into the coating evolution.

Optimizing design and fabrication of microfluidic devices for cell cultures: An effective approach to control cell microenvironment in three dimensions

The effects of gradients of bioactive molecules on the cell microenvironment are crucial in several biological processes, such as chemotaxis, angiogenesis, and tumor progression. The elucidation of the basic mechanisms regulating cell responses to gradients requires a tight control of the spatio-temporal features of such gradients. Microfluidics integrating 3D gels are useful tools to fulfill this requirement. However, even tiny flaws in the design or in the fabrication process may severely impair microenvironmental control, thus leading to inconsistent results. Here, we report a sequence of actions aimed at the design and fabrication of a reliable and robust microfluidic device integrated with collagen gel for cell culturing in 3D, subjected to a predetermined gradient of biomolecular signals. In particular, we developed a simple and effective solution to the frequently occurring technical problems of gas bubble formation and 3D matrix collapsing or detaching from the walls. The device here proposed, in Polydimethylsiloxane, was designed to improve the stability of the cell-laden hydrogel, where bubble deprived conditioning media flow laterally to the gel. We report the correct procedure to fill the device with the cell populated gel avoiding the entrapment of gas bubbles, yet maintaining cell viability. Numerical simulations and experiments with fluorescent probes demonstrated the establishment and stability of a concentration gradient across the gel. Finally, chemotaxis experiments of human Mesenchymal Stem Cells under the effects of Bone Morphogenetic Protein-2 gradients were performed in order to demonstrate the efficacy of the system in controlling cell microenvironment. The proposed procedure is sufficiently versatile and simple to be used also for different device geometries or experimental setups.

Particle tracking by full-field complex wavefront subtraction in digital holography microscopy

The 3D tracking of micro-objects, based on digital holography, is proposed through the analysis of the complex wavefront of the light scattered by the micro-samples. Exploiting the advantages of the off-axis full-field holographic interferometry, the tracking of multiple objects is achieved by a direct wavefront analysis at the focal plane overcoming the limitation of the conventional back focal plane interferometry in which only one object at a time can be tracked. Furthermore, the method proposed and demonstrated here is a step forward with respect to other holographic tracking tools. The approach is tested in two experiments, the first investigates the Brownian motion of particles trapped by holographic optical tweezers, while the second relates to the cell motility in a 3D collagen matrix, thus showing its usefulness for lab-on-chip systems in typical bioassay testing.

Digital holography as a method for 3D imaging and estimating the biovolume of motile cells

Sperm morphology is regarded as a significant prognostic factor for fertilization, as abnormal sperm structure is one of the most common factors in male infertility. Furthermore, obtaining accurate morphological information is an important issue with strong implications in zoo-technical industries, for example to perform sorting of species X from species Y. A challenging step forward would be the availability of a fast, high-throughput and label-free system for the measurement of physical parameters and visualization of the 3D shape of such biological specimens. Here we show a quantitative imaging approach to estimate simply and quickly the biovolume of sperm cells, combining the optical tweezers technique with digital holography, in a single and integrated set-up for a biotechnology assay process on the lab-on-a-chip scale. This approach can open the way for fast and high-throughput analysis in label-free microfluidic based "cytofluorimeters" and prognostic examination based on sperm morphology, thus allowing advancements in reproductive science.

Fluorescent (rhodamine), folate decorated and doxorubicin charged, PEGylated nanoparticles synthesis

PEGylated silica nanoparticles, giving very stable aqueous sols, were successfully functionalised with rhodamine, one of the more stable fluorophore; they were also decorated with the targeting agent folic acid (FA) and charged with the well known drug doxorubicin. Rhodamine functionalization required a modification of the synthesis route of the nanoparticles (NP). Functionalization with FA required activation with 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride. Folate decorated NP were easily charged with doxorubicin. The experimental results proved the successfulness of the functionalization. The bond to the NP does not reduce the therapeutic efficacy of the drug. The calculated encapsulation efficiency (32 %) was only a little lower than the value (47 %) reported for the very popular PEGylated PLGA NP.

Hyaluronic-acid-based semi-interpenetrating materials

In order to enhance the mechanical performances of hyaluronic acid (HA) without compromising its biological activity, HA has been interpenetrating with a fibrillar collagen scaffold. The semi-interpenetrating materials were obtained by mixing HA with different molecular weight and a pepsin-solubilized collagen (atelocollagen) solution, and then by inducing collagen fibrillogenesis. Results indicate that molecular weight of HA significantly influences the mechanical properties of the semi-interpenetrating materials and more specifically stronger material results from the use of low-molecular-weight (LMW) HA. According to the dynamic mechanical data the composite collagen-LMW HA has a higher elastic modulus than collagen, whereas the opposite is true for the high-molecular-weight (HMW) HA. This result highlights the role of specific interactions that occur between collagen and HA during the gel formation in controlling the network mechanical stability. LMW HA may, probably, interact more strongly with collagen during the fibrillogenesis process than HMW HA due to the higher mobility of the chains and the weaker homologous interactions. Moreover, morphological observations showed that LMW HA is intimately interdispersed within the collagen network and completely coated the fibrils, which act as mechanical support.

Solvent and melting induced microspheres sintering techniques: a comparative study of morphology and mechanical properties

In this work we propose a bottom up approach founded on the assembly of building blocks by solvent induced microparticle sintering to realize multifunctional polymer scaffolds with predefined pore dimension and fully percolative pathway, able to include interspersing microdepot for the release of bioactive molecules. The aim of this study was to develop a versatile method of microspheres sintering based on the partial dissolution of the surface of adjacent microparticles and to compare it with melting induced microspheres sintering, just developed in a previous work. The two techniques were compared in terms of morphology, porosity and mechanical properties. The high potential of customizing the sintering process by the proper selection of the sintering techniques as well as microparticles with different features (e.g., material, size, shape, inner porosity) allows obtaining a wide pattern of micro/nanostructures with bio-inspired mechanical response so satisfying all basic requirements of a "smart" scaffold for bone tissue engineering.

Tuning the microstructure and biodegradation of three-phase scaffolds for bone regeneration made of PCL, Zein, and HA

The aim of this study has been the design of novel multi-phase porous scaffolds with bi-modal pore size distributions and controlled biodegradation rate for bone tissue engineering (bTE), via a gas foaming—leaching approach. Poly( ε-caprolactone) (PCL) has been melt mixed with thermoplastic zein (TZ) and hydroxyapatite particle, to prepare multi-phase PCL—TZ and PCL—TZ—HA composites suitable to be further processed for the fabrication of 3D porous scaffolds. To this aim, these systems have been gas foamed by using CO2 as blowing agent and, subsequently, soaked in H2O to leach out the plasticizer from the TZ. This combined process allows the formation of an interpenetrated micro- and macro-porosity network within the samples. The effect of the different formulations on the micro-structural properties and in vitro biodegradation of the scaffolds has been investigated, and the results correlated to the mechanisms involved in the formation of the bi-modal pore structure. Results demonstrated that the multi-phase nature of the biomaterials prepared as well as their composition significantly affect the micro-structural properties and biodegradation rate of the scaffolds. The optimal selection of the processing conditions may allow for the design of multi-phase 3D porous scaffolds suitable for bTE.

Covalently immobilized RGD gradient on PEG hydrogel scaffold influences cell migration parameters

Understanding the influence of a controlled spatial distribution of biological cues on cell activities can be useful to design "cell instructive" materials, able to control and guide the formation of engineered tissues in vivo and in vitro. To this purpose, biochemical and mechanical properties of the resulting biomaterial must be carefully designed and controlled. In this work, the effect of covalently immobilized RGD peptide gradients on poly(ethylene glycol) diacrylate hydrogels on cell behaviour was studied. We set up a mechanical device generating gradients based on a fluidic chamber. Cell response to RGD gradients with different slope (0.7, 1 and 2 mM cm(-1)) was qualitatively and quantitatively assessed by evaluating cell adhesion and, in particular, cell migration, compared to cells seeded on hydrogels with uniform distribution of RGD peptides. To evaluate the influence of RGD gradient and to exclude any concentration effect on cell response, all analyses were carried out in a specific region of the gradients which displayed the same average concentration of RGD (1.5 mM). Results suggest that cells recognize the RGD gradient and adhere onto it assuming a stretched shape. Moreover, cells tend to migrate in the direction of the gradient, as their speed is higher than that of cells migrating on hydrogels with a uniform distribution of RGD and increases by increasing RGD gradient steepness. This increment is due to an augmentation of bias speed component of the mean squared speed, that is, the drift of the cell population migrating on the anisotropic surface provided by the RGD gradient.

Design of porous polymeric scaffolds by gas foaming of heterogeneous blends

One of the challenges in tissue engineering scaffold design is the realization of structures with a pre-defined multi-scaled porous network. Along this line, this study aimed at the design of porous scaffolds with controlled porosity and pore size distribution from blends of poly(epsilon-caprolactone) (PCL) and thermoplastic gelatin (TG), a thermoplastic natural material obtained by de novo thermoplasticization of gelatin. PCL/TG blends with composition in the range from 40/60 to 60/40 (w/w) were prepared by melt mixing process. The multi-phase microstructures of these blends were analyzed by scanning electron microscopy and dynamic mechanical analysis. Furthermore, in order to prepare open porous scaffolds for cell culture and tissue replacement, the TG and PCL were selectively extracted from the blends by the appropriate combination of solvent and extraction parameters. Finally, with the proposed combination of gas foaming and selective polymer extraction technologies, PCL and TG porous materials with multi-scaled and highly interconnected porosities were designed as novel scaffolds for new-tissue regeneration.

Toxicological properties of nanoparticles of organic compounds (NOC) from flames and vehicle exhausts

We examined the biological reactivity in vitro of nanoparticles of organic compounds (NOC) with diameters, d = 1-3 nm, a class of combustion-generated particulate relatively unstudied compared to larger more graphitic soot particles because of their small size even though they may contribute significantly to the organic fraction of PM sampled from vehicle exhausts and urban atmospheres. We tested NOC samples collected from 2004 model vehicle emissions and laboratory flames. NOC produced a dose dependent mutagenic response in Salmonella bacteria, suggesting that NOC may add significantly to the overall mutagenicity of vehicle emissions. Incubation with peptides caused agglomeration and precipitate of the otherwise stable NOC suspension, but the chemical and/or physical nature of the NOC-peptide interactions could not be resolved. A significant cytotoxic response was measured above a critical dose of NOC in mouse embryo fibroblasts NIH3T3 cells along with possible evidence of cellular uptake by optical and confocal microscopy. The toxicological assays showed that NOC collected from flames and vehicle exhausts effectively interacted in vitro with both prokaryotic and eukaryotic cells. Differences in mutagenic potencies observed for various Salmonella strains with and without metabolic activation indicate differences in the chemical composition of NOC collected from different vehicles and flames.

Gene delivery systems for gene therapy in tissue engineering and central nervous system applications

The present review aims to describe the potential applications of gene delivery systems to tissue engineering and central nervous system diseases. Some key experimental work has been done with interesting results, but the subject is far from being fully explored. The combined approach of gene therapy and material science has a huge potential to improve the therapeutic approaches now available for a wide range of medical applications. Focus is given to this multidisciplinary strategy in neurodegenerative pathologies, where the use of polymeric matrices as gene carriers might make a crucial difference.

The performance of poly-epsilon-caprolactone scaffolds in a rabbit femur model with and without autologous stromal cells and BMP4

The ability of a cellular construct to guide and promote tissue repair strongly relies on three components, namely, cell, scaffold and growth factors. We aimed to investigate the osteopromotive properties of cellular constructs composed of poly-epsilon-caprolactone (PCL) and rabbit bone marrow stromal cells (BMSCs), or BMSCs engineered to express bone morphogenetic protein 4 (BMP4). Highly porous biodegradable PCL scaffolds were obtained via phase inversion/salt leaching technique. BMSCs and transfected BMSCs were seeded within the scaffolds by using an alternate flow perfusion system and implanted into non-critical size defects in New Zealand rabbit femurs. In vivo biocompatibility, osteogenic and angiogenic effects induced by the presence of scaffolds were assessed by histology and histomorphometry of the femurs, retrieved 4 and 8 weeks after surgery. PCL without cells showed scarce bone formation at the scaffold-bone interface (29% bone/implant contact and 62% fibrous tissue/implant contact) and scarce PCL resorption (16%). Conversely, PCL seeded with autologous BMSCs stimulated new tissue formation into the macropores of the implant (20%) and neo-tissue vascularization. Finally, the BMP4-expressing BMSCs strongly favoured osteoinductivity of cellular constructs, as demonstrated by a more extensive bone/scaffold contact.

Effects of fibronectin and laminin on structural, mechanical and transport properties of 3D collageneous network

Recent studies, on cells cultured in 3D collagen gels, have shown that, beside from their well known biochemical role, fibronectin (FN) and laminin (LM) affect cell functions via a modification of mechanical and structural properties of matrix due to interaction with collagen molecules. Though biochemical properties of FN and LM have been widely studied, little is known about their role in collagen matrix assembly. The aim of this work was to characterize FN- and LM-based collagen semi-interpenetrating polymer networks (semi-IPNs), in order to understand how these biomacromolecular species can affect collagen network assembly and properties. Morphology, viscoelasticity and diffusivity of collagen gels and FN- and LM-based collagen semi-IPNs were analysed by Confocal Laser Scanning microscopy (CLSM), Environmental Scanning Electron microscopy (ESEM), Transmission Electron microscopy (TEM), Rheometry and Fluorescence Recovery After Photobleaching (FRAP) techniques. It was found that FN and LM were organized in aggregates, interspersed in collagen gel, and in thin fibrils, distributed along collagen fibres. In addition, high FN and LM concentrations affected collagen fibre assembly and structure and induced drastic effects on rheological and transport properties.

Synthesis and characterization of macroporous poly(ethylene glycol)-based hydrogels for tissue engineering application

Peptide activated poly(ethylene glycol) (PEG)-based hydrogels have received wide attention as material for tissue engineering application. However, the close structure of these materials may pose severe barriers to tissue invasion and nutrient transport. The aim of this work was to synthesize highly interconnected macroporous PEG hydrogels, suitable for use as tissue engineering scaffolds, by combining the photocrosslinking reaction with a foaming process. In particular, various porous samples, differing for both the polymer molecular weight and concentration in the starting precursor solution, have been prepared and characterized by means of scanning electron microscopy and mercury porosimetry. Moreover, water swelling properties have been evaluated and compared with those of the conventional nonporous ones, by performing both equilibrium and kinetic swelling measurements in distilled water. Results indicated that foamed hydrogels display a well-interconnected porous network, suitable for tissue invasion and free molecular trafficking within them. Pores dimension as well as swelling rate can be modulated by polymer concentrations and bubbling agent composition in the precursor solution.

Time and Space Evolution of Transport Properties in Agarose–Chondrocyte Constructs

During the development of de novo synthesized cartilage tissue engineered constructs, transport and biophysical properties are expected to change in time and space. Monitoring and control of the evolution of these parameters are of crucial importance to process biohybrid constructs in vitro. The aim of this work was to measure fluid and macromolecular transport and evolution of mechanical properties of tissue-engineered cartilage constructs as a function of culture time and extracellular matrix (ECM) production. It was found, in agreement with other literature reports, that mechanical and fluid transport properties of the constructs correlated well with time of culture and glycosaminoglycan (GAG) content. Further, diffusion coefficients of 2 probes, dextran (500 kDa) and bovine serum albumin (BSA), correlated well with GAG production. Diffusion coefficients (D) were measured with high spatial and temporal resolution by fluorescent recovery after photobleaching (FRAP). Diffusivity steadily decreases with time while it does not vary through the thickness of the specimen. On the basis of these results, an empirical relationship between diffusion coefficient and GAG content was proposed for the 2 probes analyzed. The results of this study provide useful information to optimize and control the tissue culture process in vitro.

The effective dispersion of nanovectors within the tumor microvasculature

The effective longitudinal diffusion of nanovectors along non-permeable and permeable capillaries has been studied considering the contribution of molecular and convective diffusion based on the Taylor's theory of shear dispersion. The problem is of importance in the transport of nanovectors used for the intravascular delivery of drugs and contrast agents. It has been shown that for a given capillary size and hemodynamic conditions a critical radius acr exists for which the effective longitudinal diffusion along the capillary has a minimum: Nanovectors with a < acr diffuse mainly by Brownian diffusion whereas nanovectors with a < acr diffuse mainly by convection and the effective diffusion coefficient grows with a. In permeable conduits, the effective diffusion reduces significantly compared to normal non-leaky vessels and it has been derived that acr grows almost linearly with the hydraulic permeability Lp of blood vessels. It has been shown that the blood conduits with the largest effective longitudinal diffusivity could be preferentially targeted by the circulating vectors. Based on these findings, the following strategies are proposed to increase the number of nanovectors targeting the tumor vessels: (i) The use of nanovectors with a critical radius for normal vessels, (ii) the injecting of bolus of nanovectors with different radii, and (iii) the normalization of the tumor vasculature. Finally, it has been emphasized that the size of the vector should be selected depending on the body district where the tumoral mass is developing and on the type, malignancy, and state of the tumor.

Poly-epsilon-caprolactone/hydroxyapatite composites for bone regeneration: in vitro characterization and human osteoblast response

Polycaprolactone (PCL), a semicrystalline linear resorbable aliphatic polyester, is a good candidate as a scaffold for bone tissue engineering, due to its biocompatibility and biodegradability. However, the poor mechanical properties of PCL impair its use as scaffold for hard tissue regeneration, unless mechanical reinforcement is provided. To enhance mechanical properties and promote osteoconductivity, hydroxyapatite (HA) particles were added to the PCL matrix: three PCL-based composites with different volume ratio of HA (13%, 20%, and 32%) were studied. Mechanical properties and structure were analysed, along with biocompatibility and osteoconductivity. The addition of HA particles (in particular in the range of 20% and 32%) led to a significant improvement in mechanical performance (e.g., elastic modulus) of scaffold. Saos-2 cells and osteoblasts from human trabecular bone (hOB) retrieved during total hip replacement surgery were seeded onto 3D PCL samples for 1-4 weeks. Following the assessment of cell viability, proliferation, morphology, and ALP release, HA-loaded PCL was found to improve osteoconduction compared to the PCL alone. The results indicated that PCL represents a potential candidate as an efficient substrate for bone substitution through an accurate balance between structural/ mechanical properties of polymer and biological activities.

Basic structural parameters for the design of composite structures as ligament augmentation devices

Composite structures are designed to mimic the morphology and mechanical properties of natural ligaments. Filament winding technology has been implemented in order to obtain a composite material based on a polyurethane matrix (HydroThaneTM ), reinforced with degradable and non-degradable fibers. The mechanical properties of the matrix and fiber have been analysed to define the optimal type, volume ratio and winding angle of the reinforcement. The typical J-shaped stress-strain curve, displayed by natural tendons and ligaments, is reproduced. The mechanical behaviour of HydroThaneTM reinforced with poly(ethylene terephthalate) (PET) fibers were modified by varying the winding angle of the fibers. Fibers comprising poly(l-lactic acid) (PLLA), poly(glycolic acid) (PGA) and PET, individually and in combination, were considered as candidate materials for the reinforcement of a composite ligament augmentation device (LAD). Mechanical and degradation studies demonstrated that, by combining different types of fiber, at a fixed volume fraction and winding angle (20 degrees ), it is possible to optimize mechanical properties and degradation kinetics of the device.

Cellulose Derivative−Hyaluronic Acid-Based Microporous Hydrogels Cross-Linked through Divinyl Sulfone (DVS) To Modulate Equilibrium Sorption Capacity and Network Stability

The aim of this work is to obtain a chemically cross-linked hydrogel from hyaluronic acid and cellulose derivatives that exhibits sensitivity to variation of the composition of the external absorbing medium and an equilibrium sorption capacity higher than a common hyaluronic acid-based hydrogel, in view of its potential use in prevention of postsurgical soft tissue adhesion. This has been achieved by chemical stabilization of hyaluronic acid (HA) and cellulose derivatives, hydroxyethylcellulose (HEC) and carboxymethylcellulose (CMCNa) through the difunctional cross-linker divinyl sulfone. Significant increase in sorption capacity, both in water and in water solutions at different ionic strength, has been observed for these samples in comparison with hydrogels obtained through chemical stabilization of hyaluronic acid. Moreover, different dehydration procedures adopted for the xerogel synthesis have been used, which resulted in a modulation of the equilibrium sorption capacity. Hyaluronic acid stability has been confirmed by means of NMR analysis.

Spatial and structural dependence of mechanical properties of porcine intervertebral disc

Structure-function relationship of natural tissues is crucial to design a device mimicking the structures present in human body. For this purpose, to provide guidelines to design an intervertebral disc (IVD) substitute, in this study the influence of the spatial location and structural components on the mechanical properties of porcine IVD was investigated. Local compressive stiffness (LCS) was measured on the overall disc, also constrained between the two adjacent vertebrae: the dependence on the lumbar position was evaluated. The compliance values in the anterior position (A) were higher than both in the central posterior (CP) and in the lateral-posterior (RP, LP) locations. The values of Young's Modulus (74.67+/-6.03 MPa) and compression break load (1.36x10(4)+/-0.09x10(4)N) of the disc were also evaluated by distributed compression test. The NP rheological behavior was typical of weak-gels, with elastic modulus G' always higher than viscous modulus G" all over the frequency range investigated (G' and G" respectively equal to 320 and 85 Pa at 1 Hz) and with the moduli trends were almost parallel to each other.

Chitosan-based hydrogels: synthesis and characterization

Chitosan (CHI) is a polysaccharide of beta-1,4-linked 2-amino-2-deoxy-D-glucopyranose derived by N-deacetylation of chitin in aqueous alkaline medium. The shells of crustaceans such as crabs, shrimp, and lobster are the current source of chitosan. It is known to be non-toxic, odourless, biocompatible in animal tissues and enzymatically biodegradable. For these reasons much research interest has been paid to its biomedical, ecological, and industrial applications over the past decade. However, its rigid crystalline structure, poor solubility in organic solvents and poor processability have limited its use. To broadening its range of applications, a growth research effort has been devoted to explore ways of modifying Chitosan. Here it has been reported on the synthesis of new hydrogels, obtained by self-curing chitosan with acrylic acid (AA) and methyl acrylate (MA). The hydrogels were characterized by FTIR, swelling and rheological analysis. The results of this study showed that the swelling and mechanical properties of chitosan are highly improved by the presence of poly acrylate. The swelling degree of these materials does not depend upon the ratio MA/AA. It is possible to improve and modulate the mechanical properties of the hydrogels by changing the relative MA/AA ratio.

Role of extracellular matrix assembly in interstitial transport in solid tumors

The extracellular matrix (ECM) may contribute to the drug resistance of a solid tumor by preventing the penetration of therapeutic agents. We measured differences in interstitial resistance to macromolecule (IgG) motion in four tumor types and found an unexpected correspondence between transport resistance and the mechanical stiffness. The interstitial diffusion coefficient of IgG was measured in situ by fluorescence redistribution after photobleaching. Tissue elastic modulus and hydraulic conductivity were measured by confined compression of excised tissue. In apparent contradiction to an existing paradigm, these functional properties are correlated with total tissue content of collagen, not glycosaminoglycan. An extended collagen network was observed in the more penetration-resistant tumors. Collagenase treatment of the more penetration-resistant tumors significantly increased the IgG interstitial diffusion rate. We conclude that collagen influences the tissue resistance to macromolecule transport, possibly by binding and stabilizing the glycosaminoglycan component of the ECM. These findings suggest a new method to screen tumors for potential resistance to macromolecule-based therapy. Moreover, collagen and collagen-proteoglycan bonds are identified as potential targets of treatment to improve macromolecule delivery.

Mechanics of interstitial-lymphatic fluid transport: theoretical foundation and experimental validation

Interstitial fluid movement is intrinsically linked to lymphatic drainage. However, their relationship is poorly understood, and associated pathologies are mostly untreatable. In this work we test the hypothesis that bulk tissue fluid movement can be evaluated in situ and described by a linear biphasic theory which integrates the regulatory function of the lymphatics with the mechanical stresses of the tissue. To accomplish this, we develop a novel experimental and theoretical model using the skin of the mouse tail. We then use the model to demonstrate how interstitial-lymphatic fluid movement depends on a balance between the elasticity, hydraulic conductivity, and lymphatic conductance as well as to demonstrate how chronic swelling (edema) alters the equipoise between tissue fluid balance parameters. Specifically, tissue fluid equilibrium is perturbed with a continuous interstitial infusion of saline into the tip of the tail. The resulting gradients in tissue stress are measured in terms of interstitial fluid pressure using a servo-null system. These measurements are then fit to the theory to provide in vivo estimates of the tissue hydraulic conductivity, elastic modulus, and overall resistance to lymphatic drainage. Additional experiments are performed on edematous tails to show that although chronic swelling causes an increase in the hydraulic conductivity, its greatly increased distensibility (due to matrix remodeling) dampens the driving forces for fluid movement and leads to fluid stagnation. This model is useful for examining potential treatments for edema and lymphatic disorders as well as substances which may alter tissue fluid balance and/or lymphatic drainage.

Diffusion of macromolecules in agarose gels: comparison of linear and globular configurations

The diffusion coefficients (D) of different types of macromolecules (proteins, dextrans, polymer beads, and DNA) were measured by fluorescence recovery after photobleaching (FRAP) both in solution and in 2% agarose gels to compare transport properties of these macromolecules. Diffusion measurements were conducted with concentrations low enough to avoid macromolecular interactions. For gel measurements, diffusion data were fitted according to different theories: polymer chains and spherical macromolecules were analyzed separately. As chain length increases, diffusion coefficients of DNA show a clear shift from a Rouse-like behavior (DG congruent with N0-0.5) to a reptational behavior (DG congruent with N0-2.0). The pore size, a, of a 2% agarose gel cast in a 0.1 M PBS solution was estimated. Diffusion coefficients of the proteins and the polymer beads were analyzed with the Ogston model and the effective medium model permitting the estimation of an agarose gel fiber radius and hydraulic permeability of the gels. Not only did flexible macromolecules exhibit greater mobility in the gel than did comparable-size rigid spherical particles, they also proved to be a more useful probe of available space between fibers.

Enhancement of fluid filtration across tumor vessels: implication for delivery of macromolecules

Cancer therapies using genes and other macromolecules might realize their full clinical potential if they could be delivered to tumor tissue in optimal quantities. Unfortunately, the compromised circulation within tumors poses a formidable resistance to adequate and uniform penetration of these agents. Previously, we have proposed elevated interstitial fluid pressure (IFP) as a major physiological barrier to delivery of macromolecules. Here we postulate that modulation of tumor microvascular pressure (MVP) and associated changes in IFP would enhance macromolecular delivery into a solid tumor. To test our hypothesis, we altered tumor MVP by either periodic injection or continuous infusion of angiotensin II (AII) and measured the resulting changes in IFP and uptake of macromolecules. We used the nicotinyl hydrazine derivative of human polyclonal IgG (HYNIC-IgG) as a nonspecific macromolecule and CC49 antibody as a specific macromolecule. We found that both chronic and periodic modulation of tumor MVP enhances transvascular fluid filtration, leading to a 40% increase in total uptake of the specific antibody within 4 hr of its administration. Conversely, neither continuous nor periodic infusion of AII induced any increase in uptake of nonspecific antibodies. Strategies to improve delivery of macromolecules and limitations of this approach are identified.

Intratumoral infusion of fluid: estimation of hydraulic conductivity and implications for the delivery of therapeutic agents

We have developed a new technique to measure in vivo tumour tissue fluid transport parameters (hydraulic conductivity and compliance) that influence the systemic and intratumoral delivery of therapeutic agents. An infusion needle approximating a point source was constructed to produce a radially symmetrical fluid source in the centre of human tumours in immunodeficient mice. At constant flow, the pressure gradient generated in the tumour by the infusion of fluid (Evans blue-albumin in saline) was measured as a function of the radial position with micropipettes connected to a servo-null system. To evaluate whether the fluid infused was reabsorbed by blood vessels, infusions were also performed after circulatory arrest. In the colon adenocarcinoma LS174T with a spherically symmetrical distribution of Evans blue-albumin, the median hydraulic conductivity in vivo and after circulatory arrest at a flow rate of 0.1 microl min(-1) was, respectively, 1.7x10(-7) and 2.3x10(-7) cm2 mmHg(-1) s. Compliance estimates were 35 microl mmHg(-1) in vivo, and 100 microl mmHg(-1) after circulatory arrest. In the sarcoma HSTS 26T, hydraulic conductivity and compliance were not calculated because of the asymmetric distribution of the fluid infused. The technique will be helpful in identifying strategies to improve the intratumoral and systemic delivery of gene targeting vectors and other therapeutic agents.

Viscoelastic behavior of composite ligament prostheses

Despite the compelling need for artificial connective tissue replacements for orthopedic applications, to date, there is no material which can adequately reproduce the mechanical behavior of natural tissue with necessary long-term endurance. In this work, we introduce a novel soft composite material as a more suitable candidate for connective tissue replacement. The material proposed is based on a hydrogel-polymer matrix reinforced with poly(ethylene terephthalate) fibers wound helically to mimic the architecture of the collagen fibers in natural tissue. Macroscopic behaviors such as static stress-strain, stress relaxation, and dynamic frequency responses can be modulated with choice of the components and design of the composite structure. In doing so, the mechanical characteristics of natural ligaments can be qualitatively reproduced and sustained over time.

Solid stress inhibits the growth of multicellular tumor spheroids

In normal tissues, the processes of growth, remodeling, and morphogenesis are tightly regulated by the stress field; conversely, stress may be generated by these processes. We demonstrate that solid stress inhibits tumor growth in vitro, regardless of host species, tissue of origin, or differentiation state. The inhibiting stress for multicellular tumor spheroid growth in agarose matrices was 45 to 120 mm Hg. This stress, which greatly exceeds blood pressure in tumor vessels, is sufficient to induce the collapse of vascular or lymphatic vessels in tumors in vivo and can explain impaired blood flow, poor lymphatic drainage, and suboptimal drug delivery previously reported in solid tumors. The stress-induced growth inhibition of plateau-phase spheroids was accompanied, at the cellular level, by decreased apoptosis with no significant changes in proliferation. A concomitant increase in the cellular packing density was observed, which may prevent cells from undergoing apoptosis via a cell-volume or cell-shape transduction mechanism. These results suggest that solid stress controls tumor growth at both the macroscopic and cellular levels, and thus influences tumor progression and delivery of therapeutic agents.

Transmural coupling of fluid flow in microcirculatory network and interstitium in tumors

The growth of tumors and their response to treatment are determined by delivery of diffusible substances to cancer cells and hence by their blood supply. Relative to most normal tissues, tumor blood flow is highly heterogeneous. Several hypotheses have been postulated to explain this anomalous behavior of tumor microcirculation, but the underlying mechanisms for these heterogeneities are not fully understood. In this study we consider a potential source of nonuniformity in the blood flow: the enhanced fluid exchange between the vascular and interstitial space mediated by the high leakiness of tumor vessels which could lead to a coupling between vascular, transvascular, and interstitial fluid flow. A simple network model is presented to describe the basic features of flow through a network of permeable and compliant vessels embedded in an isotropic porous medium. Two vascular geometries are considered: a regular mesh of identical vessels and a pair of countercurrent vessels of equal diameter. In each case, the flow through each vessel of the network is described by Poiseuille's law; the transmural flow between the vessels and the external porous medium is governed by Starling's law; the fluid movement through the porous medium is described by Darcy's law; and the vessel wall is assumed to be elastic. Our results show that the behavior of microcirculation may be strongly modified as a result of vascular compliance and enhanced vascular leakiness of tumor vessels. We found not only that the vascular pressure generates the well-known, high central interstitial fluid pressure, but also that the elevated interstitial pressure in turn alters the vascular pressure distribution. These changes in vascular pressure distribution result in a modification of the blood flow pattern. As the leakiness and compliance of the vessels increase, the blood is diverted away from the center of the tumor to a more peripheral path. The clinical significance of these results is that drug delivery for chemotherapy and oxygenation needed for radiotherapy may well be hampered in the central region of the tumor, despite the presence of highly permeable vessels in these regions.

Compatibility and the genesis of residual stress by volumetric growth

The equations of compatibility which are pertinant for growth strain fields are collected and examples are given in simply-connected and multiply-connected regions. Compatibility conditions for infinitesimal strains are well known and the possibilities of Volterra dislocations in multiply-connected regions are enumerated. For finite growth strains in a multiply-connected regions, each case must be examined individually and no generalizations in terms of Volterra dislocations are available. Any incompatible growth strains give rise to residual stresses which are known to occur in many tissues such as the heart, arterial wall, and solid tumors.

Effect of transvascular fluid exchange on pressure-flow relationship in tumors: a proposed mechanism for tumor blood flow heterogeneity

Tumor blood flow (TBF) is characterized by spatial and temporal heterogeneities. Despite the crucial role of TBF in tumor growth, metastasis, and therapy, the mechanisms underlying these heterogeneities are not fully understood. Tumor vessels are, in general, more leaky than normal vessels and this may enhance the efficiency of fluid exchange between the vascular and the interstitial space. The coupling between transvascular fluid exchange and hemodynamics in tumors has not been explored previously. To investigate the role of transvascular fluid exchange on afferent and efferent blood flow, we modeled the tumor vasculature as an equivalent single vessel which is permeable and deformable and embedded in a fluid medium with uniform pressure. Simulations were carried out to examine the effects of vessel leakiness, vessel compliance, and interstitial fluid pressure on (a) pressure-flow relationship, (b) arterial-venous pressure relationship, and (c) pressure profile along the vessel. Experiments suggested by model simulations required an independent control of arterial and venous pressure and tumor blood flow. To this end, we perfused tissue-isolated tumors ex vivo and obtained data on perfusate flow rate vs arterial and venous pressures. The simulations predicted the following trends as a result of an enhanced fluid filtration across the vessel wall: (a) for a fixed arterial-venous pressure difference, efferent flow decreases with increasing venous pressure, (b) changes in venous pressure are not completely transmitted to the arterial side, and (c) the pressure profile along the vessel becomes less steep. The experimental results confirmed these trends and indicated that vascular and interstitial flow are coupled in isolated tumors. The implications of this coupling for the spatial and temporal heterogeneity in TBF are discussed.

Time-dependent behavior of interstitial fluid pressure in solid tumors: implications for drug delivery

Elevated interstitial fluid pressure (IFP) may constitute a significant physiological barrier to drug delivery in solid tumors. Strategies for overcoming this barrier have not been developed to date. To identify and characterize various mechanisms regulating IFP and to develop strategies for overcoming the IFP barrier, we modeled the tumor as a poroelastic solid. We used this model to simulate the effect of changes in microvascular pressure and tumor blood flow (TBF) on IFP. To test model predictions, the effects of changes in arterial pressure and TBF on IFP were measured using a tissue-isolated tumor preparation. IFP in the center of an isolated tumor was predicted to follow variation of the arterial pressure with a time delay of the order of magnitude of 10 s, and this delay was found to be 11 +/- 6 s experimentally. Following a cessation of TBF, the time constant of the drop in IFP was predicted to be of the order of 1000 s and was found to be 1500 +/- 900 s experimentally. The former time scale is characteristic of transcapillary fluid exchange, and the latter of percolation of fluid through the interstitial matrix. Relying on the good agreement between theoretical predictions and experimental data, we estimated the effect of blood pressure modulation on macromolecular uptake in solid tumors. Our results show that no appreciable increase of macromolecular uptake should occur either by an acute or by a chronic increase of blood pressure. On the other hand, higher uptake would result from periodic modulation of blood pressure. Therefore, the effectiveness of a vasoconstrictor such as angiotensin II to increase macromolecular delivery should be significantly enhanced by periodic rather than bolus or continuous administration of the vasoactive agent.

Hydrogels as an interface between bone and an implant

The use of fully hydrated hydrogels in the body has been well established. The forces a hydrogel generates on swelling when it is placed in a constrained space were investigated with a view to providing a mechanism for fixing a prosthesis in the intramedullary cavity. A cross-linked poly(2-hydroxyethyl methacrylate) [p(HEMA)] hydrogel was investigated as a potential material. In vitro mechanical tests were carried out to determine the stresses generated in the p(HEMA) when it was placed in water and not allowed to swell. Pull out loads of up to 375 N indicated that the system could be used successfully in vivo. Consequently, the material was placed intraosseously at two sites in a rabbit animal model, in the mid-shaft (diaphysis) and the lower end (metaphysis) of the femur. Histological examination showed there was no adverse bone response; bone was growing from the endosteal surface up to and into the hydrogel in the diaphyseal implants and surrounded the hydrogel in the metaphysis. As a result of the shape and size variations in the rabbit femur, in vivo mechanical tests were found to give lower values than those obtained in vitro.