Particle diameter influences adhesion under flow

Mass transport and shear stress in a microchannel bioreactor: numerical simulation and dynamic similarity

Microchannel bioreactors have been used in many studies to manipulate and investigate the fluid microenvironment around cells. In this study, substrate concentrations and shear stresses at the base were computed from a three-dimensional numerical flow-model incorporating mass transport. Combined dimensionless parameters were developed from a simplified analysis. The numerical results of substrate concentration were well correlated by the combined parameters. The generalized results may find applications in design analysis of microchannel bioreactors. The mass transport and shear stress were related in a generalized result. Based on the generalized results and the condition of dynamic similarity, various means to isolate their respective effects on cells were considered.

The mechanism of the self-assembly of associating DNA molecules under shear flow: Brownian dynamics simulation

A shear flow induces the assembly of DNAs with the sticky spots. In order to strictly interpret the mechanism of shear-induced DNA assembly, Brownian dynamics simulations with the bead-spring model were carried out for these molecules at various ranges of the Weissenberg numbers (We). We calculate a formation time and analyze the radial distribution function of end beads and the probability distribution of fractional extension at the formation time to understand the mechanism of shear-induced assembly. At low Weissenberg number the formation time, which is defined as an elapsed time until a multimer forms for the first time, decreases rapidly, reaching a plateau at We = 1000. A shear flow changes the radial distribution of end beads, which is almost the same regardless of the Weissenberg number. A shear flow deforms and stretches the molecules and generates different distributions between end beads with a stickly spot. The fractional extension progresses rapidly in shear flow from a Gaussian-like distribution to a uniform distribution. The progress of the distribution of fractional extension increases the possibility of meeting of end beads. In shear flow, the inducement of the assembly mainly results from the progress of the probability distribution of fractional extension. We also calculate properties such as the radius of gyration, stretch, and so on. As the Weissenberg number increases, the radius of gyration at the formation time also increases rapidly, reaching a plateau at We = 1000.

PSGL-1 derived from human neutrophils is a high-efficiency ligand for endothelium-expressed E-selectin under flow

P-selectin glycoprotein ligand-1 (PSGL-1) has been proposed as an important tethering ligand for E-selectin and is expressed at a modest level on human leukocytes. Sialyl Lewis x (sLe(x))-like glycans bind to E-selectin and are expressed at a relatively high level on circulating leukocytes. It is unclear whether PSGL-1 has unique biochemical attributes that contribute to its role as an E-selectin ligand. To probe this issue, we conjugated microspheres with either sLe(x) or PSGL-1 purified from myeloid cells (neutrophils and HL-60) and compared their adhesion to endothelial expressed E-selectin under defined shear conditions. We found that both sLe(x) and PSGL-1 microspheres adhere to 4 h of IL-1beta-activated human umbilical vein endothelial cells predominantly through E-selectin. Analysis of the adhesion revealed that the rate of initial tethering of the PSGL-1 microspheres to E-selectin was significantly greater than the rate of initial tethering of the sLe(x) microspheres despite the fact that the sLe(x) microspheres tested had higher ligand densities than the PSGL-1 microspheres. We also found that pretreatment of the PSGL-1 or sLe(x) microspheres with HECA-452 had no significant effect on initial tethering to E-selectin. These results support the hypotheses that 1) PSGL-1 is a high-efficiency tethering ligand for E-selectin, 2) ligand biochemistry can significantly influence initial tethering to E-selectin, and 3) PSGL-1 tethering to E-selectin can occur via non-HECA-452 reactive epitopes.

Radiation-force assisted targeting facilitates ultrasonic molecular imaging

Ultrasonic molecular imaging employs contrast agents, such as microbubbles, nanoparticles, or liposomes, coated with ligands specific for receptors expressed on cells at sites of angiogenesis, inflammation, or thrombus. Concentration of these highly echogenic contrast agents at a target site enhances the ultrasound signal received from that site, promoting ultrasonic detection and analysis of disease states. In this article, we show that acoustic radiation force can be used to displace targeted contrast agents to a vessel wall, greatly increasing the number of agents binding to available surface receptors. We provide a theoretical evaluation of the magnitude of acoustic radiation force and show that it is possible to displace micron-sized agents physiologically relevant distances. Following this, we show in a series of experiments that acoustic radiation force can enhance the binding of targeted agents: The number of biotinylated microbubbles adherent to a synthetic vessel coated with avidin increases as much as 20-fold when acoustic radiation force is applied; the adhesion of contrast agents targeted to alpha(v)beta3 expressed on human umbilical vein endothelial cells increases 27-fold within a mimetic vessel when radiation force is applied; and finally, the image signal-to-noise ratio in a phantom vessel increases up to 25 dB using a combination of radiation force and a targeted contrast agent, over use of a targeted contrast agent alone.

Characterization of biodegradable drug delivery vehicles with the adhesive properties of leukocytes II: effect of degradation on targeting activity

The site-specific expression of selectins (P- and E-selectin) on endothelial cells of blood vessels during inflammation provides an opportunity for the targeted delivery of anti-inflammatory drugs to sites of chronic inflammation. It is well documented that the selectins mediate the initial interaction (rolling) of leukocytes in an inflamed vessel by binding to carbohydrate-presenting counter-receptors displayed on leukocytes. Previous work in our laboratory has shown that artificial capsules with the adhesive properties of leukocytes can be made by attaching leukocyte adhesive ligands to polymer microspheres (Biomaterials 23(10) (2002) 2167). Specifically, we showed that drug-loaded poly (lactic-co-glycolic-acid) (PLGA) microspheres coated with biotinylated-Sialyl LewisX (sLeX), a carbohydrate that serves as a ligand to selectins, mimic the adhesive behavior of leukocytes on selectins in flow chambers, displaying slow rolling under flow, suggesting that these drug-loaded particles can potentially target inflammatory sites in vivo. Since the effectiveness of this delivery system might depend on the degradation of polymer microspheres as well as the degradation of sLeX molecules, we measured the effect of polymer and ligand degradation on the adhesiveness of microspheres over time. We show that degrading sLeX microspheres maintain the ability to recognize selectin surfaces under flow for at least 2 weeks and that the ability to sustain recognition depends upon the extent at which microspheres are loaded. We also show that microsphere rolling velocity increases as microsphere degrade and that this increase is due to a combination of increase in average microsphere size and loss of sLeX molecules on microsphere surface--a result of microsphere degradation confirmed by flow cytometry. Control experiments show that microsphere, not sLeX, degradation limits the lifetime of our targeted delivery system; therefore, factors affecting degradation such as type of polymer, type of drug, extent of drug loading and microsphere size, provide an opportunity for engineering the time-scale of activity for the delivery system.

Nanoparticle uptake by the oral route: Fulfilling its potential?

The absorption of nanoparticles from the gastrointestinal tract is discussed first from a recent historical perspective, emphasising the crucial parameters that influence uptake, such as particle diameter, the nature of the particle and any surface modification with ligands designed to interact with receptors to facilitate targeting to and uptake into cells. Post-absorptive events, including translocation processes, are suggested here to be as important as initial uptake into the epithelial cells or M-cells of the gut associated lymphoid tissue.:

Hyaluronan microspheres for sustained gene delivery and site-specific targeting

Hyaluronan is a naturally occurring polymer that has enjoyed wide successes in biomedical and cosmetic applications as coatings, matrices, and hydrogels. For controlled delivery applications, formulating native hyaluronan into microspheres could be advantageous but has been difficult to process unless organic solvents are used or hyaluronan has been modified by etherification. Therefore, we present a novel method of preparing hyaluronan microspheres using adipic dihydrazide mediated crosslinking chemistry. To evaluate their potential for medical applications, hyaluronan microspheres are incorporated with DNA for gene delivery or conjugated with an antigen for cell-specific targeting. The results show that our method, originally developed for preparing hyaluronan hydrogels, generates robust microspheres with a size distribution of 5-20mum. The release of the encapsulated plasmid DNA can be sustained for months and is capable of transfection in vitro and in vivo. Hyaluronan microspheres, conjugated with monoclonal antibodies to E- and P-selectin, demonstrate selective binding to cells expressing these receptors. In conclusion, we have developed a novel microsphere preparation using native hyaluronan that delivers DNA at a controlled rate and adaptable for site-specific targeting.

Radiation-Force Assisted Targeting Facilitates Ultrasonic Molecular Imaging

Ultrasonic molecular imaging employs contrast agents, such as microbubbles, nanoparticles, or liposomes, coated with ligands specific for receptors expressed on cells at sites of angiogenesis, inflammation, or thrombus. Concentration of these highly echogenic contrast agents at a target site enhances the ultrasound signal received from that site, promoting ultrasonic detection and analysis of disease states. In this article, we show that acoustic radiation force can be used to displace targeted contrast agents to a vessel wall, greatly increasing the number of agents binding to available surface receptors. We provide a theoretical evaluation of the magnitude of acoustic radiation force and show that it is possible to displace micron-sized agents physiologically relevant distances. Following this, we show in a series of experiments that acoustic radiation force can enhance the binding of targeted agents: The number of biotinylated microbubbles adherent to a synthetic vessel coated with avidin increases as much as 20-fold when acoustic radiation force is applied; the adhesion of contrast agents targeted to alpha(v)beta3 expressed on human umbilical vein endothelial cells increases 27-fold within a mimetic vessel when radiation force is applied; and finally, the image signal-to-noise ratio in a phantom vessel increases up to 25 dB using a combination of radiation force and a targeted contrast agent, over use of a targeted contrast agent alone.

Computational modeling of cell adhesion and movement using a continuum-kinetics approach

Adhesion of leukocytes to substrate involves the coupling of disparate length and timescales between molecular mechanics and macroscopic transport, and existing models of cell adhesion do not use full cellular information. To address these challenges, a multiscale computational approach for studying the adhesion of a cell on a substrate is developed and assessed. The cellular level model consists of a continuum representation of the field equations and a moving boundary tracking capability to allow the cell to change its shape continuously. At the receptor-ligand level, a bond molecule is mechanically represented by a spring. Communication between the macro/micro- and nanoscale models is facilitated interactively during the computation. The computational model is assessed using an adherent cell, rolling and deforming along the vessel wall under imposed shear flows. Using this approach, we first confirm existing numerical and experimental results. In this study, the intracellular viscosity and interfacial tension are found to directly affect the rolling of a cell. Our results also show that the presence of a nucleus increases the bond lifetime, and decreases the cell rolling velocity. Furthermore, it is found that a cell with a larger diameter rolls faster, and decreases the bond lifetime. This study shows that cell rheological properties have significant effects on the adhesion process contrary to what has been hypothesized in most literature.

Leukocyte-inspired biodegradable particles that selectively and avidly adhere to inflamed endothelium in vitro and in vivo

We exploited leukocyte-endothelial cell adhesion chemistry to generate biodegradable particles that exhibit highly selective accumulation on inflamed endothelium in vitro and in vivo. Leukocyte-endothelial cell adhesive particles exhibit up to 15-fold higher adhesion to inflamed endothelium, relative to noninflamed endothelium, under in vitro flow conditions similar to that present in blood vessels, a 6-fold higher adhesion to cytokine inflamed endothelium relative to non-cytokine-treated endothelium in vivo, and a 10-fold enhancement in adhesion to trauma-induced inflamed endothelium in vivo due to the addition of a targeting ligand. The leukocyte-inspired particles have adhesion efficiencies similar to that of leukocytes and were shown to target each of the major inducible endothelial cell adhesion molecules (E-selectin, P-selectin, vascular cell adhesion molecule 1, and intercellular adhesion molecule 1) that are up-regulated at sites of pathological inflammation. The potential for targeted drug delivery to inflamed endothelium has significant implications for the improved treatment of an array of pathologies, including cardiovascular disease, arthritis, inflammatory bowel disease, and cancer.

Leukocyte adhesion: an exquisite balance of hydrodynamic and molecular forces

Leukocyte adhesion to the vascular endothelium involves a disruptive force exerted on the leukocyte by the flow of the blood and an adhesive force that forms at the leukocyte-endothelial interface. The relative strengths of these two competing forces govern leukocyte adhesion.

A proteasome inhibitor reduces concurrent, sequential, and long-term IL-1 beta- and TNF-alpha-induced ECAM expression and adhesion

A promising approach for reducing aberrant leukocyte-endothelial adhesion during pathological inflammation is to inhibit endothelial cell adhesion molecule (ECAM) expression at the transcription level. Several compounds have been shown to decrease cytokine-induced upregulation of ECAMs primarily by modulating the activity of transcription factors [e.g., nuclear factor-kappa B (NF-kappa B)]. The majority of the in vitro studies have focused on the effect of transcription inhibitors on endothelial cells exposed to a single cytokine [primarily tumor necrosis factor-alpha (TNF-alpha)] for a relatively short period of time (primarily 4-6 h). However, in the in vivo setting, multiple cytokines [e.g., interleukin-1 beta (IL-1 beta) and TNF-alpha] may be present for extended periods of time. Thus we studied the effects of a transcription inhibitor, the proteasome inhibitor lactacystin, on ECAM expression and myeloid (HL60) cell adhesion to human umbilical vein endothelial cells (HUVEC) activated by concurrent, sequential, and long-term (24 h) treatment with IL-1 beta and TNF-alpha. We show, for the first time, that lactacystin inhibits 1) 4-h concurrent IL-1 beta- and TNF-alpha-induced expression of E-selectin, VCAM-1, ICAM-1, and HL60 cell adhesion to HUVEC; 2) 4-h TNF-alpha-induced expression of E-selectin, VCAM-1, and HL60 cell adhesion to HUVEC that have become desensitized to IL-1 beta activation; 3) 24-h TNF-alpha-induced expression of E-selectin and VCAM-1 but not ICAM-1; and 4) 24-h TNF-alpha-induced HL60 cell adhesion to HUVEC. Combined, our results demonstrate that a proteasome inhibitor can reduce concurrent, sequential, and long-term IL-1 beta- and TNF-alpha-induced ECAM expression and myeloid cell adhesion.

Low venular shear rates promote leukocyte-dependent recruitment of adherent platelets

The influence of reductions in venular shear rate on platelet-endothelial (P/E) cell adhesion has not been previously addressed. The objectives of this study were to define the effects of reductions in venular shear rate on P/E cell adhesion and to determine the interdependence of P/E cell adhesion and leukocyte-endothelial (L/E) cell adhesion at low shear rates. Intravital videomicroscopy was used to quantify P/E and L/E cell adhesion in rat mesenteric venules exposed to shear rates ranging between 118 +/- 9 and 835 +/- 44 s(-1). Shear rate was altered in postcapillary venules by rapid, graded blood withdrawal, without retransfusion of shed blood. Reducing shear rate from >600 s(-1) to <200 s(-1) resulted in an eightfold increase in L/E cell adhesion, whereas P/E cell adhesion increased 18-fold. A blocking antibody directed against P-selectin blunted both the P/E and L/E cell adhesion elicited by low shear rates. Immunoneutralization of CD11/CD18 on leukocytes or rendering animals neutropenic also blocked the shear rate-dependent recruitment of both platelets and leukocytes. These findings indicate that 1) low shear rates promote P/E and L/E cell adhesion in mesenteric venules, and 2) adherent neutrophils (mediated by CD11/CD18) create a platform onto which platelets can bind to the venular wall at low shear rates.

Thermal induced modification of the contact mechanics of adhering liposomes on cationic substrate

The correlation between the mechanical property and the thermotropic transition of the phospholipid bilayer has been recently demonstrated (Chem. Phys. Lipids 110 (2001) 27). However, the role of thermal induced mechanical responses of phospholipid bilayer on the contact mechanics of liposome adhering on a cationic substrate has not been determined. In this study, confocal-reflectance interference contrast microscopy, phase contrast microscopy and contact mechanics modeling are applied to probe the adhesion mechanisms of liposomes in the presence of electrostatic interactions during the thermotropic transition of the lipid bilayer. When temperature increases from 23 to 49 degrees C at pH 7.4, the degree of liposome deformation (a/R) and adhesion energy of dipalmitoyl-sn-glycero-3-phosphocholine liposome increases by 10% and remains constant, respectively, on 3-amino-propyl-triethoxy-silane (APTES) modified substrate. The extents of increase in these two parameters are highly dependent on the physicochemical properties of the rigid substrate. At pH 4, the adhesion energies above and below the phase transition temperature (T(m)) are increased by one order of magnitude due to the formation of the free silanol groups on APTES substrate. In hypotonic condition, the degree of vesicle deformation remains constant and the adhesion energy reduces by 20% during sample heating. Under all conditions, the adhesion energy of the adhering liposome spans a few orders of magnitude against the increase of liposome size as the surface area to volume ratio is maximized in smallest vesicle.

Estimating the efficiency of cell capture and arrest in flow chambers: study of neutrophil binding via E-selectin and ICAM-1

A mathematical model was developed to quantify the efficiency of cell-substrate attachment in the parallel-plate flow chamber. The model decouples the physical features of the system that affect cell-substrate collision rates from the biological features that influence cellular adhesivity. Thus, experimental data on cell rolling and adhesion density are converted into "frequency" parameters that quantify the "efficiency" with which cells in the flow chamber progress from the free stream to rolling, and transition from rolling to firm arrest. The model was partially validated by comparing simulation results with experiments where neutrophils rolled and adhered onto substrates composed of cotransfected cells bearing E-selectin and intercellular adhesion molecule-1 (ICAM-1). Results suggest that: 1) Neutrophils contact the E-selectin substrate on average for 4-8.5s before tethering. This contact duration is insensitive to applied shear stress. 2) At 2 dyn/cm(2), approximately 28% of the collisions between the cells and substrate result in primary capture. Also, approximately 5-7% of collisions between neutrophils in the free stream and previously recruited neutrophils bound on the substrate result in secondary capture. These percentages were higher at lower shears. 3) An adherent cell may influence the flow streams in its vicinity up to a distance of 2.5 cell diameters away. 4) Our estimates of selectin on-rate in cellular systems compare favorably with data from reconstituted systems with immobilized soluble E-selectin. In magnitude, the observed on-rates occur in the order, L-selectin > P-selectin > E-selectin.

Role of glycocalyx in leukocyte-endothelial cell adhesion

The binding of fluorescently labeled microspheres (FLMs, 0.1-microm diameter) coated with antibody (1a29) to ICAM-1 was studied in postcapillary venules during topical application of the chemoattractant N-formylmethionyl-leucyl-phenylalanine (fMLP). FLM adhesion to endothelial cells (ECs) increased dramatically from 50 to 150 spheres per 100-microm length of venule after superfusion of the mesentery with fMLP and equaled or exceeded levels of leukocyte (WBC) adhesion. Removal of the EC glycocalyx by micropipette infusion of the venule with heparinase increased FLM-EC adhesion to levels attained with fMLP. Subsequent application of fMLP did not increase FLM adhesion further, suggesting that the FLMs saturated all ICAM-1 binding sites. Perfusion with heparinase after suffusion with fMLP significantly increased FLM-EC adhesion above levels attained with fMLP. However, WBC adhesion fell because of possible removal of selectins necessary to maintain WBC rolling at the wall. It is concluded that the glycocalyx serves as a barrier to adhesion and that its shedding during natural activation of ECs may be an essential part of the inflammatory response.

Targeting microparticles to select tissue via radiation-induced upregulation of endothelial cell adhesion molecules

PURPOSE

Certain endothelial cell adhesion molecules are up regulated in tissue that has been irradiated for therapeutic purposes. This up-regulation of adhesion molecules provides a potential avenue for targeting drugs to select tissues.

METHODS

Microspheres were coated with a mAb to ICAM-1 and the level of adhesion of the anti-ICAM-1 microspheres to irradiated tissue in vitro and in vivo was quantified.

RESULTS

Under in vitro flow conditions, the number of adherent microspheres on irradiated HUVEC was 4.8 +/- 0.9 times that of control; the adhesion of anti-ICAM-1 microspheres on irradiated HUVEC could be enhanced by more than 170% in the presence of RBC (20% hematocrit) in the medium. In vivo in a rat cranial window model, the number of adherent anti-ICAM-1 microspheres in locally irradiated cerebral tissue was 8 and 13 times that of IgG microspheres at 24 h and 48 h post-irradiation, respectively and returned to baseline 7 days post-irradiation. In locally irradiated animals, the number of adhering microspheres in unirradiated tissue remained at the basal level.

CONCLUSIONS

Radiation-induced up-regulation of endothelial cell adhesion molecules may be exploited to target drugs and/or genes to select segments of the endothelium.

The N-terminal peptide of PSGL-1 can mediate adhesion to trauma-activated endothelium via P-selectin in vivo

P-selectin glycoprotein ligand-1 (PSGL-1) is present on leukocytes and is the major ligand for endothelial expressed P-selectin. A variety of studies strongly suggests that the N-terminal region of PSGL-1 contains the binding site for P-selectin. We hypothesized that this relatively small N-terminal peptide of PSGL-1 is sufficient to support adhesion to P-selectin in vivo. To test this hypothesis, we coated 2 microm-diameter microspheres with a recombinant PSGL-1 construct, termed 19.ek.Fc. The 19.ek.Fc construct consists of the first 19 N-terminal amino acids of mature PSGL-1 linked to an enterokinase cleavage site that, in turn, is linked to human immunoglobulin G Fc. The 19.ek.Fc-coated microspheres were injected into the jugular vein of mice. Intravital microscopy of postcapillary venules within the cremaster muscle of mice revealed that a significantly greater number of 19.ek.Fc microspheres rolled compared with control microspheres. The number of rolling 19.ek.Fc microspheres was significantly diminished by pretreatment of the mice with a monoclonal antibody to P-selectin or by pretreatment of the 19.ek.Fc microspheres with a monoclonal antibody to PSGL-1. Combined, the results indicate that the N-terminal peptide of PSGL-1 can mediate adhesion to trauma-activated microvascular endothelium via P-selectin in vivo.

Characterization of biodegradable drug delivery vehicles with the adhesive properties of leukocytes

The site-specific expression of selectins (E- and P-selectin) on endothelial cells of blood vessels during inflammation provides an opportunity for the targeted delivery of anti-inflammatory drugs to inflammatory sites. Previous work in our laboratory has shown that artificial capsules with the adhesive properties of leukocytes can be made by attaching leukocyte adhesive ligands to polystyrene microspheres. In this work, we have adapted this technology to create a targeted delivery system using biodegradable, poly lactic-co-glycolic-acid (PLGA) microspheres. Biotinylated-Sialyl Lewis(x) (sLe(x)), a carbohydrate that serves as a ligand to selectins, was attached to the surface of avidin-linked PLGA microspheres. These carbohydrate-coated microspheres mimic the adhesive behavior of leukocytes on selectins in flow chambers, displaying slow rolling under flow. The rolling velocities displayed by sLe(x)-coated microspheres were similar to those displayed by leukocytes rolling on P- or E-selectin coated surfaces, and these rolling velocities, which relate to the residence time of the capsules, can be tuned by changing the density of carbohydrate residues on microsphere surfaces. We have also demonstrated that these microspheres will release model drugs on a time scale of several days. Therefore, we have made a targeted drug delivery vehicle that mimics the adhesive properties of leukocytes and is biodegradable.

Comparison of PSGL-1 microbead and neutrophil rolling: microvillus elongation stabilizes P-selectin bond clusters

A cell-scaled microbead system was used to analyze the force-dependent kinetics of P-selectin adhesive bonds independent of micromechanical properties of the neutrophil's surface microvilli, an elastic structure on which P-selectin ligand glycoprotein-1 (PSGL-1) is localized. Microvillus extension has been hypothesized in contributing to the dynamic range of leukocyte rolling observed in vivo during inflammatory processes. To evaluate PSGL-1/P-selectin bond kinetics of microbeads and neutrophils, rolling and tethering on P-selectin-coated substrates were compared in a parallel-plate flow chamber. The dissociation rates for PSGL-1 microbeads on P-selectin were briefer than those of neutrophils for any wall shear stress, and increased more rapidly with increasing flow. The microvillus length necessary to reconcile dissociation constants of PSGL-1 microbeads and neutrophils on P-selectin was 0.21 microm at 0.4 dyn/cm2, and increased to 1.58 microm at 2 dyn/cm2. The apparent elastic spring constant of the microvillus ranged from 1340 to 152 pN/microm at 0.4 and 2.0 dyn/cm2 wall shear stress. Scanning electron micrographs of neutrophils rolling on P-selectin confirmed the existence of micrometer-scaled tethers. Fixation of neutrophils to abrogate microvillus elasticity resulted in rolling behavior similar to PSGL-1 microbeads. Our results suggest that microvillus extension during transient PSGL-1/P-selectin bonding may enhance the robustness of neutrophil rolling interactions.

Transcytosis of nanoparticle and dendrimer delivery systems: evolving vistas

The translocation of particulate matter across the gastrointestinal tract is now a well documented phenomenon offering new potential for the delivery of drugs with poor dissolution profiles and labile chemistries via encapsulation in biodegradable nanoparticles. The last few years have seen an acceleration in the number of publications describing the varying facets of this approach and the multidisciplinary nature of this field. This review delineates data from this rather fragmented area and from cognate fields to provide a physicochemical viewpoint of the importance of surface chemistries of oral drug delivery vehicles and their interactions in and with gut contents prior to uptake. The role of lymphoid and non-lymphoid tissues is examined, and the role of bioadhesion is discussed. The exciting potential of molecular encapsulation of drugs via dendrimers and star branched molecules is discussed in the context of nanotechnological applications for the oral route. Evolving vistas include a better understanding of the plasticity of the intestinal epithelium and M-cell induction as well as the influence of disease states on particulate uptake. In this review we address a number of issues deemed vital to an understanding of the subject including (i) some background knowledge on particulate uptake (the subject of several reviews), (ii) factors affecting uptake such as diameter and surface charge and character, (iii) the dynamic nature of particle interactions in the gut, (iv) the dynamic nature of the processes of capture, adhesion, uptake, transcytosis and translocation, and (v) the influence of surface ligands.