In vivo time-course biocompatibility assessment of biomagnetic nanoparticles-based biomaterials for tissue engineering applications

Novel artificial tissues with potential usefulness in local-based therapies have been generated by tissue engineering using magnetic-responsive nanoparticles (MNPs). In this study, we performed a comprehensive in vivo characterization of bioengineered magnetic fibrin-agarose tissue-like biomaterials. First, in vitro analyses were performed and the cytocompatibility of MNPs was demonstrated. Then, bioartificial tissues were generated and subcutaneously implanted in Wistar rats and their biodistribution, biocompatibility and functionality were analysed at the morphological, histological, haematological and biochemical levels as compared to injected MNPs. Magnetic Resonance Image (MRI), histology and magnetometry confirmed the presence of MNPs restricted to the grafting area after 12 weeks. Histologically, we found a local initial inflammatory response that decreased with time. Structural, ultrastructural, haematological and biochemical analyses of vital organs showed absence of damage or failure. This study demonstrated that the novel magnetic tissue-like biomaterials with improved biomechanical properties fulfil the biosafety and biocompatibility requirements for future clinical use and support the use of these biomaterials as an alternative delivery route for magnetic nanoparticles.

In vitro characterization of a novel magnetic fibrin-agarose hydrogel for cartilage tissue engineering

The encapsulation of cells into biopolymer matrices enables the preparation of engineered substitute tissues. Here we report the generation of novel 3D magnetic biomaterials by encapsulation of magnetic nanoparticles and human hyaline chondrocytes within fibrin-agarose hydrogels, with potential use as articular hyaline cartilage-like tissues. By rheological measurements we observed that, (i) the incorporation of magnetic nanoparticles resulted in increased values of the storage and loss moduli for the different times of cell culture; and (ii) the incorporation of human hyaline chondrocytes into nonmagnetic and magnetic fibrin-agarose biomaterials produced a control of their swelling capacity in comparison with acellular nonmagnetic and magnetic fibrin-agarose biomaterials. Interestingly, the in vitro viability and proliferation results showed that the inclusion of magnetic nanoparticles did not affect the cytocompatibility of the biomaterials. What is more, immunohistochemistry showed that the inclusion of magnetic nanoparticles did not negatively affect the expression of type II collagen of the human hyaline chondrocytes. Summarizing, our results suggest that the generation of engineered hyaline cartilage-like tissues by using magnetic fibrin-agarose hydrogels is feasible. The resulting artificial tissues combine a stronger and stable mechanical response, with promising in vitro cytocompatibility. Further research would be required to elucidate if for longer culture times additional features typical of the extracellular matrix of cartilage could be expressed by human hyaline chondrocytes within magnetic fibrin-agarose hydrogels.

Ex vivo characterization of a novel tissue-like cross-linked fibrin-agarose hydrogel for tissue engineering applications

The generation of biomaterials with adequate biomechanical and structural properties remains a challenge in tissue engineering and regenerative medicine. Earlier research has shown that nanostructuration and cross-linking techniques improved the biomechanical and structural properties of different biomaterials. Currently, uncompressed and nanostructured fibrin-agarose hydrogels (FAH and NFAH, respectively) have been used successfully in tissue engineering. The aim of this study was to investigate the possibility of improving the structural and biomechanical properties of FAH and NFAH by using 0.25% and 0.5% (v/v) glutaraldehyde (GA) as a cross-linker. These non-cross-linked and cross-linked hydrogels were subjected to structural, rheological and ex vivo biocompatibility analyses. Our results showed that GA cross-linking induced structural changes and significantly improved the rheological properties of FAH and NFAH. In addition, ex vivo biocompatibility analyses demonstrated viable cells in all conditions, although viability was more compromised when 0.5% GA was used. Our study demonstrates that it is possible to control fiber density and hydrogel porosity of FAH and NFAH by using nanostructuration or GA cross-linking techniques. In conclusion, hydrogels cross-linked with 0.25% GA showed promising structural, biochemical and biological properties for use in tissue engineering.

Inverse magnetorheological fluids

We report a new kind of field-responsive fluid consisting of suspensions of diamagnetic (DM) and ferromagnetic (FM) microparticles in ferrofluids. We designate them as inverse magnetorheological (IMR) fluids for analogy with inverse ferrofluids (IFFs). Observations on the particle self-assembly in IMR fluids upon magnetic field application showed that DM and FM microparticles were assembled into alternating chains oriented along the field direction. We explain such assembly on the basis of the dipolar interaction energy between particles. We also present results on the rheological properties of IMR fluids and, for comparison, those of IFFs and bidispersed magnetorheological (MR) fluids. Interestingly, we found that upon magnetic field application, the rheological properties of IMR fluids were enhanced with respect to bidispersed MR fluids with the same FM particle concentration, by an amount greater than the sum of the isolated contribution of DM particles. Furthermore, the field-induced yield stress was moderately increased when up to 30% of the total FM particle content was replaced with DM particles. Beyond this point, the dependence of the yield stress on the DM content was non-monotonic, as expected for FM concentrations decreasing to zero. We explain these synergistic results by two separate phenomena: the formation of exclusion areas for FM particles due to the perturbation of the magnetic field by DM particles and the dipole-dipole interaction between DM and FM particles, which enhances the field-induced structures. Based on the second phenomenon, we present a theoretical model for the yield stress that semi-quantitatively predicts the experimental results.

Optimizing the magnetic response of suspensions by tailoring the spatial distribution of the particle magnetic material

We report an experimental enhancement of the magnetic susceptibility of suspensions of particles that is related to the spatial distribution of the magnetic phase in the particles. At low field, the susceptibility of suspensions of nickel-coated diamagnetic spheres was approximately 75% higher than that of suspensions of solid nickel spheres with the same nickel content. This result was corroborated by magnetostatics theory and simulation. The distribution of the magnetic phase in a shell also led to an improvement of the field-induced rheological response of the suspensions.

Effect of the hydration on the biomechanical properties in a fibrin-agarose tissue-like model

The effect of hydration on the biomechanical properties of fibrin and fibrin-agarose (FA) tissue-like hydrogels is reported. Native hydrogels with approximately 99.5% of water content and hydrogels with water content reduced until 90% and 80% by means of plastic compression (nanostructuration) were generated. The biomechanical properties of the hydrogels were investigated by tensile, compressive, and shear tests. Experimental results indicate that nanostructuration enhances the biomechanical properties of the hydrogels. This improvement is due to the partial draining of the water that fills the porous network of fibers that the plastic compression generates, which produces a denser material, as confirmed by scanning electron microscopy. Results also indicate that the characteristic compressive and shear parameters increase with agarose concentration, very likely due to the high water holding capacity of agarose, which reduces the compressibility and gives consistency to the hydrogels. However, results of tensile tests indicate a weakening of the hydrogels as agarose concentration increases, which evidences the anisotropic nature of these biomaterials. Interestingly, we found that by adjusting the water and agarose contents it is possible to tune the biomechanical properties of FA hydrogels for a broad range, within which the properties of many native tissues fall.

Colloids on the frontier of ferrofluids. Rheological properties

This paper is devoted to the steady-state rheological properties of two new kinds of ferrofluids. One of these was constituted by CoNi nanospheres of 24 nm in diameter, whereas the other by CoNi nanofibers of 56 nm in length and 6.6 nm in width. These ferrofluids were subjected to shear rate ramps under the presence of magnetic fields of different intensity, and the corresponding shear stress values were measured. From the obtained rheograms (shear stress vs shear rate curves) the values of both the static and the dynamic yield stresses were obtained as a function of the magnetic field. The magnetoviscous effect was also obtained as a function of both the shear rate and the magnetic field. The experimental results demonstrate that upon magnetic field application these new ferrofluids develop yield stresses and magnetoviscous effects much greater than those of conventional ferrofluids, based on nanospheres of approximately 10 nm in diameter. Besides some expected differences, such as the stronger magnetorheological effect in the case of ferrofluids based on nanofibers, some intriguing differences are found between the rheological behaviors of nanofiber ferrofluids and nanosphere ferrofluid. First, upon field application the rheograms of nanofiber ferrofluids present N-shaped dependence of the shear stress on the shear rate. The decreasing part of the rheograms takes place at low shear rate. These regions of negative differential viscosity, and therefore, unstable flow is not observed in the case of nanosphere ferrofluids. The second intriguing difference concerns the curvature of the yield stress vs magnetic field curves. This curvature is negative in the case of nanosphere ferrofluid, giving rise to saturation of the yield stress at medium field, as expected. However, in the case of nanofiber ferrofluid this curvature is positive, which means a faster increase of the yield stress with the magnetic field the higher the magnitude of the latter. These interesting differences may be due to the existence of strong interparticle solid friction in the case of nanofiber ferrofluids. Finally, theoretical models for the static yield stress of the ferrofluids were developed. These models consider that upon field application the ferrofluid nanoparticles are condensed in drops of dense phase. These drops tend to be aligned along the field direction, opposing the flow of the ferrofluids and being responsible for the static quasielastic deformation and the yield-stress phenomena. By considering the existence of interparticle dry friction only in the case of nanofiber ferrofluids, the developed models predicted quite well not only the magnitude of the static yield stress but also the differences in curvature of the yield stress vs magnetic field curves.

Dynamic characterization of extremely bidisperse magnetorheological fluids

In this work, we investigate the stability and redispersibility of magnetorheological fluids (MRFs). These are disperse systems where the solid is constituted by ferro- or ferri-magnetic microparticles. Upon the application of external magnetic field, they experience rapid and reversible increases in yield stress and viscosity. The problem considered here is first of all the determination of their stability against sedimentation, an essential issue in their practical application. Although this problem is typically faced through the addition of thixotropic agents to the liquid medium, in this work, we propose the investigation of the effect of magnetic nanoparticles addition, so that the dispersion medium is in reality a ferrofluid. It is found that a volume fraction of nanoparticles not higher than 3% is enough to provide a long-lasting stabilization to MRFs containing above 30% iron microparticles. In the, in fact unavoidable, event of settling, the important point is the ease of redispersion of the sediment. This is indirectly evaluated in the present investigation by measuring the penetration force in the suspension, using a standard hardness needle. Again, it is found that the nanoparticles addition produces soft sediments by avoiding short-range attractions between the large iron particles. Finally, the performance of the designed MRFs is evaluated by obtaining their steady-state rheograms for different volume fractions of magnetite and different magnetic field strengths. The yield stress is found to be strongly field-dependent, and it can achieve the high values expected in standard magnetorheological fluids but with improved stability and redispersibility.

Stability and magnetorheological behaviour of magnetic fluids based on ionic liquids

This paper reports the preparation of magnetic fluids consisting of magnetite nanoparticles dispersed in an ionic liquid. Different additives were used in order to stabilize the fluids. Colloidal stability was checked by magnetic sedimentation, centrifugation and direct observation. The results of these tests showed that a true ferrofluid was only obtained when the nanoparticles were coated with a layer of surfactant compatible with the ionic liquid. These experiments also showed that stability could not be reached just by electrostatic repulsion. The conclusions of the stability tests were confirmed by calculations of the interparticle energies of interaction. The rheological behaviour of the magnetic fluids upon magnetic field application was also investigated. The experimental magnetoviscous response was fitted by a microstructural model. The model considered that the fluids consisted of two populations of particles, one with a magnetic core diameter of 9 nm, and another with a larger diameter. Upon field application chain-like structures are supposed to be induced. According to estimations particles of 9 nm are too small to aggregate upon field application. The results of the calculations showed that the intensity of the magnetoviscous response depends on the concentration and size of the large particles, and on the thickness of the surfactant layers.

Description and performance of a fully automatic device for the study of the sedimentation of magnetic suspensions

In this paper we describe an experimental setup for the automatic determination of the sedimentation behavior of magnetic suspensions (i.e., disperse systems consisting on ferro- or ferri-magnetic particles in a suitable fluid) of arbitrary volume fraction of solids. The device is based on the evaluation of the inductance of a thin coil surrounding the test tube containing the sample. The inductance L is evaluated from the measurement of the resonant frequency of a parallel LC circuit constructed with the coil and a capacitor of known capacitance. The coil can be moved vertically along the tube at specified steps and time intervals, and from the knowledge of L as a function of the vertical position and time, one can get an image of the particle concentration profiles at given instants of time. The performance of the device is tested against suspensions of spherical iron particles in the micrometer size range dispersed in silicone oil, with various initial concentrations of solids. The sedimentation profiles are then compared with the predictions of existing models for the settling of disperse systems of non-interacting particles.

Study of the magnetorheology of aqueous suspensions of extremely bimodal magnetite particles

In this paper we describe the magnetorheological behavior of aqueous suspensions consisting of magnetite particles of two size populations, in the micrometer and nanometer scale, respectively. Previous works on the magnetorheology of oil-based fluids demonstrated that the addition of nanoparticles has a very significant effect on the intensity of the magnetorheological effect. The present contribution confirms such results in the case of aqueous fluids, based on the dependence of the yield stress and the viscosity of the bimodal suspensions on both the composition of the mixtures and the magnetic field strength. It is demonstrated that for a given concentration of micrometer particles, increasing the amount of nanometer magnetite provokes a clear enhancement in the yield stress for all the magnetic fields applied. This is proposed to be due to the formation of heterogeneous aggregates that improve the stability of the suspensions and ease the building of well-arranged field-induced structures. The behavior of both the yield stress and the post-yield viscosity agrees better with the predictions of standard chain models when the relative proportion of both types of particles confers optimum stability to the bimodal dispersions.

Magnetic colloids as drug vehicles

This review article is a description of the present status of magnetic drug delivery systems (DDS). These are colloidal dispersions of composite nanoparticles consisting of a (polymeric or inorganic) biocompatible matrix and magnetic units, and designed to load and release therapeutic drugs. The matrix, together perhaps with adsorbed polymers or polyelectrolytes, provides the DDS with additional colloidal stability and eventually control of the immune response, and the magnetic inclusions have the goal of providing magnetic guidance. The techniques used in the production of the particles are described. The large surface/volume ratio of the particles brings about a superlative importance of the interface aspects, which are depicted in some detail. Attention is also paid to the possibilities that magnetic DDS offer to be guided by magnetic fields, and to their fate upon entering in contact with the blood proteins and the tumor cells. A description of in vitro and in vivo biodistribution experiments helps in this description. The number of animal experiments performed using magnetic DDS is rather large, but results in humans are far from being sufficient in number, something easily understood. The hopes for improvement and the challenges that must be overcome are described in the closing section.

Preparation and characterization of iron-based magnetorheological fluids stabilized by addition of organoclay particles

Suspensions of micrometer-sized iron particles (10 vol %) dispersed in kerosene and stabilized by addition of organoclay particles were prepared. The magnetization curves of these suspensions were measured, and their sedimentation and redispersion behaviors were analyzed as a function of clay concentration by means of optical and rheological methods. Furthermore, their magnetorheological properties were investigated using a controlled rate magnetorheometer and the effect of clay concentration on these properties was also analyzed. These experiments showed that the addition of clay slows down iron particle settling and eases the redispersion of the iron-based suspensions without masking their magnetorheological properties. Two mechanisms were found to be involved in this behavior: (i) the formation of a clay gel network and (ii) the presence of heterogeneous iron-clay adhesion.

An experimental method for the measurement of the stability of concentrated magnetic fluids

In this paper we present a device and method suited to the experimental determination of the sedimentation rate of concentrated suspensions of magnetic particles. The method is based on the measurement of the inductance of one or more sensing coils located at specified positions around a test tube containing the suspension. Such measurement is made possible by the determination of the resonant frequency of a parallel LC circuit in which L is the inductance of the sensing coil and C is the capacity of a capacitor chosen in such a way that the resonant frequency is easily measured. Upon calibration it is possible to relate the resonant frequency to the volume fraction of the particles at the coil location. The method is applied in the present work to the evaluation of the sedimentation kinetics of iron suspensions in base fluids of viscosities ranging from 0.3 to 100 mPa s and volume fractions of solids between 2.5 and 25%. Both if a single coil is used and if a set of three coils at different positions are employed, it is possible to detect the rate of accumulation of particles at the bottom of the container as well as a phenomenon of buoyancy of the largest particles brought about by the hydrostatic push of a dense fluid consisting of the smallest particles in the supporting liquid.

Study of the colloidal stability of concentrated bimodal magnetic fluids

In this paper, we describe an investigation of the stability and sedimentation behavior of moderately concentrated suspensions of extremely bimodal magnetite particles, including micro- (diameter 1450 nm) and nano- (diameter 8 nm) units. An original method is used, based on the determination of the time dependence of the inductance of a coil surrounding the suspensions. The method proves to be very useful for the determination of the volume fraction of magnetic material in the sensed volume. The observed changes in the resonant frequency of a parallel LC circuit demonstrate that the addition of the magnetite nanoparticles improves the stability and slows down the settling rate of the mixed suspensions. It is proposed that the observed behavior is the result of competition between two processes. One is the formation of a cloud of nanoparticles around the large magnetite units, by virtue of which the latter are maintained at distances beyond the range of DLVO and magnetic attractive interactions. At long times, these composite units will eventually sediment when some critical size is reached, as the small particles are progressively associated with the large ones. The second mechanism is mainly predominant at short times and is related to the higher viscosity of the dispersion medium (the nanoparticles dispersed in the base fluid) for higher nanoparticle concentrations. The stability of the suspensions is discussed in terms of the competition between the two mechanisms.

Stability and magnetic characterization of oleate-covered magnetite ferrofluids in different nonpolar carriers

This work describes the preparation and stability evaluation of suspensions consisting of hydrophobic magnetite nanoparticles dispersed in different organic solvents. The ferrite particles are covered by a shell of chemisorbed oleate ions following a procedure that is described in detail. The oleate-covered particles were dispersed in different organic solvents with dielectric constants, epsilon(r), ranging between 1.8 and 9, and the centrifugal field strength needed to remove particle aggregates formed during the synthesis was determined for the different liquid carriers used. A thermodynamic analysis demonstrated that the observed stability of the suspensions in liquids with epsilon(r) < 5 is well correlated with the very low lyophobic attraction between the particles. This can easily be surmounted by thermal agitation, since the van der Waals attraction is negligible. In contrast, for liquids with epsilon(r) > 9, the suspensions become unstable because of the combined action of the van der Waals and lyophobic attractions, the latter being dominant for very polar solvents. Finally, a complete magnetic characterization of the oleate-magnetite powder, as well as of several stable ferrofluids prepared with it, was carried out. From this characterization, the magnetic diameters and magnetic moments of the particles immersed in the different liquid carriers were estimated and compared to those corresponding to the dry magnetic particles. This made it possible to estimate the thickness of the nonmagnetic layer on the particles.

Optical properties of dilute hematite/silicone oil suspensions under low electric fields

Electrorheology (ER) is the name given to a set of phenomena related to the significant changes experienced by the rheological properties of certain fluids and suspensions upon application of external electric fields. It is mostly explained in terms of the formation of particle aggregates as a consequence of field-induced particle-particle interactions. In this work, we explore such structures by investigating the changes in optical absorbance of hematite/silicone oil suspensions associated to the application of an electric field. We have studied the effect of particle concentration, phi, electric field strength, E(0), and viscosity, eta(m), of the liquid medium on the absorbance-time behavior of the suspensions. Photographs of the electrified suspensions helped in elucidating the structures formed. At low phi values, the absorbance A of electrified suspensions dramatically decreases with time until a constant plateau is reached. The absorbance fall is faster the higher the field, although at long times curves corresponding to different fields tend to merge. In these dilute suspensions particles are observed to migrate toward the electrodes thus clarifying the medium and reducing A. When the concentration of particles is increased, fibrils stretching between the electrodes can be observed in addition to particle deposition on them, as long as the field is kept low. At high fields, migration of the particles to the electrodes occurs whatever the volume fraction. Two mechanisms producing particle-particle interactions are suggested by these data: the conductivity mismatch between the particles and the medium brings about an interfacial or Maxwell-Wagner polarization of the particles; in addition, solids can acquire a net charge provoked by injection from the electrodes. The first mechanism will produce attractive dipole-dipole interactions and hence columns or fibrils. The second one should lead to electrophoretic migration. Structural observations suggest that the latter predominates at high fields. If the viscosity of the fluid phase is increased, the critical electric field values separating both regimes also increase: the electrophoretic motion is hindered and the particle-particle aggregation is enhanced.

Preparation and sedimentation behavior in magnetic fields of magnetite-covered clay particles

This work is devoted to the preparation of magnetite-covered clay particles in aqueous medium. For this purpose, magnetite nanoparticles were synthesized by a coprecipitation method. These magnetic particles are adhered to sodium montmorillonite (NaMt) particles in aqueous suspensions of both materials, by appropriate control of the electrolyte concentrations. The best condition to produce such heteroaggregation corresponds to acid pH and approximately 1 mol/L ionic strength, when the electrokinetic potentials (zeta-potential) of both NaMt and Fe3O4 particles have high enough and opposite sign, as demonstrated from electrophoresis measurements. When a layer of magnetite re-covers the clay particles, the application of an external magnetic field induces a magnetic moment in clay-magnetite particles parallel to the external magnetic flux density. The sedimentation behavior of such magnetic particles is studied in the absence or presence of an external magnetic field in a vertical direction. The whole sedimentation behavior is also strongly affected by the formation of big flocculi in the suspensions under the action of internal colloidal interactions. van der Waals and dipole-dipole magnetic attractions between magnetite-covered clay particles dominate the flocculation processes. The different relative orientation of the clay-magnetite particles (edge-to-edge, face-to-edge, and face-to-face) are discussed in order to predict the most favored flocculi configuration.

Stabilization of magnetorheological suspensions by polyacrylic acid polymers

This work is devoted to the synthesis and stabilization of magnetorheological suspensions constituted by monodisperse micrometer-sized magnetite spheres in aqueous media. The electrical double-layer characteristics of the solid/liquid interface were studied in the absence and presence of adsorbed layers of high molecular weight polyacrylic acids (PAA; Carbopol). Since the Carbopol-covered particles can be thought of as "soft" colloids, Ohshima's theory was used to gain information of the surface potential and the charge density of the polymer layer. The effect of the pH of the solution on the double-layer characteristics is related to the different conformations of the adsorbed molecules provoked by the dissociation of the acrylic groups present in polymer molecules. The stability of the suspensions was experimentally studied for different pH and polymer concentrations, and in the absence or presence of a weak magnetic field applied. The stability of the suspensions was explained using the classical DLVO theory of colloidal stability extended to account for hydration, steric, and magnetic interactions between particles. Diagrams of potential energy vs interparticle distance show the predominant effect of steric, hydrophilic/hydrophobic, and magnetic interactions on the whole stability of the system. The best conditions to obtain stable suspensions were found when strong steric and hydrophilic repulsions hinder the coagulation between polymer-covered particles, simultaneously avoiding sedimentation by the thickening effect of the polymer solution. When a not too high molecular weight PAA was employed in a low concentration, the task of a long-time antisettling effect compatible with the desired magnetic response of the fluid was achieved.

A slender-body micromechanical model for viscoelasticity of magnetic colloids: Comparison with preliminary experimental data

The storage modulus, G', together with the yield stress, is an essential quantity characterizing the rheological properties of magnetic field-responsive suspensions (magnetorheological fluids or MRF). In this work, we present both experimental and theoretical results on the viscoelastic properties of MRFs. Two MRFs are used: In one the solid phase consists of cobalt ferrite particles + silica gel, with silicone oil as liquid phase. The second system is formed by carbonyl iron + silica gel also dispersed in silicone oil. The cobalt ferrite particles are synthesized as monodisperse colloidal spheres with an average diameter of 850 nm. We describe a new model based on the slender-body approach for hydrodynamic interactions. The predictions of the model are compared to preliminary experimental G' data obtained in a controlled stress plate-plate rheometer. It is found that the model gives the correct order of magnitude for the highest fields in iron suspensions, but underestimates the experimental results obtained in ferrite ones. In the case of high permeability materials such as carbonyl iron, by the inclusion of high-order multipolar interactions and saturation effects we also predict the order of magnitude of the experimental results. When dealing with low permeability cobalt ferrite based MRFs, other effects, such as remanence (at low fields) and saturation (at high fields), must be considered.

Interfacial and rheological properties of humic acid/hematite suspensions

This work deals with the effect of humic acid (HA) adsorption on the interfacial properties, the stability, and the rheology of aqueous iron oxide (hematite) suspensions. It is first of all demonstrated that HA effectively adsorbs onto hematite, mainly at acid pH. Since the charge of the HA chains is negative, it will be electrostatically attracted to the hematite surface below the point of zero charge of the particles, when they are positively charged. Electrophoresis measurements of hematite suspensions as a function of pH in the presence and absence of HA clearly demonstrate the adsorption of negatively charged entities on the oxide. Since the HA-covered particles can be thought of as "soft" colloids, Ohshima's theory was used to gain information on the surface potential and the charge density of the HA layer (H. Ohshima, in: A.V. Delgado (Ed.), Interfacial Electrokinetics and Electrophoresis, Dekker, New York, 2002, p. 123). A different procedure was also used to ascertain the degree of modification experienced by the hematite surface when placed in contact with HA solutions. The contact angles of selected liquids on pretreated hematite layers lead to the conclusion that the humic acid molecules impart to the particles a significant electron-donor character, in turn increasing their hydrophilicity. All this amount of information is used in the work for the interpretation of the rheological properties of hematite suspensions; the results are consistent with a stabilizing effect of HA adsorption on the suspensions, mainly as a consequence of the increased electrostatic repulsion between particles.