Microspheres for biomedical applications: preparation of reactive and labelled microspheres

Microparticle sampling by electrowetting-actuated droplet sweeping

This paper describes a new microparticle sampler where particles can be efficiently swept from a solid surface and sampled into a liquid medium using moving droplets actuated by the electrowetting principle. We successfully demonstrate that super hydrophilic (2 microm and 7.9 microm diameter glass beads of about 14 degrees contact angle), intermediate hydrophilic (7.5 microm diameter polystyrene beads of about 70 degrees contact angle), and super hydrophobic (7.9 microm diameter Teflon-coated glass beads and 3 microm size PTFE particles of over 110 degrees contact angles) particles on a solid surface are picked up by electrowetting-actuated moving droplets. For the glass beads as well as the polystyrene beads, the sampling efficiencies are over 93%, in particular over 98% for the 7.9 microm glass beads. For the PTFE particles, however, the sampling efficiency is measured at around 70%, relatively lower than that of the glass and polystyrene beads. This is due mainly to the non-uniformity in particle size and the particle hydrophobicity. In this case, the collected particles staying (adsorbing) on the air-to-water interface hinder the droplet from advancing. This particle sampler requires an extremely small amount of liquid volume (about 500 nanoliters) and will thus be highly compatible and easily integrated with lab-on-a-chip systems for follow-up biological/chemical analyses.

Transcription of T7 DNA immobilised on latex beads and Langmuir–Blodgett film

The recognition of DNA is the first and most important condition for biological applications, including transcription and translation regulators and DNA sensors. For this purpose, we have developed few systems where we were able to immobilize long double-stranded DNA (dsDNA) successfully to the surfaces of different solid substrates. To achieve this, we have chosen polystyrene beads and standard Langmuir-Blodgett monolayer of Zn-arachidate. In the first attempt, variant of T7 DNA containing one strong promoter A1 for Escherichia coli RNA polymerase was immobilised on uniform polystyrene microspheres (0.31 microm diameter) by covalent grafting. In the latter case, Zn(II) is bound to arachidic acid through charge neutralization. Since tetrahedral Zn(II) participates in DNA recognition through coordination, we have been able to layer DNA over the Zn-arachidate monolayer. The successful immobilization of DNAs on these different substrates was visualized under fluorescence microscope. These immobilized DNAs were used as a template to study in vitro transcription reaction and thus we introduce a new strategy for the study of transcription in heterogeneous phase.

Formation of a DNA layer on Langmuir-Blodgett films and its enzymatic digestion

Here, we report a system we have developed where long double-stranded DNAs (dsDNAs) are immobilized on a monolayer of Zn-arachidate. We have applied the Langmuir-Blodgett technique to form the monolayer of Zn-arachidate where Zn(II) is bound to arachidic acid through charge neutralization. Because tetrahedral Zn(II) participates in DNA recognition through coordination, we have been able to layer DNA over the Zn-arachidate monolayer. The DNA layer shows a typical compression and expansion cycle in a concentration-dependent fashion. Interestingly, the DNA monolayer is available for enzymatic degradation by DNaseI. The detection of DNA and its accessibility towards biological reaction is demonstrated by imaging through fluorescence microscopy. The conformation of the DNA, immobilized on the monolayer, was studied with the help of atomic force microscopy (AFM). We observed that the dsDNAs were aligned in a stretched manner on the surface. To investigate further, we also demonstrate here that the small single-stranded DNA (ssDNA) immobilized on the air-water interface can act as a target molecule for the complementary ssDNA present in the subphase. The study of DNA hybridization done with the help of fluorescence spectroscopy clearly supports the AFM characterization.

Segmented detachable structure of cochlear-implant electrodes for close-hugging engagement with the modiolus

Surface and bulk modification techniques of polydimethylsiloxane (PDMS) polymers were used to develop a new intracochlear electrode that can closely hug the inner wall of scala tympani. Laser-induced surface grafting of poly (2-hydroxyethyl methacrylate) (PHEMA) and sequential method for preparation of interpenetrating polymer networks (IPNs) of PDMS/PHEMA were, respectively, used for surface and bulk modifications. The hydrogel content and water-uptake capability of the modified samples were optimized by Taguchi method for experimental design. The modified PDMS samples were examined by performing attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, water contact-angle measurement, and peel strength tests. The performances of actual-sized fabricated electrodes were assessed inside a transparent model of scala tympani, which was filled with saline. After a swelling process, the hydrophilic branch begins to elongate and detach from hydrophobic branch and pushes it toward the inner wall.

Resonance energy transfer from dibucaine to acriflavine in polystyrene latex dispersions

Resonance energy transfer from dibucaine (DC) to acriflavine (AF) has been investigated both in homogeneous aqueous solutions and in polystyrene latex dispersions. The energy transfer reaction is observed by monitoring fluorescence quenching of DC as well as sensitized emission of AF. It is found that the energy transfer from DC to AF is remarkably enhanced on going from the aqueous homogeneous solution to the latex dispersion. This is mainly attributed to the fact that both the donor and acceptor are effectively adsorbed onto the latex particles, as evidenced by the measurement of adsorption isotherms. From the adsorption experiments, it is also elucidated that electrostatic interaction is significant at low DC concentration, while hydrophobic interaction dominates at the higher concentration especially at higher pH.

Covalent grafting of polythymidylic acid onto amine-containing polystyrene latex particles

The covalent grafting of a 35-base poly(thymidylic) acid (dT35) onto aminated polystyrene latex has been studied. The oligonucleotide was activated by 1,4-phenylene diisothiocyanate (PDC) prior to grafting reaction. The influence of various experimental conditions related either to the medium (pH, ionic strength) or to the latex (nature and surface charge density, presence of adsorbed nonionic surfactant) was studied. Discrimination between covalent grafting and adsorption was achieved by desorbing the non-covalently grafted dT35 induced by consecutive washings with a suitable buffer solution. The amount of covalently grafted oligonucleotide directly correlated to the concentration and accessible functional groups at the particle surface.

Dynamic behavior of glucose oxidase-containing microparticles of poly(ethylene glycol)-grafted cationic hydrogels in an environment of changing pH

Poly(diethylaminoethyl-g-ethylene glycol) microparticles were prepared by suspension polymerization of diethylaminoethyl methacrylate, poly(ethylene glycol) monomethacrylate and the crosslinking agent tetra(ethylene glycol) dimethacrylate in silicone oil using redox initiators. Particles of different sizes, crosslinking ratios and graft molecular weights were prepared. The changes in the swelling of the particles were studied as the pH was changed between 3.0 and 7.4. The particles showed rapid swelling/deswelling dynamics in response to changes in pH. It was evident that faster response could be obtained from smaller particles. Changing the crosslinking ratio resulted in changes in the extent of swelling, as well as the speed of response. It was also found that longer graft lengths were responsible for increasing the effect of relaxation of the swelling of the network.

Hydrophilic and cationic latex particles for the specific extraction of nucleic acids

The adsorption of BSA and RNA onto hydrophilic and thermosensitive poly(N-isopropyl-acrylamide) (NIPAM) latex particles was described as a function of pH, ionic strength and temperature. The hydrogel poly(NIPAM) latex was synthesized by precipitation polymerization in the presence of a cationic amino-containing monomer. The latex obtained was characterized in terms of particle size, and electrophoretic mobility as a function of pertinent variables: pH, temperature and ionic strength. The adsorption of BSA onto the latex was investigated to identify the conditions at which the adsorbed amount of BSA was negligible. The adsorption of RNA was studied to establish the conditions which give rise to maximal adsorption of RNA. In order to favor the desorption of RNA, desorption was investigated by changing the pH, ionic strength, and temperature. The adsorption of BSA was found to be lower at 20 than at 40 degrees C. However, the adsorption of RNA is drastically affected by the pH and the ionic strength of the medium. Maximal adsorbed amounts were obtained at acidic pH, 20 degrees C, and low ionic strength. The adsorption is shown to decrease when the pH, temperature and ionic strength increase, implying that the adsorption was mainly governed by electrostatic interactions. Maximal release of RNA molecules was obtained at high ionic strength and basic pH.

Controlled release of human immunoglobulin G. 2. Morphological characterization

Human immunoglobulin G (IgG) serves as an important chemotherapeutic agent for a number of immunological ailments and as a carrier in the targeted delivery of other therapeutic agents. This paper deals with the characterization of IgG-dispersed monolithic matrixes of different geometries, prepared using a nonbiodegradable polymer carrier EVAc. The morphological changes associated with the matrix during drug release was studied using scanning electron microscopy, polarizing microscopy, atomic force microscopy, and X-ray photoelectron microscopy, and the results were compared. The study answered the burst effect problem significantly and illustrated the potential of these techniques in understanding the morphological structure of matrixes and mode of release kinetics.

Biocompatible magnetic polymer carriers for in vivo radionuclide delivery

The magnetic guidance of antiplastic and antibacterial agents as well as x-ray and MRI contrast substances in vivo by means of magnetic particles has been attempted repeatedly during the last 2 decades with more or less success. When using microparticles, the circulation time in the blood, the biodistribution, and to a greater or lesser extent, the specific targeting are determined by the uniformity of size, chemical composition, surface modification, and the electric surface charge. The electrophoretic mobility is an important parameter for the prediction of the usefulness of the prepared particle, modified by chemical and biological molecules. For its success, radionuclide therapy depends on the critical relationship between the amount of radioactive isotopes in the target tissue and in critical normal tissue. Because the implementation of radioimmunotherapy for the treatment of cancer has proven to be considerably more difficult than initially anticipated, we propose the use of magnetic nanospheres for the well directed delivery of radionuclides to a tumor after the intravenous administration of the biodegradable colloidal suspension.

Acid-Base Approach to Latex Particles Containing Specific Groups Based on Wettability Measurements

Polar properties of latex particles containing specific groups with various surface charge densities were examined from contact angle measurements using "old" polar/dispersive models. It was determined that the Fowkes approach is quite sufficient for classifying our functionalized polystyrene latices. The pertinence to our systems of the acid-base approach, which has a more pronounced chemical background, was then investigated, and the strong dependence of the acid-base parameters (gamma plus sign in circle, gammaTheta) on the acid-base scale chosen for the reference liquid (water) was shown. Finally, using the receding angle values revealed the outermost surface buried moieties in the latex particles, which were not seen in the advancing angle measurements. Copyright 1998 Academic Press.

Implantable bioelectric interfaces for lost nerve functions

Neuronal cells are unique within the organism. In addition to forming long-distance connections with other nerve cells and non-neuronal targets, they lose the ability to regenerate their neurites and to divide during maturation. Consequently, external violations like trauma or disease frequently lead to their disappearance and replacement by non-neuronal, and thus not properly functioning cells. The advent of microtechnology and construction of artificial implants prompted to create particular devices for specialised regions of the nervous system, in order to compensate for the loss of function. The scope of the present work is to review the current devices in connection with their applicability and functional perspectives. (1) Successful implants like the cochlea implant and peripherally implantable stimulators are discussed. (2) Less developed and not yet applicable devices like retinal or cortical implants are introduced, with particular emphasis given to the reasons for their failure to replace very complex functions like vision. (3) Material research is presented both from the technological aspect and from their biocompatibility as prerequisite of any implantation. (4) Finally, basic studies are presented, which deal with methods of shaping the implants, procedures of testing biocompatibility and modification of improving the interfaces between a technical device and the biological environment. The review ends by pointing to future perspectives in neuroimplantation and restoration of interrupted neuronal pathways.

in vivo targeting of colloidal carriers by novel graft copolymers

One of the major obstacles to the targeted delivery of colloidal carriers (nanocapsules) is the body's own defence mechanism in capturing foreign particles by the reticuloendothelial system (RES). This means that following intravenous administration, practically all nanometer size particles are captured by the RES (mainly the liver). This paper draws attention to a recent initiative on the design of 'macromolecular homing devices' which seem to disguise nanoparticles from the RES, and hence are of potential interest for the targeted delivery of nanocapsular carriers. The idea is based on a graft copolymer model embodying a link site for attachment (binding) to the carrier, a floating pad for maintaining the particles afloat in the blood stream, an affinity ligand for site-specific delivery and a structural tune for balancing the overall structure of the homing device. A general synthetic scheme for the preparation of such graft copolymers is given, and preliminary biological evaluations relating to the floating pad concept are discussed.

Preparation and characterization of high-surface-area polymer substrates for microcalorimetry

The preparation and characterization of high-surface-area polymeric substrates suitable for the microcalorimetry of protein adsorption are described. High-surface-area polystyrene, poly(styrene-co-butyl methacrylate) and poly(styrene-co-allyl alcohol) were prepared by adsorbing polymer from solution onto fumed silica. Verification of adsorption of polystyrene by silica was determined by noting peak shifts of the surface silanol group in the infrared. The amount of polymer adsorbed was determined from adsorption isotherms. The minimum thickness of polystyrene required to mask silicon oxide properties was found to be that thickness at which contact angles became constant, about 35 A. Polymer densities were measured. Water contact angles on each polymer surface indicate that poly(styrene-co-allyl alcohol) has the surface most wettable by water. Polymer-water interfacial energies were estimated from pendant drop results and a harmonic mean equation along with contact angles. Two methods were used to estimate the polar and dispersion components of the three polymers. Both methods predicted polystyrene to have the highest interfacial energy against water, and one method predicted poly(styrene-co-allyl alcohol) to have the lowest. A Wilhelmy plate study verified the change in interfacial properties as a function of contact time with water. A study of the heats of adsorption of lysozyme by each substrate using a modified Tien-Calvet microcalorimeter demonstrated the suitability of the substrates for microcalorimetry.

Recent developments of collagen-based materials for medical applications and drug delivery systems

In this review, an attempt was made to summarize some of the recent developments in the application of collagen as a biomaterial and in drug delivery systems. The main applications covered include: collagen for burn/wound cover dressings; osteogenic and bone filling materials; antithrombogenic surfaces; and immobilization of therapeutic enzymes. Recently, collagen used as a carrier for drug delivery has attracted many researchers throughout the world. The use of collagen for various drug delivery systems has also been reviewed in this article. Collagen-based drug delivery systems include: injectable microspheres based on gelatin (degraded form of collagen); implantable collagen-synthetic polymer hydrogels; interpenetrating networks of collagen; and synthetic polymers collagen membranes for ophthalmic delivery. Recent efforts to use collagen-liposomal composites for controlled drug delivery, as well as collagen as controlling membranes for transdermal delivery, were also reviewed. In this review, the main emphasis was on the work done in our laboratory.

Prolonged blood circulation in rats of nanospheres surface-modified with semitelechelic poly[N-(2-hydroxypropyl)methacrylamide]

Semitelechelic poly[N-(2-hydroxypropyl)methacrylamide]s (ST-PHPMA) containing one amino end-group and differing in molecular weight were synthesized by radical polymerization in the presence of 2-aminoethanethiol (AET) as chain transfer agent. These polymers were covalently attached via amide bonds to the surface of nanospheres based on a copolymer of methyl methacrylate, maleic anhydride, and methacrylic acid. When compared to unmodified nanospheres, those with the surface modified with ST-PHPMA possessed a decreased protein (albumin, IgG, fibrinogen) adsorption in vitro, an increased intravascular half-life as well as a decreased accumulation in the liver after intravenous administration into rats. The higher the molecular weight of the ST-PHPMA, the more pronounced the changes in these properties. The results obtained have clearly demonstrated that covalently attached ST-PHPMA chains are efficient in decreasing the biorecognition of negatively charged (hydrophilic) polymer surfaces.

Biodegradable microspheres: a new delivery system for growth hormone

A drug delivery system for biologically active agents targeted to specific cells could be used to improve tissue repair in orthopaedics. The system should be controllable and capable of drug release over an extended period of time. Biodegradable, membrane-moderated, monolithic microspheres for the controlled release of growth hormone (GH) were developed and the release of GH was monitored in vitro. Cross-linked gelatin microspheres were used as the vehicle, with the drug dispersed within the gelatin. The amount of GH released from the microspheres was increased following ultrasonication. The release of growth hormone was monitored in phosphate buffered saline and horse serum. Interestingly, a higher level of GH was detected in the phosphate buffered saline than in serum. In addition, both pH and enzyme-induced degradation had an effect on the swelling kinetics of the gelatin microspheres. The release of GH from the microspheres was diffusion controlled, during the time period studied.

Influence of the structure of support microspheres on leucocyte activation by RGDS-carrying microspheres

A cell adhesive peptide, Arg-Gly-Asp-Ser (RGDS), enhances leucocyte response to stimuli when insolubilized or conjugated with proteins. In order to investigate the contribution of the microsphere structure to leucocyte response. RGDS was immobilized onto several polymeric microspheres. The respiratory burst of polymorphonuclear leucocytes (PMNs) contacting contacting microspheres was enhanced by RGDS in cases where it was immobilized on polystyrene and, more effectively, a positively charged one. This was not observed in polyacrylamide-based or negative-charged microsphere systems. These results demonstrate that the property of supports strongly affects the regulation of the PMN response via an adhesive interaction.