Synthesis and characterization of a hydrogel with controllable electroosmosis: a potential brain tissue surrogate for electrokinetic transport

Electrokinetic convection-enhanced delivery for infusion into the brain from a hydrogel reservoir

Electrokinetic convection-enhanced delivery (ECED) utilizes an external electric field to drive the delivery of molecules and bioactive substances to local regions of the brain through electroosmosis and electrophoresis, without the need for an applied pressure. We characterize the implementation of ECED to direct a neutrally charged fluorophore (3 kDa) from a doped biocompatible acrylic acid/acrylamide hydrogel placed on the cortical surface. We compare fluorophore infusion profiles using ECED (time = 30 min, current = 50 µA) and diffusion-only control trials, for ex vivo (N = 18) and in vivo (N = 12) experiments. The linear intensity profile of infusion to the brain is significantly higher in ECED compared to control trials, both for in vivo and ex vivo. The linear distance of infusion, area of infusion, and the displacement of peak fluorescence intensity along the direction of infusion in ECED trials compared to control trials are significantly larger for in vivo trials, but not for ex vivo trials. These results demonstrate the effectiveness of ECED to direct a solute from a surface hydrogel towards inside the brain parenchyma based predominantly on the electroosmotic vector.

Anionic polymers amplify electrokinetic perfusion through extracellular matrices

Electrical stimulation (ES) promotes healing of chronic epidermal wounds and delays degeneration of articular cartilage. Despite electrotherapeutic treatment of these non-excitable tissues, the mechanisms by which ES promotes repair are unknown. We hypothesize that a beneficial role of ES is dependent on electrokinetic perfusion in the extracellular space and that it mimics the effects of interstitial flow. In vivo, the extracellular space contains mixtures of extracellular proteins and negatively charged glycosaminoglycans and proteoglycans surrounding cells. While these anionic macromolecules promote water retention and increase mechanical support under compression, in the presence of ES they should also enhance electro-osmotic flow (EOF) to a greater extent than proteins alone. To test this hypothesis, we compare EOF rates between artificial matrices of gelatin (denatured collagen) with matrices of gelatin mixed with anionic polymers to mimic endogenous charged macromolecules. We report that addition of anionic polymers amplifies EOF and that a matrix comprised of 0.5% polyacrylate and 1.5% gelatin generates EOF with similar rates to those reported in cartilage. The enhanced EOF reduces mortality of cells at lower applied voltage compared to gelatin matrices alone. We also use modeling to describe the range of thermal changes that occur during these electrokinetic experiments and during electrokinetic perfusion of soft tissues. We conclude that the negative charge density of native extracellular matrices promotes electrokinetic perfusion during electrical therapies in soft tissues and may promote survival of artificial tissues and organs prior to vascularization and during transplantation.

Electrokinetic Perfusion Through Three-Dimensional Culture Reduces Cell Mortality

Cell proliferation and survival are dependent on mass transfer. In vivo, fluid flow promotes mass transfer through the vasculature and interstitial space, providing a continuous supply of nutrients and removal of cellular waste products. In the absence of sufficient flow, mass transfer is limited by diffusion and poses significant challenges to cell survival during tissue engineering, tissue transplantation, and treatment of degenerative diseases. Artificial perfusion may overcome these challenges. In this work, we compare the efficacy of pressure driven perfusion (PDP) with electrokinetic perfusion (EKP) toward reducing cell mortality in three-dimensional cultures of Matrigel extracellular matrix. We characterize electro-osmotic flow through Matrigel to identify conditions that generate similar interstitial flow rates to those induced by pressure. We also compare changes in cell mortality induced by continuous or pulsed EKP. We report that continuous EKP significantly reduced mortality throughout the perfusion channels more consistently than PDP at similar flow rates, and pulsed EKP decreased mortality just as effectively as continuous EKP. We conclude that EKP has significant advantages over PDP for promoting tissue survival before neovascularization and angiogenesis. Impact statement Interstitial flow helps promote mass transfer and cell survival in tissues and organs. This study generated interstitial flow using pressure driven perfusion (PDP) or electrokinetic perfusion (EKP) to promote cell viability in three-dimensional cultures. EKP through charged extracellular matrices possesses significant advantages over PDP and may promote cell survival during tissue engineering, transplantations, and treatment of degenerative diseases.

Convection-Enhanced Delivery and Principles of Extracellular Transport in the Brain

Stereotactic neurosurgery involves a targeted intervention based on congruence of image guidance to a reference fiducial system. This discipline has widespread applications in radiosurgery, tumor therapy, drug delivery, functional lesioning, and neuromodulation. In this article, we focused on convection-enhanced delivery to deliver therapeutic agents to the brain addressing areas of research and clinical development. We performed a robust literature review of all relevant articles highlighting current efforts and challenges of making this delivery technique more widely understood. We further described key biophysical properties of molecular transport in the extracellular space that may impact the efficacy and control of drug delivery using stereotactic methods. Understanding these principles is critical for further refinement of predictive models that can inform advances in stereotactic techniques for convection-enhanced delivery of therapeutic agents to the brain.

Electrostatic wrapping of a microfiber around a curved particle

The dynamics of the wrapping of a charged flexible microfiber around an oppositely charged curved particle immersed in a viscous fluid is investigated. We observe that the wrapping behavior varies with the radius and Young's modulus of the fiber, the radius of the particle, and the ionic strength of the surrounding solution. We find that wrapping is primarily a function of the favorable interaction energy due to electrostatics and the unfavorable deformation energy needed to conform the fiber to the curvature of the particle. We perform an energy balance to predict the critical particle radius for wrapping, finding reasonably good agreement with experimental observations. In addition, we use mathematical modeling and observations of the deflected shape of the free end of the fiber during wrapping to extract a measurement of the Young's modulus of the fiber. We evaluate the accuracy and potential limitations of this in situ measurement when compared to independent mechanical tests.

Electroosmotic Perfusion-Microdialysis Probe Created by Direct Laser Writing for Quantitative Assessment of Leucine Enkephalin Hydrolysis by Insulin-Regulated Aminopeptidase in Vivo

There are many processes that actively alter the concentrations of solutes in the extracellular space. Enzymatic reactions, either by soluble enzymes or membrane-bound ectoenzymes, and uptake or clearance are two such processes. Investigations of ectoenzymatic reactions in vivo is challenging, particularly in the brain. Studies using microdialysis have revealed some qualitative information about what enzymes may be present, but microdialysis is a sampling technique so it is not designed to control conditions such as a substrate concentration outside the probe. Micropush-pull perfusion has been used to determine which nitric oxide synthase enzymes are active in discrete regions of the rat retina. Ectopeptidases are a particularly important class of ectoenzymes. As far as it is known, the extracellular activity of active peptides in the brain is controlled by ectopeptidases. To understand ectopeptidase activity, we developed a physical probe and an accompanying method. The probe has a two-channel source that supplies substrate or substrate plus inhibitor using electroosmotic perfusion (EOP). It also has a microdialysis probe to collect products and unreacted substrate. The method provides quantitative estimates of substrate-to-product conversion and the influence of inhibitors on this process. The quantitative estimates are made possible by including a d-amino acid-containing peptide analog of the substrate in the substrate-containing solution infused. Quantitative analysis of substrate, substrate analog, and products is carried out by quantitative, online capillary liquid chromatography-tandem mass spectrometry. The electroosmotic perfusion-microdialysis probe and associated method were used to determine the effect of the selective inhibitor HFI-419 on insulin-regulated aminopeptidase (EC 3.4.11.3) in the rat neocortex.

Electrokinetic Convection-Enhanced Delivery of Solutes to the Brain

Pressure-induced infusion of solutions into brain tissue is used both in research and in medicine. In medicine, convection enhanced delivery (CED) may be used to deliver agents to localized areas of the brain, such as with gene therapy to functional targets or with deep tumors not readily amenable to resection. However, clinical trials have demonstrated mixed results from CED. CED is limited by a lack of control of the infusion flow path and may cause damage or even neurological deficits due to neuronal distortion. In laboratory research, infusions may be achieved using pressure or using brief bursts of electrical current in iontophoresis. Electrokinetic convection enhanced delivery (ECED) has the potential to deliver drugs and other bioactive substances to local regions in the brain with improved control and lower applied pressures than pressure-based CED. ECED improves control over the infusion profile because the fluid follows the electrical current path and thus can be directed. Both small molecules and macromolecules can be delivered. Here we demonstrate proof-of-principal that electrokinetic (electroosmosis and electrophoresis) convection-enhanced delivery is a viable means for delivering solutes to the brain. We assessed the volume of tissue exposed to the infusates tris(2,2'-bipyridine)ruthenium(II) and fluorescent dextrans. Control of the direction of the transport was also achieved over distances ranging from several hundred micrometers to more than 4 mm. Electrokinetic delivery has the potential to improve control over infusions.

Cellulose Nanocrystal-Polyelectrolyte Hybrids for Bentonite Water-Based Drilling Fluids

Cellulose nanocrystals (CNCs), with their rodlike shape and nanoscale dimensions, greatly improve the filtration performance of bentonite-containing, water-based drilling fluids (BT-WDFs) through interactions with the BT platelets. When these WDFs are exposed to high salt concentrations, though, their fluid retention properties are greatly diminished due to reduced CNC-BT interaction and BT aggregation/flocculation. Consequently, we reduce BT-BT interaction at high salt by grafting polyelectrolytes (PE) to CNC particles (CNC-PE) to enhance CNC-BT interactions when incorporating these hybrid particles with BT-WDFs. The particles sterically and electrostatically screen BT platelets from associating, thus improving fluid filtration performance at high salt. Three types of CNC modifications were carried out: grafting from direct surface initiation, modification with vinyl-terminated glycidyl methacrylate (GMA) before grafting, and physical mixing of CNC with a polymer. These modifications were performed using three polyelectrolyte materials: anionic polystyrene sulfonate (PSS), cationic polyacrylamide (PAM), and a random copolymer of PSS and PAM (PSS-co-PAM). Formulations containing CNC-PEs prepared by covalent grafting exhibited superior filtration properties compared to those in which CNCs and PEs were physically mixed. The higher graft loading achieved with the GMA method resulted in poorer filtration results compared to the direct grafting method due to CNC-PE interparticle cross-linking. PSS-modified CNC-PEs appeared to attach to BT edges, while PAM-modified CNC-PEs attached to the BT faces. These interactions disrupted BT aggregation, with the PSS-co-PAM CNC hybrid displaying the most desired filtration properties. The results highlight the importance of steric and charge stabilization of the BT particle edges and faces to achieve high-performance WDFs for well excavation.

Electrokinetic infusions into hydrogels and brain tissue: Control of direction and magnitude of solute delivery

BACKGROUND

Delivering solutes to a particular region of the brain is currently achieved by iontophoresis for very small volumes and by diffusion from a microdialysis probe for larger volumes. There is a need to deliver solutes to particular areas with more control than is possible with existing methods.

NEW METHOD

Electrokinetic infusions of solutes were performed into hydrogels and organotypic hippocampal slice cultures. Application of an electrical current creates electroosmotic flow and electrophoresis of a dicationic fluorescent solute through organotypic hippocampal tissue cultures or larger hydrogels. Transport was recorded with fluorescence microscopy imaging in real-time.

RESULTS

Electrokinetic transport in brain tissue slice cultures and hydrogels occurs along an electrical current path and allows for anisotropic delivery over distances from several hundred micrometers to millimeters. Directional transport may be controlled by altering the current path. The applied electrical current linearly affects the measured solute fluorescence in our model system following infusions.

COMPARISON WITH EXISTING METHODS

Localized drug delivery involves iontophoresis, with diffusion primarily occurring beyond infusion capillaries under current protocols. Pressure-driven infusions for intraparenchymal targets have also been conducted. Superfusion across a tissue surface provides modest penetration, however is unable to impact deeper targets. In general, control over intraparenchymal drug delivery has been difficult to achieve. Electrokinetic transport provides an alternative to deliver solutes along an electrical current path in tissue.

CONCLUSIONS

Electrokinetic transport may be applied to living systems for molecular transport. It may be used to improve upon the control of solute delivery over that of pressure-driven transport.

Micromechanical properties of biomedical hydrogel for application as microchannel elastomer

Polymers are believed to be the building blocks for the creation of the next generation of materials and devices in practically all areas of biomedical research. There are a number of polymers that are being employed in varied applications in microfluidic platform due to the tremendous possibilities for soft matter based elastomers especially in biomedical applications. Polymeric hydrogels have been used as building block in micro-confinements and for specified function such as flow control. The need exists to suitably determine the mechanical characteristics of gel-based materials for possible use as a microchannel elastomer. In this investigation, we describe synthesis procedure, morphological, wettability characterization of hydrogel elastomer synthesized by free-radical polymerization crosslinked over varying acrylamide composition for 10% w/v: 25% w/w, 15% w/v: 25% w/w, 20% w/v: 25% w/w and 25% w/v: 25% w/w respectively. Micromechanical properties such as surface morphology, wettability, and micro-rheological behaviour of hydrogel elastomer using standard protocols was undertaken to determine roughness, contact angle, loss modulus and storage modulus over varied cross-linking of the constituent monomers. The impact of these parameters on flow transport and microchannel structural stability is well delineated in this report. We established that polymeric hydrogel could be a candidate for whole microchannel elastomer with suitable application in areas of tissues and biomedical engineering to mimic native biological transport conduits.

IPN hydrogel nanocomposites based on agarose and ZnO with antifouling and bactericidal properties

Nanocomposite hydrogels with interpenetrating polymer network (IPN) structure based on poly(ethylene glycol) methyl ether methacrylate modified ZnO (ZnO-PEGMA) and 4-azidobenzoic agarose (AG-N3) were prepared by a one-pot strategy under UV irradiation. The hydrogels exhibited a highly macroporous spongelike structure, and the pore size decreased with the increase of the ZnO-PEGMA content. Due to the entanglement and favorable interactions between the two crosslinked networks, the IPN hydrogels exhibited excellent mechanical strength and light transmittance. The maximum compressive and tensile strengths of the IPN hydrogels reached 24.8 and 1.98 MPa respectively. The transparent IPN hydrogels transmitted more than 85% of visible light at all wavelengths (400-800 nm). The IPN hydrogels exhibited anti-adhesive property towards Gram-negative Escherichia coli (E. coli) and Gram-positive Staphylococcus aureus (S. aureus), and the bactericidal activity increased with the ZnO-PEGMA content. The incorporation of ZnO-PEGMA did not reduce the biocompatibility of the IPN hydrogels and all the IPN nanocomposites showed negligible cytotoxicity. The present study not only provided a facile method for preparing hydrogel nanocomposites with IPN structure but also developed a new hydrogel material which might be an excellent candidate for wound dressings.

Dextrin and poly(acrylic acid)-based biodegradable, non-cytotoxic, chemically cross-linked hydrogel for sustained release of ornidazole and ciprofloxacin

Herein, novel biodegradable, stimulus-responsive, chemically cross-linked and porous hydrogel has been synthesized to evaluate its applicability as an efficient carrier for sustained release of ornidazole and ciprofloxacin. The cross-linked hydrogel (c-Dxt/pAA) has been developed from dextrin and poly(acrylic acid) using N,N'-methylene bis(acrylamide) cross-linker via Michael-type addition reaction. With the variation of reaction parameters, various c-Dxt/pAA hydrogels have been synthesized to optimize the best one. c-Dxt/pAA hydrogel has been characterized using various physicochemical characterization techniques. The hydrogel demonstrates significant pH and temperature sensitivity. Gel characteristics and gel kinetics have been performed through the measurement of rheological parameters. The hydrogel shows noncytotoxic behavior toward human mesenchymal stem cells. Biodegradation study predicts that c-Dxt/pAA is degradable in nature. The in vitro release of ornidazole and ciprofloxacin suggests that the hydrogel released both the drugs in a controlled manner with extensive stability up to 3 months. The results suggest that c-Dxt/pAA is probably a promising candidate for controlled release of ornidazole and ciprofloxacin.

Modified biopolymer-dextrin based crosslinked hydrogels: application in controlled drug delivery

This review describes hydrogels and their classifications along with the synthesis and properties of biopolymer-dextrin based crosslinked hydrogels towards potential application in controlled drug delivery.

Electroosmotic perfusion of tissue: sampling the extracellular space and quantitative assessment of membrane-bound enzyme activity in organotypic hippocampal slice cultures

This review covers recent advances in sampling fluid from the extracellular space of brain tissue by electroosmosis (EO). Two techniques, EO sampling with a single fused-silica capillary and EO push-pull perfusion, have been developed. These tools were used to investigate the function of membrane-bound enzymes with outward-facing active sites, or ectoenzymes, in modulating the activity of the neuropeptides leu-enkephalin and galanin in organotypic-hippocampal-slice cultures (OHSCs). In addition, the approach was used to determine the endogenous concentration of a thiol, cysteamine, in OHSCs. We have also investigated the degradation of coenzyme A in the extracellular space. The approach provides information on ectoenzyme activity, including Michaelis constants, in tissue, which, as far as we are aware, has not been done before. On the basis of computational evidence, EO push-pull perfusion can distinguish ectoenzyme activity with a ~100 μm spatial resolution, which is important for studies of enzyme kinetics in adjacent regions of the rat hippocampus.

Electroosmotic push-pull perfusion: description and application to qualitative analysis of the hydrolysis of exogenous galanin in organotypic hippocampal slice cultures

We demonstrate here a method that perfuses a small region of an organotypic hippocampal culture with a solution containing an enzyme substrate, a neuropeptide. Perfusate containing hydrolysis products is continually collected and subsequently analyzed for the products of the enzymatic degradation of the peptide substrate. The driving force for perfusion is an electric field. The fused silica capillaries used as "push" and "pull" or "source" and "collection" capillaries have a ζ-potential that is negative and greater in magnitude than the tissue's ζ-potential. Thus, depending on the magnitudes of particular dimensions, the electroosmotic flow in the capillaries augments the fluid velocity in the tissue. The flow rate is not directly measured; however, we determine it using a finite-element approach. We have determined the collection efficiency of the system using an all d-amino acid internal standard. The flow rates are low, in the nL/min range, and adjustable by controlling the current or voltage in the system. The collection efficiency of the d-amino acid peptide internal standard is variable, increasing with increased current and thus electroosmotic flow rate. The collection efficiency can be rationalized in the context of a Peclet number. Electroosmotic push-pull perfusion of the neuropeptide galanin (gal1-29) through the extracellular space of an organotypic hippocampal culture results in its hydrolysis by ectopeptidase reactions occurring in the extracellular space. The products of hydrolysis were identified by MALDI-MS. Experiments at two levels of current (8-12 μA and 19-40 μA) show that the probability of seeing hydrolysis products (apparently from aminopeptidases) is greater in the Cornu Ammonis area 3 (CA3) than in the Cornu Ammonis area 1 (CA1) in the higher current experiments. In the lower current experiments, shorter peptide products of aminopeptidases (gal13-29 to gal20-19) are seen with greater frequency in CA3 than in CA1 but there is no statistically significant difference for longer peptides (gal3-29 to gal12-29).

Bioresponsive hydrogels

Bioresponsive hydrogels are emerging with technological significance in targeted drug delivery, biosensors, and regenerative medicine. Their ability to respond to specific biologically derived stimuli creates a design challenge in effectively linking the conferred biospecificity with an engineered response tailored to the needs of a particular application. Moreover, the fundamental phenomena governing the response must support an appropriate dynamic range, limit of detection, and the potential for feedback control. The design of these systems is inherently complicated due to the high interdependency of the governing phenomena that guide sensing, transduction, and actuation of the hydrogel. Future advancements in bioresponsive hydrogels will out of necessity contain control loops similar to synthetic metabolic pathways. The use of these materials will continue to expand as they become coupled and integrated with new technologies.

Iontophoresis from a micropipet into a porous medium depends on the ζ-potential of the medium

Iontophoresis uses electricity to deliver solutes into living tissue. Often, iontophoretic ejections from micropipets into brain tissue are confined to millisecond pulses for highly localized delivery, but longer pulses are common. As hippocampal tissue has a ζ-potential of approximately -22 mV, we hypothesized that, in the presence of the electric field resulting from the iontophoretic current, electroosmotic flow in the tissue would carry solutes considerably farther than diffusion alone. A steady state solution to this mass transport problem predicts a spherically symmetrical solute concentration profile with the characteristic distance of the profile depending on the ζ-potential of the medium, the current density at the tip, the tip size, and the solute electrophoretic mobility and diffusion coefficient. Of course, the ζ-potential of the tissue is defined by immobilized components of the extracellular matrix as well as cell-surface functional groups. As such, it cannot be changed at will. Therefore, the effect of the ζ-potential of the porous medium on ejections is examined using poly(acrylamide-co-acrylic acid) hydrogels with various magnitudes of ζ-potential, including that similar to hippocampal brain tissue. We demonstrated that nearly neutral fluorescent dextran (3 and 70 kD) solute penetration distance in the hydrogels and OHSCs depends on the magnitude of the applied current, solute properties, and, in the case of the hydrogels, the ζ-potential of the matrix. Steady state solute ejection profiles in gels and cultures of hippocampus can be predicted semiquantitatively.