Disassembly of layer-by-layer films of plasmid DNA and reducible TAT polypeptide

Polymeric Delivery of Therapeutic Nucleic Acids

The advent of genome editing has transformed the therapeutic landscape for several debilitating diseases, and the clinical outlook for gene therapeutics has never been more promising. The therapeutic potential of nucleic acids has been limited by a reliance on engineered viral vectors for delivery. Chemically defined polymers can remediate technological, regulatory, and clinical challenges associated with viral modes of gene delivery. Because of their scalability, versatility, and exquisite tunability, polymers are ideal biomaterial platforms for delivering nucleic acid payloads efficiently while minimizing immune response and cellular toxicity. While polymeric gene delivery has progressed significantly in the past four decades, clinical translation of polymeric vehicles faces several formidable challenges. The aim of our Account is to illustrate diverse concepts in designing polymeric vectors towards meeting therapeutic goals of in vivo and ex vivo gene therapy. Here, we highlight several classes of polymers employed in gene delivery and summarize the recent work on understanding the contributions of chemical and architectural design parameters. We touch upon characterization methods used to visualize and understand events transpiring at the interfaces between polymer, nucleic acids, and the physiological environment. We conclude that interdisciplinary approaches and methodologies motivated by fundamental questions are key to designing high-performing polymeric vehicles for gene therapy.

Disulfide Bridging Strategies in Viral and Nonviral Platforms for Nucleic Acid Delivery

Self-assembled nanostructures that are sensitive to environmental stimuli are promising nanomaterials for drug delivery. In this class, disulfide-containing redox-sensitive strategies have gained enormous attention because of their wide applicability and simplicity of nanoparticle design. In the context of nucleic acid delivery, numerous disulfide-based materials have been designed by relying on covalent or noncovalent interactions. In this review, we highlight major advances in the design of disulfide-containing materials for nucleic acid encapsulation, including covalent nucleic acid conjugates, viral vectors or virus-like particles, dendrimers, peptides, polymers, lipids, hydrogels, inorganic nanoparticles, and nucleic acid nanostructures. Our discussion will focus on the context of the design of materials and their impact on addressing the current shortcomings in the intracellular delivery of nucleic acids.

Programmable multi-DNA release from multilayered polyelectrolytes using gigahertz nano-electromechanical resonator

BACKGROUND

Controllable and multiple DNA release is critical in modern gene-based therapies. Current approaches require complex assistant molecules for combined release. To overcome the restrictions on the materials and environment, a novel and versatile DNA release method using a nano-electromechanical (NEMS) hypersonic resonator of gigahertz (GHz) frequency is developed.

RESULTS

The micro-vortexes excited by ultra-high frequency acoustic wave can generate tunable shear stress at solid-liquid interface, thereby disrupting molecular interactions in immobilized multilayered polyelectrolyte thin films and releasing embedded DNA strands in a controlled fashion. Both finite element model analysis and experiment results verify the feasibility of this method. The release rate and released amount are confirmed to be well tuned. Owing to the different forces generated at different depth of the films, release of two types of DNA molecules with different velocities is achieved, which further explores its application in combined gene therapy.

CONCLUSIONS

Our research confirmed that this novel platform based on a nano-electromechanical hypersonic resonator works well for controllable single and multi-DNA release. In addition, the unique features of this resonator such as miniaturization and batch manufacturing open its possibility to be developed into a high-throughput, implantable and site targeting DNA release and delivery system.

Dendrimer-grafted bioreducible polycation/DNA multilayered films with low cytotoxicity and high transfection ability

Controlled release of incorporated foreign DNA from multilayered films plays an important role in surface-mediated gene delivery. Herein, multilayered polyelectrolyte complex thin films, composed of dendrimer-grafted bio-reducible cationic poly(disulfide amine) and plasmid DNA, were fabricated via layer-by-layer (LBL) assembly for in vitro localized gene delivery. The UV absorbance and thickness of the LBL films were found to have linear correlation with the numbers of poly(disulfide amine)/DNA bilayers. Although LBL films were stable in PBS buffer, their degradation could be triggered by reducing agents (i.e. glutathione, GSH). The degradation rate of the films is directly proportional to the GSH concentration, which in turn affected the corresponding gene expression. All poly(disulfide amine)/DNA films exhibited lower cytotoxicity and higher transfection activity in comparison with PEI/DNA multilayered films. Moreover, LBL films showed the highest transfection efficiency in the presence of 2.5 mM GSH when cultured with 293T cells, with ~36% GFP-positive 293T cells after 5-days of co-culture. These DNA-containing reducible films could potentially be useful in gene therapy and tissue engineering by controlling the release of incorporated DNA.

Layer-by-layer DNA films incorporating highly transfecting bioreducible poly(amido amine) and polyethylenimine for sequential gene delivery

Background

The layer-by-layer (LbL) assembly method offers a molecular level control of the amount and spatial distribution of bioactive molecules. However, successful clinical translation of LbL film technology will most certainly require a better understanding and control of not only the film assembly process, but also film disassembly kinetics in physiologic conditions.

Purpose

This work focuses on the understanding and control of degradation properties of LbL films for localized gene delivery.

Methods

Bioreducible poly(amido amine)s (PAAs) containing cystaminebisacrylamide (CBA), methylenebisacrylamide, and 5-amino-1-pentanol (APOL) were synthesized by Michael addition polymerization for the construction of bioreducible LbL films capable of sequential gene delivery.

Results

The synthesized PAAs were screened for desirable buffering capacity, cell transfection, and cytotoxicity characteristics together with 25 kDa branched polyethylenimine (PEI) and cross-linked 800 Da PEI. By screening the various polycations we were able to identify a copolymer of CBA and APOL for the subsequent construction of the LbL films. By incorporating a highly transfecting polycation and a nondiffusing polycation we were able to improve the overall transfection of HEK293 and MC3T3 cells from the bioreducible LbL films. We also demonstrated the dual-stage release and transfection of two different DNAs from the LbL films.

Conclusion

The results indicate that LbL films consisting of bioreducible PAAs and non-diffusing polyelectrolytes have excellent degradation properties for the development of LbL coating technology for localized gene delivery applications.

Cells capture and antimicrobial effect of hydrophobically modified chitosan coating on Escherichia coli

Hydrophobically modified chitosan (HMCS), prepared by reacting alkyl aldehyde with chitosan was demonstrated to be an effective antimicrobial and transparent coating. The grafted alkyl chains exist as protruded hydrophobic tails on the HMCS coating surface. In contact with E. coli cells, HMCS coating captured and immobilized the cells via these hydrophobic tails. The hydrophobic tails could also kill the cells captured on the coating surface as visualized by fluorescence microscope. More than 50% of the initially loaded cells (2.5×10⁴ CFU) could be killed after 2h contact with HMCS coating. The cells capture and killing effects of the coating surface could be completely neutralized by treating with α-cyclodextrin to sequester the protruded hydrophobic tails. The facile coating of antimicrobial HMCS on surface also enabled the easy fabrication of patterned E. coli cells arrays.

pH-Degradable antioxidant nanoparticles based on hydrogen-bonded tannic acid assembly

Hydrogen-bonded polyphenol-based assemblies have attracted increasing interest for biomedical applications.

Triggerable Degradation of Polyurethanes for Tissue Engineering Applications

Tissue engineered and bioactive scaffolds with different degradation rates are required for the regeneration of diverse tissues/organs. To optimize tissue regeneration in different tissues, it is desirable that the degradation rate of scaffolds can be manipulated to comply with various stages of tissue regeneration. Unfortunately, the degradation of most degradable polymers relies solely on passive controlled degradation mechanisms. To overcome this challenge, we report a new family of reduction-sensitive biodegradable elastomeric polyurethanes containing various amounts of disulfide bonds (PU-SS), in which degradation can be initiated and accelerated with the supplement of a biological product: antioxidant-glutathione (GSH). The polyurethanes can be processed into films and electrospun fibrous scaffolds. Synthesized materials exhibited robust mechanical properties and high elasticity. Accelerated degradation of the materials was observed in the presence of GSH, and the rate of such degradation depends on the amount of disulfide present in the polymer backbone. The polymers and their degradation products exhibited no apparent cell toxicity while the electrospun scaffolds supported fibroblast growth in vitro. The in vivo subcutaneous implantation model showed that the polymers prompt minimal inflammatory responses, and as anticipated, the polymer with the higher disulfide bond amount had faster degradation in vivo. This new family of polyurethanes offers tremendous potential for directed scaffold degradation to promote maximal tissue regeneration.

Multilayered thin films from poly(amido amine)s and DNA

Dip-coated multilayered thin films of poly(amido amine)s (PAAs) and DNA have been developed to provide surfaces with cell-transfecting capabilities. Three types of PAAs, differing in side chain functional groups, were synthesized and characterized for their properties in forming multilayered structures with ultrasonicated calf thymus DNA (CTDNA) as model DNA. All three polymers display a multilayer build-up in linear profiles as demonstrated by UV spectroscopy. More highly charged side chains were found to provide the lowest deposition of DNA. Surface profiles of the obtained films were investigated by atomic force microscopy (AFM) and static water contact angle measurements to reveal complete surface coverage after at least four layer pair depositions, where alternating patterns of surface profiles were observed depending on whether the cationic polymer or the anionic DNA layer was on top. The stability of the formed surfaces was investigated in vitro under physiological and reductive conditions. Owing to the presence of disulfide bonds in the PAA main chain, the films were readily degraded in the presence of 1mM of DTT in vitro. Under non-reductive physiological conditions, two of the thicker films underwent thermodynamic rearrangement, which resulted in release of approximately half of the incorporated material within 1h, which was caused by the physiological salt concentration. Further, this unpacking phenomenon proved useful in transfecting COS-7 cells seeded on top of these multilayers containing functional plasmid DNA encoding for green fluorescence protein (GFP). Two out of the three different multilayers facilitated good COS-7 cell attachment, proliferation, and transfection in vitro within 2d ays of culture. Fluorescence staining further revealed the presence of DNA-containing released film material among cultured cells. The present work demonstrates the possibility of coating surfaces with thin films that are conveniently adjustable in thickness and amount of active agent to provide cell-transfecting functionality. In this manner transfection can be achieved by simply culturing cells on a multilayer-coated surface in their optimal culture condition (in the presence of serum) and without the need of removing the transfection agent to avoid cytotoxicity.

Flipped polyelectrolyte multilayer films: accessing the buried interface

Little is known concerning the interface between a polyelectrolyte multilayer, PEMU, and its substrate. Recent models suggest that excess polymer charge, compensated by counterions, remains buried within the PEMU, especially for thicker films having a nonlinear component to their growth. We report a novel approach for making free-standing multilayers of poly(diallyldimethylammonium) (PDADMA) and poly(styrenesulfonate) (PSS): after assembly on aluminum substrates, films were released by brief immersion in aqueous alkali. The multilayers were then flipped, allowing access to the initially buried substrate/PEMU interface. Experiments were performed to show that this method of release, one of many established for PEMUs, perturbed the surface and bulk of the film minimally. Film/solution and film/substrate interfaces were compared using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). AFM was used to record topography and perform nanoindentation, while XPS provided surface elemental composition. All three methods revealed data consistent with an excess of PDADMA at the buried interface. This excess PDADMA was then complexed with additional PSS to yield "nanosandwiches" of nonstoichiometric PEMU between layers of stoichiometric PEMU.

Bioresorbable polyelectrolytes for smuggling drugs into cells

There is ample evidence that biodegradable polyelectrolyte nanocapsules are multifunctional vehicles which can smuggle drugs into cells, and release them upon endogenous activation. A large number of endogenous stimuli have already been tested in vitro, and in vivo research is escalating. Thus, the interest in the design of intelligent polyelectrolyte multilayer (PEM) drug delivery systems is clear. The need of the hour is a systematic translation of PEM-based drug delivery systems from the lab to clinical studies. Reviews on multifarious stimuli that can trigger the release of drugs from such systems already exist. This review summarizes the available literature, with emphasis on the recent progress in PEM-based drug delivery systems that are receptive in the presence of endogenous stimuli, including enzymes, glucose, glutathione, pH, and temperature, and addresses different active and passive drug targeting strategies. Insights into the current knowledge on the diversified endogenous approaches and methodological challenges may bring inspiration to resolve issues that currently bottleneck the successful implementation of polyelectrolytes into the catalog of third-generation drug delivery systems.

Bioreducible polycations in nucleic acid delivery: past, present, and future trends

Polycations that are degradable by reduction of disulfide bonds are developed for applications in delivery of nucleic acids. This Feature Article surveys methods of synthesis of bioreducible polycations and discusses current understanding of the mechanism of action of bioreducible polyplexes. Emphasis is placed on the relationship between the biological redox environment and toxicity, trafficking, transfection activity, and in vivo behavior of bioreducible polycations and polyplexes.

Biotin-triggered release and transfection of DNA complexes immobilized on a substrate via biotin–avidin interaction

In this study, the highly specific biotin–avidin interaction was used to immobilize DNA complexes to a substrate. An excess of biotin was added to trigger the dissociation of DNA complexes from the substrate to mediate their release and transfection. Biotin-grafted-polyethyleneimine/DNA complexes with N/P ratio of 5 were prepared with diameter of 170.2 nm and ζ-potential of 16.1 mV. The DNA immobilized substrates were fabricated using a biotin–avidin–biotin sandwich model, which were characterized by atom force microscope and fluorescent microscope. Compared to DNA immobilization by physical adsorption, a higher DNA density of 935 ng/cm2 was observed on biotinylated substrates. Based on the in vitro release profiles, the DNA complexes immobilized on silanized substrate released faster than those on biotinylated substrate. Triggered by the addition of extra biotin, more DNA complexes were released. The transfection efficiencies of the DNA complexes immobilized on different substrates were assayed on HEK-293T cells. The highest transfection efficiency was obtained in the group of biotinylated substrate with the trigger of extra biotin. Thus, the system of demobilized DNA complexes onto a substrate by the biotin–avidin interaction and the dissociation of DNA complexes from a substrate triggered by the extra biotin provides a promising strategy for the realization of the controlled release and enhanced transgene expression of genes.

Release of polyphenolic drugs from dynamically bonded layer-by-layer films

Layer-by-layer (LbL) assembled films have been exploited for surface-mediated drug delivery. The drugs loaded in the films were usually released via diffusion or the degradation of one of the film components. Here we demonstrate that drug release can also be achieved by exploiting the dynamic nature of hydrogen-bonded LbL films. The films were fabricated from tannic acid (TA), a model polyphenolic drug, and poly(vinyl pyrrolidone) (PVPON). The driving force for the film buildup is the hydrogen bonding between the two components, which was confirmed by Fourier transform infrared (FTIR) spectra. The film growth is linear, and the growth rate of the film decreases with increasing assembly temperature. Because of the reversible/dynamic nature of hydrogen bonding, when soaked in aqueous solutions, the PVPON/TA films disassemble gradually and thus release TA to the media. The release rate of TA increases with increasing pH and temperature but decreases with increasing ionic strength. Scanning electron microscopy (SEM) studies on the surface morphology of the film during TA release reveal that the film surface becomes smoother and then rougher again because of the dewetting of the film. The released TA can scavenge ABTS(+•) cation radicals, indicating it retains its antioxidant activity, a major biological activity of polyphenols.

Addressing the use of PDIF-CN2 molecules in the development of n-type organic field-effect transistors for biosensing applications

BACKGROUND

There is no doubt that future discoveries in the field of biochemistry will depend on the implementation of novel biosensing techniques, able to record biophysiological events with minimal biological interference. In this respect, organic electronics may represent an important new tool for the analysis of structures ranging from single molecules up to cellular events. Specifically, organic field-effect transistors (OFET) are potentially powerful devices for the real-time detection/transduction of bio-signals. Despite this interest, up to date, the experimental data useful to support the development of OFET-based biosensors are still few and, in particular, n-type (electron-transporting) devices, being fundamental to develop highly-performing circuits, have been scarcely investigated.

METHODS

Here, films of N,N'-1H,1H-perfluorobutyldicyanoperylene-carboxydi-imide (PDIF-CN2) molecules, a recently-introduced and very promising n-type semiconductor, have been evaporated on glass and silicon dioxide substrates to test the biocompatibility of this compound and its capability to stay electrically-active even in liquid environments.

RESULTS

We found that PDIF-CN2 transistors can work steadily in water for several hours. Biocompatibility tests, based on in-vitro cell cultivation, remark the need to functionalize the PDIF-CN2 hydrophobic surface by extra-coating layers (i.e. poly-l-lysine) to favor the growth of confluent cellular populations.

CONCLUSIONS

Our experimental data demonstrate that PDIF-CN2 compound is an interesting organic semiconductor to develop electronic devices to be used in the biological field.

GENERAL SIGNIFICANCE

This work contributes to define a possible strategy for the fabrication of low-cost and flexible biosensors, based on complex organic complementary metal-oxide-semiconductor (CMOS) circuitry including both p- (hole-transporting) and n-type transistors. This article is part of a Special Issue entitled Organic Bioelectronics-Novel Applications in Biomedicine.

Layer thickness of hydrophobin films leads to oscillation in wettability

In nanobiotechnology, the properties of surfaces are often key to sensor applications. If analytes possess a low tolerance or affinity regarding the sensory substrate (surface), then the setup of mediators may be indicated. Hydrophobins enable biocompatible surface functionalization without significant restrictions of the physicochemical substrate properties. Because of the imperfect formation of hydrophobin films, a high variation in surface properties is observed. In this study, we report on the relation between the film thickness of hydrophobin-coated solid surfaces and their wettability. We found that the wettability of protein-coated surfaces strictly depends on the amount of adsorbed protein, as reflected in an oscillation of the contact angles of hydrophobin-coated silicon wafers. Fusion proteins of Ccg2 and HFBI, representatives of class I and II hydrophobins, document the influence of fused peptide tags on the wettability. The orientation of the first crystal nuclei plays a decisive role in the formation of the growing hydrophobin layers. Here, a simple method of deducing the film thickness of hydrophobin assemblies on solid surfaces is presented. The determination of the static contact angle allows the prediction of which part of the protein is exposed to possible analytes.

Rapid release of plasmid DNA from surfaces coated with polyelectrolyte multilayers promoted by the application of electrochemical potentials

We report an approach to the rapid release of DNA based on the application of electrochemical potentials to surfaces coated with polyelectrolyte-based thin films. We fabricated multilayered polyelectrolyte films (or "polyelectrolyte multilayers", PEMs) using plasmid DNA and a model hydrolytically degradable cationic poly(β-amino ester) (polymer 1) on stainless steel substrates using a layer-by-layer approach. The application of continuous reduction potentials in the range of -1.1 to -0.7 V (vs a Ag/AgCl electrode) to film-coated electrodes in PBS at 37 °C resulted in the complete release of DNA over a period of 1-2 min. Film-coated electrodes incubated under identical conditions in the absence of applied potentials required 1-2 days for complete release. Control over the magnitude of the applied potential provided control over the rate at which DNA was released. The results of these and additional physical characterization experiments are consistent with a mechanism of film disruption that is promoted by local increases in pH at the film/electrode interface (resulting from electrochemical reduction of water or dissolved oxygen) that disrupt ionic interactions in these materials. The results of cell-based experiments demonstrated that DNA was released in a form that remains intact and able to promote transgene expression in mammalian cells. Finally, we demonstrate that short-term (i.e., non-continuous) electrochemical treatments can also be used to promote faster film erosion (e.g., over 1-2 h) once the potential is removed. Past studies demonstrate that PEMs fabricated using polymer 1 can promote surface-mediated transfection of cells and tissues in vitro and in vivo. With further development, the electrochemical approaches reported here could thus provide new methods for the rapid, triggered, or spatially patterned transfer of DNA (or other agents) from surfaces of interest in a variety of fundamental and applied contexts.

Characterization of degradable polyelectrolyte multilayers fabricated using DNA and a fluorescently-labeled poly(β-amino ester): shedding light on the role of the cationic polymer in promoting surface-mediated gene delivery

Polyelectrolyte multilayers (PEMs) fabricated from cationic polymers and DNA have been investigated broadly as materials for surface-mediated DNA delivery. One attractive aspect of this "multilayered" approach is the potential to exploit the presence of cationic polymer "layers" in these films to deliver DNA to cells more effectively. Past studies demonstrate that these films can promote transgene expression in vitro and in vivo, but significant questions remain regarding roles that the cationic polymers could play in promoting the internalization and processing of DNA. Here, we report physicochemical and in vitro cell-based characterization of DNA-containing PEMs fabricated using fluorescently end-labeled derivatives of a degradable polycation (polymer 1) used in past studies of surface-mediated transfection. This approach permitted simultaneous characterization of polymer and DNA in solution and in cells using fluorescence-based techniques, and provided information about the locations and behaviors of polymer 1 that could not be obtained using other methods. LSCM and flow cytometry experiments revealed that polymer 1 and DNA released from film-coated objects were both internalized extensively by cells and that they were colocalized to a significant extent inside cells (e.g., ~58% of DNA was colocalized with polymer). Fluorescence anisotropy measurements of solutions containing partially eroded films were also consistent with the presence of aggregates of polymer 1 and DNA in solution (e.g., after release from surfaces, but prior to internalization by cells). Our results support the view that polymer 1, which is incorporated into these materials as "layers" rather than as part of optimized, preformed "polyplexes", can act to promote or enhance surface-mediated DNA delivery. More broadly, our results suggest opportunities to improve the delivery properties of DNA-containing PEMs by incorporation of additional "layers" of other conventional cationic polymers designed to address specific intracellular barriers to transfection, such as endosomal escape, more effectively.

Tunable disintegration of layer-by-layer assembly multilayer films based on hydrolytical-polybetaine at wide-range time

A cationic hydrolytical-polycarboxybetaine (HPCB), poly(N-ethyl acetate-4-vinylpyridinium bromide) was synthesized by incorporating ester group into the side chain of polycarboxybetaine (PCB). The hydrolytic behaviors of HPCB samples in pH 7.4 phosphate buffer saline (PBS) were investigated by FT-IR and (1)H NMR. The layer-by-layer (LbL) assembly of HPCB/poly (sodium 4-styrenesulfonate) PSS and the disintegration of HPCB/PSS multilayer films were monitored by UV-vis absorption spectroscopy, quartz crystal microbalance (QCM) and atomic force microscopy (AFM). The disintegrated behavior of multilayer films was studied in detail by changing the cationic degree of HPCB and the pH of the immersion solution (PBS) in the disintegration process. The disintegration time of HPCB/PSS multilayer films could be controlled widely from 2 min to 30 days in PBS.

Polymer assemblies for controlled delivery of bioactive molecules from surfaces

Localized delivery of bioactive compounds from surfaces of biomedical devices affords significant therapeutic benefits, and often relies on the capability of surface coatings to provide spatial and temporal control over release rate. The layer-by-layer technique presents a unique means to construct surface coatings that can conform to a variety of biomaterial surfaces and serve as matrices enabling controlled delivery of bioactive molecules from surfaces. The versatility of layer-by-layer assembly enables construction of surface coatings of diverse chemistry and internal architecture with controlled release properties. This review focuses on recent developments in constructing such layered matrices using linear polymers, polymer nanoparticles and block copolymer micelles, including micelles with stimuli-responsive cores, as film building blocks and in controlling release rate of therapeutics from these matrices via degradation, application of pH, ionic strength, temperature, light, electric field and chemical or biological stimuli. Challenges and opportunities associated with fabrication of stratified multilayer films capable of multi-stage delivery of multiple drugs are also discussed.

Release of DNA from polyelectrolyte multilayers fabricated using 'charge-shifting' cationic polymers: tunable temporal control and sequential, multi-agent release

We report an approach to the design of multilayered polyelectrolyte thin films (or 'polyelectrolyte multilayers', PEMs) that can be used to provide tunable control over the release of plasmid DNA (or multiple different DNA constructs) from film-coated surfaces. Our approach is based upon methods for the layer-by-layer assembly of DNA-containing thin films, and exploits the properties of a new class of cationic 'charge-shifting' polymers (amine functionalized polymers that undergo gradual changes in net charge upon side chain ester hydrolysis) to provide control over the rates at which these films erode and release DNA. We synthesized two 'charge-shifting' polymers (polymers 1 and 2) containing different side chain structures by ring-opening reactions of poly(2-alkenyl azlactone)s with two different tertiary amine functionalized alcohols (3-dimethylamino-1-propanol and 2-dimethylaminoethanol, respectively). Subsequent characterization revealed large changes in the rates of side chain ester hydrolysis for these two polymers; whereas the half-life for the hydrolysis of the esters in polymer 1 was ~200 days, the half-life for polymer 2 was ~6 days. We demonstrate that these large differences in side chain hydrolysis make possible the design of PEMs that erode and promote the surface-mediated release of DNA either rapidly (e.g., over ~3 days for films fabricated using polymer 2) or slowly (e.g., over ~1 month for films fabricated using polymer 1). We demonstrate further that it is possible to design films with release profiles that are intermediate to these two extremes by fabricating films using solutions containing different mixtures of these two polymers. This approach can thus expand the usefulness of these two polymers and achieve a broader range of DNA release profiles without the need to synthesize polymers with new structures or properties. Finally, we demonstrate that polymers 1 and 2 can be used to fabricate multilayered films with hierarchical structures that promote the sequential release of two different DNA constructs with separate and distinct release profiles (e.g., the release of a first construct over a period of ~3 days, followed by the sustained release of a second for a period of ~70 days). With further development, this approach could contribute to the design of functional thin films and surface coatings that provide sophisticated control over the timing and the order of the release of two or more DNA constructs (or other agents) of interest in a range of biomedical contexts.

A reducible polycationic gene vector derived from thiolated low molecular weight branched polyethyleneimine linked by 2-iminothiolane

To improve transfection efficiency and reduce the cytotoxicity of polymeric gene vectors, reducible polycations (RPC) were synthesized from low molecular weight (MW) branched polyethyleneimine (bPEI) via thiolation and oxidation. RPC (RPC-bPEI(0.8 kDa)) possessed MW of 5 kDa-80 kDa, and 50%-70% of the original proton buffering capacity of bPEI(0.8 kDa) was preserved in the final product. The cytotoxicity of RPC-bPEI(0.8 kDa) was 8-19 times less than that of the gold standard of polymeric transfection reagents, bPEI(25 kDa). Although bPEI(0.8 kDa) exhibited poor gene condensing capacities (∼2 μm at a weight ratio (WR) of 40), RPC-bPEI(0.8 kDa) effectively condensed plasmid DNA (pDNA) at a WR of 2. Moreover, RPC-bPEI(0.8 kDa)/pDNA (WR ≥2) formed 100-200 nm-sized particles with positively charged surfaces (20-35 mV). In addition, the results of the present study indicated that thiol/polyanions triggered the release of pDNA from RPC-bPEI(0.8 kDa)/pDNA via the fragmentation of RPC-bPEI(0.8 kDa) and ion-exchange. With negligible polyplex-mediated cytotoxicity, the transfection efficiencies of RPC-bPEI(0.8 kDa)/pDNA were approximately 1200-1500-fold greater than that of bPEI(0.8 kDa)/pDNA and were equivalent or superior (∼7-fold) to that of bPEI(25 kDa)/pDNA. Interestingly, the distribution of high MW RPC-bPEI(0.8 kDa)/pDNA in the nucleus of the cell was higher than that of low MW RPC-bPEI(0.8 kDa)/pDNA. Thus, the results of the present study suggest that RPC-bPEI(0.8 kDa) has the potential to effectively deliver genetic materials with lower levels of toxicity.

Layer-by-layer-assembled capsules and films for therapeutic delivery

Polymeric materials formed via layer-by-layer (LbL) assembly have promise for use as drug delivery vehicles. These multilayered materials, both as capsules and thin fi lms, can encapsulate a high payload of toxic or sensitive drugs, and can be readily engineered and functionalized with specific properties. This review highlights important and recent studies that advance the use of LbL-assembled materials as therapeutic devices. It also seeks to identify areas that require additional investigation for future development of the field. A variety of drug-loading methods and delivery routes are discussed. The biological barriers to successful delivery are identified, and possible solutions to these problems are discussed. Finally, state-of-the-art degradation and cargo release mechanisms are also presented.

Biocleavable Polycationic Micelles as Highly Efficient Gene Delivery Vectors

An amphiphilic disulfide-containing polyamidoamine was synthesized by Michael-type polyaddition reaction of piperazine to equimolar N, N'-bis(acryloyl)cystamine with 90% yield. The polycationic micelles (198 nm, 32.5 mV), prepared from the amphiphilic polyamidoamine by dialysis method, can condense foreign plasmid DNA to form nanosized polycationic micelles/DNA polyelectrolyte complexes with positive charges, which transfected 293T cells with high efficiency. Under optimized conditions, the transfection efficiencies of polycationic micelles/DNA complexes are comparable to, or even higher than that of commercially available branched PEI (Mw 25 kDa).

DNA release dynamics from bioreducible layer-by-layer films

DNA release dynamics from layer-by-layer (LbL) films is an important aspect to consider with regards to localized gene delivery systems. The rate of DNA release and the condensation state of DNA during release are of particular interest in the field of gene delivery. A hyperbranched poly(amido amine) (RHB) containing bioreducible disulfide bonds is used to form interpolyelectrolyte complexes with DNA during LbL film assembly. During film disassembly, DNA is released in physiologic conditions due to the reducing nature of the RHB. Uncondensed DNA deposited on the surface was compared to DNA condensed by RHB in polyplex form by using two types of LbL films, RHB/DNA/RHB and polyplex terminated films, RHB/DNA/polyplex. LbL films with up to three layers are used in order to facilitate high-resolution atomic force microscopy (AFM) imaging. X-ray reflectivity, ellipsometry, and Fourier transform infrared spectroscopy are also used. The film disassembly, rearrangement, and release of molecules from the surface due to thiol-disulfide exchange is conducted in reducing dithiothreitol (DTT) solutions. Salt is found to accelerate the overall rate of film disassembly. Additionally, it was found that the polyplex layer disassembles faster than the DNA layer. The predominant intermediate structure is the toroid structure for the polyplex layer and the fiber bundle structure for the DNA layer during film disassembly. This study offers a simple means to modulate DNA release from LbL films by utilizing both condensed and uncondensed DNA in different layers. The study highlights nanostructures, toroids, and bundles as dominant intermediate DNA structures during DNA release from LbL films.

Drug releasing polymer thin films: new era of surface-mediated drug delivery

Polymer films and coatings are among the popular and most successful tools to modulate surface properties of biomaterials, specifically tissue responses and fouling behavior. Over the past decade, a novel opportunity has been widely investigated, namely utility of surface coatings in surface-mediated drug delivery. In these applications, deposited polymer films act as both a coating to modulate surface properties and a reservoir for active therapeutic cargo. The field has recently accelerated beyond the proof-of-concept reports toward delivering practical solutions and established technologies for biomedical applications. This review briefly summarizes the recent successes of polymer thin films, specifically those constructed by sequential polymer deposition technique, in surface-mediated drug delivery.

Cross-linked bioreducible layer-by-layer films for increased cell adhesion and transgene expression

The effect of cross-linking layer-by-layer (LbL) films consisting of bioreducible poly(2-dimethylaminoethyl methacrylate) (rPDMAEMA) and DNA is examined with regard to rigidity, biodegradability, cell adhesion, and transfection activity using 1,5-diiodopentane (DIP) cross-linker. DIP chemically reacts with the tertiary amines of rPDMAEMA, altering the chemical composition of these LbL films. The result is a change in surface morphology, film swelling behavior, and film rigidity, measured with AFM and ellipsometry. It is found that the apparent Young's modulus is increased more than 4 times its original value upon cross-linking. Cross-linking mass is additionally confirmed with a quartz crystal microbalance with dissipation (QCM-D). Comprehensive analyses of these experimental values were investigated to calculate the degree of cross-linking using the rubber elasticity theory and the Flory-Rehner theory. Additionally, the Flory-Huggins parameter, chi, was calculated. Good agreement in the two methods yields a cross-linking density of approximately 0.82 mmol/cm(3). The Flory-Huggins parameter increased upon cross-linking from 1.07 to 1.2, indicating increased hydrophobicity of the network and formation of bulk water droplets within the films. In addition, the effects of cross-linking on film disassembly by 1,4-dithiothreitol (DTT) are found to be insignificant despite the alteration in film rigidity. Mouse fibroblast cells and smooth muscle cells are used to study the effect of cross-linking on cell adhesion and cell transfection activity. In vitro transfection activity up to seven days is quantified using secreted alkaline phosphatase (SEAP) DNA. Film cross-linking is found to enhance cell adhesion and prolong the duration of cellular transfection. These results contribute to the development of bioreducible polymer coatings for localized gene delivery.

DNA release dynamics from bioreducible poly(amido amine) polyplexes

The DNA release dynamics of bioreducible poly(amido amine) polyplexes were studied in real time by atomic force microscopy (AFM). DNA release is triggered by a depolymerization of high-molecular-weight polycations into low-molecular-weight oligocations that occurs by means of the thiol and disulfide exchange reaction mechanism. AFM images were captured in a simulated physiological reducing environment that used dithiothreitol. Distinctive stages of disassembly are common among various polyplexes that have different disulfide content, molecular weight, and polymer architecture, while the DNA release rate depends upon the disulfide content. In the first stage, polyplexes evolve from metastable structures into the more stable toroid structure upon the depolymerization. In the second stage, toroids either aggregate or fuse into larger toroids. In the last stage, DNA wormlike chains and loops are held by a central compact core. The results confirm the prospect of bioreducible poly(amido amine)s as controlled DNA delivery vectors. The study offers new physical insights into the DNA release pathway including intermediate structures that have a high degree of structural heterogeneity and disassembly induced particle growth. The study identifies disassembly induced colloidal and morphological instability as an important issue to be addressed.

Multiple functionalities of polyelectrolyte multilayer films: new biomedical applications

The design of advanced functional materials with nanometer- and micrometer-scale control over their properties is of considerable interest for both fundamental and applied studies because of the many potential applications for these materials in the fields of biomedical materials, tissue engineering, and regenerative medicine. The layer-by-layer deposition technique introduced in the early 1990s by Decher, Moehwald, and Lvov is a versatile technique, which has attracted an increasing number of researchers in recent years due to its wide range of advantages for biomedical applications: ease of preparation under "mild" conditions compatible with physiological media, capability of incorporating bioactive molecules, extra-cellular matrix components and biopolymers in the films, tunable mechanical properties, and spatio-temporal control over film organization. The last few years have seen a significant increase in reports exploring the possibilities offered by diffusing molecules into films to control their internal structures or design "reservoirs," as well as control their mechanical properties. Such properties, associated with the chemical properties of films, are particularly important for designing biomedical devices that contain bioactive molecules. In this review, we highlight recent work on designing and controlling film properties at the nanometer and micrometer scales with a view to developing new biomaterial coatings, tissue engineered constructs that could mimic in vivo cellular microenvironments, and stem cell "niches."

Tuning the formation and degradation of layer-by-layer assembled polymer hydrogel microcapsules

Engineered polymer capsules are finding widespread importance in the delivery of encapsulated toxic or fragile drugs. The effectiveness of polymer capsules as therapeutic delivery vehicles is often dependent on the degradation behavior of the capsules because it is often necessary to release the encapsulated drugs at specific times and in certain locations. Herein we investigate the parameters that govern the formation and degradation of a recently introduced new class of polymer hydrogel capsules based on disulfide cross-linked poly(methacrylic acid). We report a new and efficient method for the synthesis of thiol-functionalized poly(methacrylic acid) (PMA(SH)), the main component of the capsules. Polymeric capsules were synthesized by the layer-by-layer deposition of PMA(SH) and poly(vinylpyrrolidone) (PVPON) on silica particle templates, followed by cross-linking the PMA(SH) layers and removing PVPON and the template particles. The disulfide cross-links provided a redox-active trigger for degradation that was initiated by a cellular concentration of glutathione. We demonstrate that increasing the degree of PMA(SH) thiol modification affords direct control over the thickness of the polymer film and the degradation rate of the polymer capsules. Furthermore, the degradation rate of the PMA(SH) capsules was independent of film thickness, suggesting a bulk erosion process.

Postdiffusion of oligo-peptide within exponential growth multilayer films for localized peptide delivery

The multilayers of poly(L-lysine) (PLL) and hyaluronic acid (HA) were constructed by alternating deposition of PLL at high pH and HA at low pH. The exponential growth of the multilayer was proved to be amplified by increasing the pH difference between the two deposition solutions. The exponential growth multilayers of PLL/HA assembled at different pH were utilized as reservoirs for loading a trans-activating transcriptional factor (TAT) peptide. The confocal laser scanning microscopy (CLSM) results indicated that the FITC-labeled TAT could diffuse throughout the exponentially growing PLL/HA film. The amount of peptide embedded within multilayer could be adjusted by both multilayer assembly pH and the TAT loading pH. Compared with (PLL/HA 6.5/6.5)5 multilayer (PLL/HA a/b means that the multilayer film was constructed by using PLL at pH a and HA at pH b), the (PLL/HA 9.5/2.9)5 film can be loaded with more TAT peptide at the same loading pH 6.5. The excess of positively charged TAT peptide within (PLL/HA 9.5/2.9)5 film could not only be ascribed to its extraordinary thickness but also be attributed to its uncompensated negative charge density enhanced by the pH difference between film buildup and peptide loading process. Increasing of the TAT loading pH from 6.5 to 9.5, which increases the pH difference between multilayer assembly and peptide loading process, enhances the uncompensated charge density within (PLL/HA 9.5/2.9)5 film and elevates the peptide density from 13.8 to 25.0 microg/cm2. Compared with direct layer-by-layer assembly of TAT and HA, the postdiffusion of TAT into (PLL/HA 9.5/2.9)5 film was loaded much more peptide. The postdiffusion of peptide into a rapid growth multilayer can be more favorable to load and sustainedly release functional oligo-peptide. The cell culture results indicated that the TAT embedded within the film maintained the ability to traverse across the Hep G2 cell membrane. The functionalized (PLL/HA 9.5/2.9)5 TAT 9.5 film was more efficient than the equivalent amount of free TAT peptide in the TAT uptake test. The postdiffusion of oligo-peptide within an exponential growth multilayer can serve as an effective approach for localized and sustained peptide delivery.

Influence of the molecular weight of bioreducible oligoethylenimine conjugates on the polyplex transfection properties

The purpose of the present study was to investigate the influence of molecular weights on the chemical, biophysical, and biological properties of bioreducible oligoethylenimine conjugates. The cationic conjugates were synthesized by polyaddition between branched oligoethylenimine 800 Da (OEI) and the disulfide bond containing N,N'-cystamine bisacrylamide (CBA) linker. A correlation between the copolymer molecular weights and the polyplex transfection properties was found. The OEI-CBA copolymers differing in molecular weights (from 8.6 to 37 kDa) showed good plasmid DNA binding ability resulting in compact 90- to 150-nm-sized polyplexes. Colloidal stability of the polyplexes was lost in reductive environment. A low concentration of dithiothreitol of 5 microM was sufficient to render polyplexes unstable in size. Reducing conditions at physiological salt concentration triggered polyplex dissociation. The bioreducible polymers displayed much lower cytotoxicity (IC(50) approximately 100 microg/mL in cell culture) than branched polyethylenimine 25 kDa (BPEI) and linear polyethylenimine 22 kDa (LPEI). Reporter gene transfection experiments were done with CHO-K1 and B16-F10 cells. The largest (37 kDa) copolymer HC-6-8 demonstrated highest transfection levels among all the bioreducible copolymers, which was comparable with LPEI and much more effective than BPEI.

Surface-Mediated Delivery of DNA: Cationic Polymers Take Charge

The last several years have seen a significant increase in the number of reports describing the use of cationic polymers to design new materials and nanoscale assemblies that promote the surface-mediated delivery of DNA to cells and tissues. In general, these approaches fall into one of two broad categories: (i) methods based upon the physical adsorption of preformed, colloidal aggregates of cationic polymer and DNA (polyplexes) to surfaces, and (ii) methods for the layer-by-layer adsorption of DNA and cationic polymers on surfaces to fabricate multilayered thin films that provide control over the release of DNA into solution or to cells. In this Opinion, we discuss several recent examples of each of these approaches and provide commentary on ways in which the physical and chemical behaviours of cationic polymers have played key roles and present future opportunities to develop new methods for localized and surface-mediated cell transfection in vitro and in vivo.