Synthesis and characterization of stimuli-sensitive hydrogels having a different length of ethylene glycol chains carrying phosphate groups: loading and release of lysozyme

Effective pH-regulated release of covalently conjugated antibiotics from antibacterial hydrogels

pH-regulated release of antibiotics is achieved by conjugation with the hydrogel matrix through the reversible imine bond.

Responsive Polymers as Smart Nanomaterials Enable Diverse Applications

Responsive polymers undergo reversible or irreversible physical or chemical modifications in response to a change in environment or stimulus, e.g., temperature, pH, light, and magnetic or electric fields. Polymeric nanoparticles (NPs), which constitute a diverse set of morphologies, including micelles, vesicles, and core-shell geometries, have been successfully prepared from responsive polymers and have shown great promise in applications ranging from drug delivery to catalysis. In this review, we summarize pH, thermo-, photo-, and enzymatic responsiveness for a selection of polymers. We then discuss the formation of NPs made from responsive polymers. Finally, we highlight how NPs and other nanomaterials are enabling a wide range of smart applications with improved efficiency, as well as improved sustainability and recyclability of polymeric systems.

Sustained tobramycin release from polyphosphate double network hydrogels

Sustained local delivery of antibiotics from a drug reservoir to treat or prevent bacterial infections can avoid many of the drawbacks of systemic administration of antibiotics. Prolonged local release of high concentrations of antibiotics may also be more effective at treating bacteria in established biofilm populations that are resistant to systemic antibiotics. A double network hydrogel comprising an organic polyphosphate pre-polymer network polymerized within a polyacrylamide network de-swelled to about 50% of its initial volume when the polyphosphate network was crosslinked with polycationic tobramycin, an aminoglycoside antibiotic. The antibiotic-loaded hydrogels contained approximately 200mg/ml of tobramycin. The hydrogels continuously released daily amounts of tobramycin above the Pseudomonas aeruginosa minimal bactericidal concentration for greater than 50days, over the pH range 6.0-8.0, and completely eradicated established P. aeruginosa biofilms within 72h in a flow cell bioreactor. The presence of physiological concentrations of Mg²⁺ and Ca²⁺ ions doubled the cumulative release over 60days. The polyphosphate hydrogels show promise as materials for sustained localized tobramycin delivery to prevent post-operative P. aeruginosa infections including infections established in biofilms.

STATEMENT OF SIGNIFICANCE

Polyphosphate hydrogels were loaded with high concentrations of tobramycin. The hydrogels provided sustained release of bactericidal concentrations of tobramycin for 50days, and were capable of completely eradicating P. aeruginosa in established biofilms. The hydrogels have potential for localized prevention or treatment of P. aeruginosa infections.

pH-Responsive polymers

This review summarizes pH-responsive monomers, polymers and their derivative nano- and micro-structures including micelles, cross-linked micelles, microgels and hydrogels.

Covalently-crosslinked mucin biopolymer hydrogels for sustained drug delivery

The sustained delivery of both hydrophobic and hydrophilic drugs from hydrogels has remained a challenge requiring the design and scalable production of complex multifunctional synthetic polymers. Here, we demonstrate that mucin glycoproteins, the gel-forming constituents of native mucus, are suitable for assembly into robust hydrogels capable of facilitating the sustained release of hydrophobic and hydrophilic drugs. Covalently-crosslinked mucin hydrogels were generated via exposure of methacrylated mucin to ultraviolet light in the presence of a free radical photoinitiator. The hydrogels exhibited an elastic modulus similar to that of soft mammalian tissue and were sensitive to proteolytic degradation by pronase. Paclitaxel, a hydrophobic anti-cancer drug, and polymyxin B, a positively-charged hydrophilic antibacterial drug, were retained in the hydrogels and released linearly with time over seven days. After four weeks of drug release, the hydrogels continued to release sufficient amounts of active paclitaxel to reduce HeLa cell viability and sufficient amounts of active polymyxin B to prevent bacterial proliferation. Along with previously-established anti-inflammatory, anti-viral, and hydrocarbon-solubilizing properties of mucin, the results of this study establish mucin as a readily-available, chemically-versatile, naturally-biocompatible alternative to complex multifunctional synthetic polymers as building blocks in the design of biomaterials for sustained drug delivery.

Mechanical & cell culture properties of elastin-like polypeptide, collagen, bioglass, and carbon nanosphere composites

Collagen, the most commonly used extra-cellular matrix protein for tissue engineering applications, displays poor mechanical properties. Here, we report on the preparation and characterization of novel multi-component composite systems that incorporate a genetically engineered, biocompatible polymer (elastin-like polypeptide, ELP), biodegradable ceramic (45S5 bioglass), carbon nanosphere chains (CNSC), and minimal amount (~25% w/w) of collagen. We hypothesized that incorporation of bioglass and CNSC would improve mechanical properties of the composites. Our results showed that the tensile strength and elastic modulus nearly doubled after addition of the bioglass and CNSC compared to the control ELP-collagen hydrogels. Further, MC3T3-E1 pre-osteoblasts were cultured within the composite hydrogels and a thorough biochemical and morphological characterization was performed. Live/dead assay confirmed high cell viability (>95%) for all hydrogels by day 21 of culture. Alkaline phosphatase (ALP) activity and osteocalcin (OCN) production assessed the pre-osteoblast differentiation. Normalized ALP activity was highest for the cells cultured within ELP-bioglass-collagen hydrogels, while normalized OCN production was equivalent for all hydrogels. Alizarin red staining confirmed the mineral deposition by the cells within all hydrogels. Thus, the multi-component composite hydrogels displayed improved mechanical and cell culture properties and may be suitable scaffold materials for bone tissue engineering.

Polyalkylcyanoacrylates as in situ formed diffusion barriers in multimaterial drug carriers

Polymeric hydrogels typically release their drug payload rapidly due to their high water content and the diffusivity for drug molecules. This study proposes a multimaterial system to sustain the release by covering the hydrogel with a poly(alkyl-2-cyanoacrylate) [PACA]-based film, which should be formed by an in situ polymerization on the hydrogel surface initiated upon contact with water. A series of PACA-hydrogel hybrid systems with increasing PACA side chain hydrophobicity was prepared using physically crosslinked alginate films and hydrophilic diclofenac sodium as model hydrogel/drug system. Successful synthesis of PACA at the hydrogel surface was confirmed and the PACA layer was identified to be most homogeneous for poly(n-butyl-2-cyanoacrylate) on both the micro- and nanolevel. At the same time, the diclofenac release from the hybrid systems was substantially sustained from ~1day for unmodified hydrogels up to >14days depending on the type of PACA employed as diffusion barrier. Overall, in situ polymerized PACA films on hydrogels may be widely applicable to various hydrogel matrices, different matrix sizes as well as more complex shaped hydrogel carriers.

Preparation and characterization of elastin-like polypeptide scaffolds for local delivery of antibiotics and proteins

Tissue engineering applications could benefit from simultaneous release of growth factors, signaling molecules, and antibiotics to obtain optimal healing of tissues. Elastin-like polypeptides (ELPs) are genetically engineered polymers that possess good biocompatibility, are biodegradable, and exhibit mechanical properties similar to natural elastin. In addition, ELPs exhibit a characteristic inverse phase transition temperature (T(t)). This T(t) behavior is widely exploited in hyperthermia mediated drug delivery. The objectives of this research were to prepare ELP hydrogel scaffolds using a novel ultrasonication method and to investigate the release of a model protein (bovine serum albumin, BSA) and a commonly used antibiotic in periodontal therapy (doxycycline) from the scaffolds at two different temperatures (25 °C <T(t) vs. 37 °C >T(t)). Both BSA and doxycycline showed a gradual time dependent release and showed a trend of higher release fractions with higher loading doses. Based on the comparison between the release profiles at the two selected temperatures, the release was higher at 37 °C compared to that at 25 °C for both the loading concentrations of doxycycline (0.05 and 0.1 % v/v) and only one of the loading concentrations of BSA (0.5 % v/v) studied, while the release was higher at 25 °C compared to that at 37 °C only for the other loading concentration of BSA (1 % v/v) studied. These results suggested that the drug molecular weight and loading concentration were significant factors that affected the release kinetics. The experiments in this study demonstrated that the ELP hydrogel scaffolds can successfully release proteins and antibiotics critical to tissue engineering.

Development of swellable local implants of a polyethyleneimine-poly(vinyl pyrrolidone) (PEI-PVP) hydrogel as a socket filler

In this study, hydrogels composed of polyethyleneimine (PEI) and poly(vinyl pyrrolidone) K90 (PVP) cross-linked with various concentrations (0, 0.125, 0.25 and 0.5%) of glutaraldehyde were evaluated as a hydrogel filler for the local delivery of lidocaine after tooth extraction. The drug-release kinetics, swellability, cytotoxicity and wound healing after tooth extraction of these non-cross-linked and cross-linked PEI-PVP hydrogels were examined in male beagles and compared to values using Spongostan(®). Results demonstrated that the extent of cross-linking influenced the swelling of the resulting hydrogel, but the drug-release rates were similar. No significant changes were observed in gingival fibroblasts in contact with the PEI- PVP hydrogels or Spongostan(®). In the in vivo study, PEI-PVP hydrogels showed good retention in the socket for 2 days and showed comparable wound-healing rates within 2 weeks with those of Spongostan(®). In conclusion, PEI-PVP hydrogels are suitable for use as socket-dressing materials, and the release of local anaesthesia from PEI-PVP hydrogels can be sustained for a desirable period of time to prevent pain after a tooth extraction.

Time controlled release of arabinofuranosylcytosine (Ara-C) from agarose hydrogels using layer-by-layer assembly: an in vitro study

Experimentally induced axonal regeneration is compromised by glial scar formation arising from leptomeningeal fibroblasts cells in and around the hydrogel scaffold implanted for nerve repair. Strategies are needed to prevent such fibroblastic reactive cell layer formation for enhanced axonal regeneration. Here, we implement the technique of layer-by-layer assembled degradable, hydrogen bonded multilayers on agarose hydrogels to incorporate an anti-mitotic drug (1-β-D-arabinofuranosylcytosine (Ara-C)) within the agarose hydrogels. We show controlled release of Ara-C under physiological conditions over a period of days. The concentrations of Ara-C released from agarose at the different time points were sufficient to inhibit fibroblast growth in vitro, while not adversely affecting the viability of the neuronal cells.

Time Controlled Protein Release from Layer-by-Layer Assembled Multilayer Functionalized Agarose Hydrogels

Axons of the adult central nervous system exhibit an extremely limited ability to regenerate after spinal cord injury. Experimentally generated patterns of axon growth are typically disorganized and randomly oriented. Support of linear axonal growth into spinal cord lesion sites has been demonstrated using arrays of uniaxial channels, templated with agarose hydrogel, and containing genetically engineered cells that secrete brain-derived neurotrophic factor (BDNF). However, immobilizing neurotrophic factors secreting cells within a scaffold is relatively cumbersome, and alternative strategies are needed to provide sustained release of BDNF from templated agarose scaffolds. Existing methods of loading the drug or protein into hydrogels cannot provide sustained release from templated agarose hydrogels. Alternatively, here it is shown that pH-responsive H-bonded poly(ethylene glycol)(PEG)/poly(acrylic acid)(PAA)/protein hybrid layer-by-layer (LbL) thin films, when prepared over agarose, provided sustained release of protein under physiological conditions for more than four weeks. Lysozyme, a protein similar in size and isoelectric point to BDNF, is released from the multilayers on the agarose and is biologically active during the earlier time points, with decreasing activity at later time points. This is the first demonstration of month-long sustained protein release from an agarose hydrogel, whereby the drug/protein is loaded separately from the agarose hydrogel fabrication process.

Loading dependent swelling and release properties of novel biodegradable, elastic and environmental stimuli-sensitive polyurethanes

A novel degradable, elastic, anionic, and linear polyurethane was synthesized from hexamethylene diisocyanate, polycaprolactone diol, and a bicine chain extender. The chemical structure, mechanical properties, degradation rate, and swelling ratio were characterized by comparing the polymer with a polyurethane containing a 2,2-(methylimino) diethanol chain extender. Due to the incorporation of negatively charged carboxyl side groups, the bicine extended polymers exhibited higher micro-phase separation, better mechanical properties in dry condition, and better sensitivity to environmental stimuli than controls, as demonstrated by its high swelling ratio at elevated pH, lower ionic strength, or higher temperature. The swelling ratio of membranes showed reversible change as the function of pH at 37 degrees C, the membranes becoming fully water soluble at pH above 8.3. Nile blue chloride and lysozyme were selected to study their release from this polymer. The release rates of both compounds were significantly influenced by the pH and ionic strength. The swelling ratios were also influenced by lysozyme loading at low pH. The pH dependent properties were used to fabricate scaffolds by drop-on-demand printing. Bicine extended polyurethanes may be of interest for possible drug delivery applications, customizable scaffold fabrication and other potential biomedical applications.

Release characteristics of lidocaine from local implant of polyanionic and polycationic hydrogels

In this study, the release characteristics of lidocaine conveyed in base (LB) and salt (LS) forms from an anionic hydrogel composed of carbopol and a cationic hydrogel composed of chitosans were examined for optimizing hydrogel formulation as a sponge filler to stop the bleeding and as a carrier for delivering lidocaine to relief pain after a tooth extraction. A Franz cell was used to simulate the in vivo environment and evaluate the drug release kinetics. It was confirmed that the release profiles of LB and LS from both the carbopol and chitosan hydrogels were best described by the Higuchi model, and values of the release rate constant (K) calculated from the slope of the linear portion of the plot were compared. Results demonstrated that the K value increased with increasing LB concentration at the same three carbopol levels of the hydrogels, whereas it increased with a decreasing level of carbopol for the same concentration of LB in the hydrogels. A minimum in the value of K was observed near neutral pH, which was attributed to two influencing factors of the viscosity and the complexing effect of carbopol gel. However, K values at the same concentration of lidocaine were larger for those formulations using the salt form compared to those using the base form. Results further revealed that the K value increased with an increasing amount of LB added to chitosan hydrogels with the same 0.5% concentration of C1000 (Chitosan, viscosity 1000 cps). K values increased with a decreasing MW of chitosan at the same level in the same concentration of an acetic acid solution with the same amount of LB added. K values for the release from chitosan hydrogels prepared at a lower level were lower than those of hydrogels prepared at a higher level. However, similarities in the release profiles between LB and LS were observed. In conclusion, the viscosity of the gel matrix and the ionic complexing effect between the anionic acid groups of hydrogels and basic groups with lidocaine were two main factors influencing regulation of the diffusion coefficient for controlling drug release.