Controlled and triggered small molecule release from a confined polymer film

A pH and Temperature Dual-Responsive Microgel-Embedded, Adhesive, and Tough Hydrogel for Drug Delivery and Wound Healing

Stimuli-responsive hydrogels have attracted much attention over the past decade for potential bioengineering applications such as wound dressing and drug delivery. In this work, a pH and temperature dual-responsive microgel-embedded hydrogel has been fabricated by incorporating poly(N-isopropylacrylamide-co-acrylic acid) (PNIPAAm-co-AAc) based microgel particles into polyacrylamide (PAAm)/chitosan (CS) semi-interpenetrating polymer network (semi-IPN), denoted as microgel@PAM/CS. The resultant hydrogel possesses excellent mechanical properties including stretchability, compressibility, and elasticity. In addition, the microgel@PAM/CS hydrogels can tightly adhere to the surfaces of a variety of tissues such as porcine skin, kidney, intestine, liver, and heart. Moreover, it shows controlled dual-drug release profile of both bovine serum albumin (BSA) (as a model protein) and sulfamethoxazole (SMZ), an antibiotic. Excellent antimicrobial properties are obtained for SMZ-loaded microgel@PAM/CS hydrogels. Compared with traditional drug administration methods such as by mouth, injection, and inhalation, the microgel@PAM/CS hydrogels possess advantages such as higher drug loading efficiency (by more than 80%) and controllable and sustained (over 48 h) release. The microgel@PAM/CS hydrogels can significantly enhance the wound healing process. This work provides a facile approach for the fabrication of multifunctional stimuli-responsive microparticle-embedded hydrogels with semi-IPN structures, and the as-prepared microgel@PAM/CS hydrogels have great potential for applications as smart wound dressing materials in biomedical engineering.

Design of hydrogel-microgel composites with tailored small molecule release profiles

Stimuli-responsive hydrogel-microgel composites (HMC) were prepared by embedding poly(N-isopropylacrylamide)-based microgels in a poly(N-isopropylacrylamide)-based hydrogel. When the microgels were pre-loaded with the small molecule model drug crystal violet (CV) via electrostatics, the HMC was able to release the CV in a pH-triggered fashion. We found that the CV release rate was dependent on the solution temperature and the dimension of the material. Also, by changing the chemical composition and/or pore size of the hydrogel matrix, the CV release kinetics can be tuned. Moreover, when multiple microgels loaded with different model drugs were embedded in a single HMC, the HMC can be used to control the release rate of each drug analog individually in a pH-dependent fashion. By understanding how properties of a hydrogel can alter the release of small molecules from embedded microgels, new materials capable of controlled and triggered release of multiple small molecule drugs can be designed with myriad uses in the biomedical field.

Two-dimensional colloidal crystal of soft microgel spheres: Development, preparation and applications

Two-dimensional (2D) colloidal crystals are ordered monolayer arrays of colloidal sphere particles assembled on the substrates or at phase interfaces. Owing to their unique periodic structure and fascinating properties, 2D colloidal crystals have aroused considerable interest because of their potential applications. Among them, 2D colloidal crystals self-assembled from soft microgel spheres stand out particularly. The 2D colloidal crystals of soft microgel spheres combine the advantages of monolayer colloidal crystals and sensitive microgels, which have a good application prospect in biomedical area. In this article, we provide a systematic overview of 2D colloidal crystals of soft microgel spheres related to their development, preparation and applications. First, various preparation methods of 2D colloidal crystal of microgels are introduced, including dip-coating, drop-coating, spin-coating, interface assembly, surface reaction-assisted assembly, and so forth. Second, representative biomedical applications consisting of optical sensor, drug delivery, antibacterial coating, cell culture, and colloidal template are also exemplified to show the high performance of 2D colloidal crystals of soft microgel spheres. In addition, we also present prospects of future developments of 2D microgel colloidal crystals.

Controlled Osteogenic Differentiation of Human Mesenchymal Stem Cells Using Dexamethasone-Loaded Light-Responsive Microgels

Human mesenchymal stem cells (hMSCs), which have the ability to differentiate into osteoblasts, show promise for bone tissue engineering and bone defect treatment. While there are a number of approaches currently available to accomplish this, e.g., utilizing biodegradable materials loaded with the synthetic glucocorticoid osteogenic inducer dexamethasone (DEX), there are still many disadvantages with the current technologies. Here, we generated light-responsive microgels that we showed are capable of loading and releasing DEX in a light-triggered fashion, with the released DEX being able to induce hMSC differentiation into osteoblasts. Specifically, light-responsive poly(N-isopropylacrylamide-co-nitrobenzyl methacrylate) (pNIPAm-co-NBMA) microgels were synthesized via free radical precipitation polymerization and their size, morphology, and chemical composition were characterized. We then went on to show that the microgels could be loaded with DEX (via what we think are hydrophobic interactions) and released upon exposure to UV light. We went on to show that the DEX released from the microgels was still capable of inducing osteogenic differentiation of hMSCs using an alamarBlue assay and normalized alkaline phosphatase (ALP) activity assay. We also investigated how hMSC differentiation was impacted by intermittent DEX released from UV-exposed microgels. Finally, we confirmed that the microgels themselves were not cytotoxic to hMSCs. Taken together, the DEX-loaded light-responsive microgels reported here may find a use for niche clinical applications, e.g., bone tissue repair.

Microgel-Based Stretchable Reservoir Devices for Elongation Enhanced Small Molecule Release Rate

Stretchable poly(N-isopropylacrylamide)-co-acrylic acid (pNIPAm-co-10% AAc) microgel-based reservoir devices were fabricated and used to control the release rate of the small molecule model drug tris(4-(dimethylamino)phenyl)methylium chloride (crystal violet, CV) to solution by varying the Au layer thickness coating the microgels and device elongation. Specifically, we showed that CV could be loaded into the microgel layer of the devices via electrostatic interactions at pH 6.5, and the release could be triggered upon exposure to a pH 3.0 solution, which breaks the microgel-CV electrostatic interactions. We demonstrated that the rate of release could be increased by decreasing the Au layer thickness coating microgels and by stretching, that is, thin Au and high elongation promoted the relatively fast release of CV from the device. We found that the Au overlayer thickness (and porosity) dominated the observed release rate profiles when the device was not stretched (or at low elongation), while elongation-induced cracks dominated the release rate at high elongation. We also showed that the CV release kinetics could transition from low ("off") to high ("on"), which enhanced when the devices are stretched. This behavior could be exploited in the future for autonomous release systems that release small molecules when stretched by natural processes, for example, movement of joints and muscles.

Novel Platform for Surface-Mediated Gene Delivery Assisted with Visible-Light Illumination

Surface-mediated gene delivery has attracted more and more attentions in biomedical research and applications because of its characteristics of low toxicity and localized delivery. Herein, a novel visible-light-regulated, surface-mediated gene-delivery platform is exhibited, arising from the photoinduced surface-charge accumulation on silicon. Silicon with a pn junction is used and tested subsequently for the behavior of surface-mediated gene delivery under visible-light illumination. It is found that positive-charge accumulation under light illumination changes the surface potential and then facilitates the delivery of gene-loaded carriers. As a result, the gene-expression efficiency shows a significant improvement from 6% to 28% under a 10 min visible-light illumination. Such improvement is ascribed to the increase in surface potential caused by light illumination, which promotes both the release of gene-loaded carriers and the cellular uptake. This work suggests that silicon with photovoltaic effect could offer a new strategy for surface-mediated, gene-delivery-related biomedical research and applications.

Thermoresponsive Diblock Copolymer Films with a Linear Shrinkage Behavior and Its Potential Application in Temperature Sensors

The linear shrinkage behavior in thermoresponsive diblock copolymer films and its potential application in temperature sensors are investigated. The copolymer is composed of two thermoresponsive blocks with different transition temperatures (TTs): di(ethylene glycol) methyl ether methacrylate (MEO₂MA; TT₁ = 25 °C) and poly(ethylene glycol) methyl ether methacrylate (OEGMA₃₀₀; TT₂ = 60 °C) with a molar ratio of 1:1. Aqueous solutions of PMEO₂MA-b-POEGMA₃₀₀ show a three-stage transition upon heating as seen with optical transmittance and small-angle X-ray scattering: dissolution (T < TT₁), self-assembled micelles with core-shell structure (TT₁ < T < TT₂), and aggregation of collapsed micelles (T > TT₂). Due to the restrictions in the polymer chain arrangement introduced by the solid Si substrate, spin-coated PMEO₂MA-b-POEGMA₃₀₀ films exhibit an entirely different internal structure and transition behavior. Neutron reflectivity shows the absence of an ordered structure normal to the Si substrate in as-prepared PMEO₂MA-b-POEGMA₃₀₀ films. After exposure to D₂O vapor for 3 h and then increasing the temperature above its TT₁ and TT₂, the ordered structure is still not observed. Only a D₂O enrichment layer is formed close to the hydrophilic Si substrate. Such PMEO₂MA-b-POEGMA₃₀₀ films show a linear shrinkage between TT₁ and TT₂ in a D₂O vapor atmosphere. This special behavior can be attributed to the synergistic effect between the restrained collapse of the PMEO₂MA blocks by the still swollen POEGMA₃₀₀ blocks and the impedance of chain arrangement by the Si substrate. Based on this unique behavior, spin-coated PMEO₂MA-b-POEGMA₃₀₀ films are further prepared into a temperature sensor by implementing Ag electrodes. Its resistance decreases linearly with temperature between TT₁ and TT₂.

Alkanethiol Molecular Barriers for Controlling Small Molecule Release Kinetics from a Microgel-Based Reservoir Device

Poly(N-isopropylacrylamide)-co-acrylic acid microgel-based reservoir devices were constructed by "sandwiching" a single layer of microgels between two thin Au layers (all on a glass support). The microgels were loaded with the model drug crystal violet (CV) utilizing the electrostatic interactions between deprotonated acrylic acid (AAc) and the positively charged CV; release can be triggered from the microgels by neutralizing the deprotonated AAc groups at acidic conditions. Alkanethiols of different alkyl chain lengths and polarities were immobilized on the upper Au layer of the device, and the release rate of the model drug CV from the microgel layer, after acid neutralization, was assessed. We found that the CV release rate was the highest when the alkyl chain length was short and contained a hydrophilic moiety. Conversely, the release rate was hindered by the presence of thiols with long alkyl chain lengths and with no hydrophilic moiety. We explain this phenomenon by quantifying the thiol's ability to hinder acid penetration into the microgel layer, and the ability of free CV to pass through the upper Au layer and into the solution. Utilizing various thiols and mixed thiol layers, we are able to tune release profiles from these reservoir devices to potentially achieve array devices with precisely tuned small molecule release profiles.

Effect of Divalent Cation on Swelling Behavior of Anionic Microgels: Quantification and Dynamics of Ion Uptake and Release

Poly(N-vinylcaprolactam-co-itaconate) (P(VCL-co-IADME) microgels were synthesized varying the molar ratio between VCL and IADME via free radical precipitation polymerization in the presence of quaternary ammonium surfactant. In order to determine the effect of the divalent metal ions on the structure and the swelling behavior of the microgel systems, both neutral and charged forms of the hydrogels after hydrolysis were investigated. The triggered gel collapse caused by the divalent metal ion together with the quantification of the metal ion uptake was studied in detail by titration and ion chromatography methods and revealed the minimum concentration around 0.1 mM to trigger gel collapse on the treated gels. Uptake and release dynamics of the gels were followed by turbidity measurements and were in the time-range of 2 and 17 s, depending on the composition and the concentrations.

Isolation of RNA from a mixture and its detection by utilizing a microgel-based optical device

In this investigation, we show that RNA can be separated from a solution containing DNA and RNA and the isolated RNA can be detected using poly (N-isopropylacrylamide-co-N-(3-aminopropyl) methacrylamide hydrochloride) microgel-based optical devices (etalons). The isolation of RNA was accomplished by using hairpin-functionalized magnetic beads (MMPDNA) and differential melting, based on the fact that the DNA–RNA hybrid duplex is stronger (i.e., high melting temperature) than the DNA–DNA duplex (i.e., low melting temperature). By performing concurrent etalon sensing and fluorescent studies, we found that the MMPDNA combined with differential melting was capable of selectively separating RNA from DNA. This selective separation and simple colorimetric detection of RNA from a mixture will help lead to future RNA-based disease diagnostic devices.

Electrically Triggered Small Molecule Release from Poly( N-Isopropylacrylamide- co-Acrylic Acid) Microgel-Modified Electrodes

A monolithic layer of poly( N-isopropylacrylamide- co-acrylic acid) microgels was deposited on an Au electrode and used for electrically triggered release of the small molecule crystal violet (CV), which was used as a model drug. CV was loaded into the surface-bound microgels by exposing them to a CV solution at pH 6.5, where the microgels are negatively charged and the CV is positively charged. The electrostatic attraction holds the CV inside of the microgels, while a decrease of the solution pH can neutralize the microgels and allow for CV release. In this investigation, we show that when CV-loaded microgels are deposited on the anode in an electrochemical cell and an appropriate voltage applied, there is a decrease in the solution pH near the anode surface that allows for CV release. We also show that removing the applied potential allows the solution pH near the anode to return to pH 6.5, which halts the release. We show that the release rate from the microgel-modified anodes could be controlled by the magnitude of the applied voltage and by pulsing the applied voltage or applying a continuous voltage. Furthermore, we showed that the microgel-modified anodes can be reloaded with CV and used to release CV to a system many times. Such devices could be used as implantable drug delivery devices, as well as for industrial applications, where small molecules need to be released to systems in response to their chemical status.

Rapidly Responding pH- and Temperature-Responsive Poly (N-Isopropylacrylamide)-Based Microgels and Assemblies

Rapidly responding stimuli-responsive materials can have a benefit in a myriad of applications, for example, sensing and biosensing, actuation, and in drug delivery systems. Thermo- and pH-responsive materials have been among the most widely studied, and can be triggered at physiologically relevant temperatures and pH. Here, we have used a "homologous series" of acids based on the acrylic acid (AAc) backbone and incorporated them into N-isopropylacrylamide (NIPAm)-based microgels. Specifically, the acids used were AAc, methacrylic acid (MAAc), ethylacrylic acid (EAAc), and butylacrylic acid (BAAc), which have pK a's in the range of 4.25-7.4. The resultant microgels were characterized by optical microscopy, and their responsivity to temperature and pH studied by dynamic light scattering. The microgels were subsequently used to generate optical devices (etalons) and their pH and temperature response was also investigated. We found that the devices composed of BAAc-modified microgels exhibit unusually fast response kinetics relative to those of the rest of the devices. We also found that the speed of the response decreased as the length of the acid pendant group decreased, with AAc-modified microgel-based devices exhibiting the slowest response kinetics. Finally, we showed that the kinetics of the device's temperature response also decreased as the length of the acid pendant group decreased, which we hypothesize is a consequence of the hydrophobicity of the acid groups, that is, increased hydrophobicity leads to faster responses. Understanding this behavior can lead to the rational design of fast responding materials for the applications mentioned above.

Stimuli-responsive polymers and their applications

Responsive polymer-based materials are capable of altering their chemical and/or physical properties upon exposure to external stimuli. This review highlights their use for sensing and biosensing, drug delivery, and artificial muscles/actuators.

Supramolecular Microgels Fabricated from Supramonomers

This letter describes a new method for fabricating supramolecular microgels from supramonomers. To this end, we designed and assembled supramonomers with one acrylate moiety on each end on the basis of noncovalent host-guest interactions, which could be utilized as a cross-linker. Then supramolecular microgels were fabricated through the copolymerization of supramonomers and N-isopropylacrylamide (NIPAm). The supramolecular microgels not only showed temperature-responsive properties as expected from conventional PNIPAm-based microgels but also exhibited stimuli-responsive and degradable properties benefiting from the dynamic nature of supramonomers. In addition, it was found that the degradation kinetics of the supramolecular microgels was related greatly to the structure of the microgels, providing a way to tune the degradation kinetics of the supramolecular microgels. Various supramolecular microgels with desired structure and function are supposed to be facilely fabricated from supramonomers. It is anticipated that the supramolecular microgels can enrich the application of microgels by easily endowing the microgels with stimuli-responsive and degradable properties.

Stimuli-Responsive Assemblies for Sensing Applications

Poly (N-isopropylacrylamide) (pNIPAm)-based hydrogels and hydrogel particles (microgels) have been extensively studied since their discovery a number of decades ago. While their utility seems to have no limit, this feature article is focused on their development and application for sensing small molecules, macromolecules, and biomolecules. We highlight hydrogel/microgel-based photonic materials that have order in one, two, or three dimensions, which exhibit optical properties that depend on the presence and concentration of various analytes. A particular focus is put on one-dimensional materials developed in the Serpe Group.

Sequential and controlled release of small molecules from poly(N-isopropylacrylamide) microgel-based reservoir devices

Devices capable of releasing two different small molecules independently, at defined release kinetics, were prepared and their behavior characterized.

Stimuli-responsive microgel-based etalons for optical sensing

Responsive polymers have found numerous applications over the years. This review highlights their use as components of photonic materials, with emphasis on responsive polymer-based etalons. The use of these materials for sensing and biosensing is detailed.

Thickness and substrate dependences of phase transition, drug release and antibacterial properties of PNIPAm-co-AAc films

Normalized absorption coefficient at 590 nm of solutions of extracted CV from 50 nm and 500 nm thick PNIPAm-co-AAc films deposited on Au substrates. Releases were measured for temperatures below (25 °C) and above (40 °C) the LCST.

Controlled release kinetics from a surface modified microgel-based reservoir device

Deposition of Si-based layers on top of a polymer-based “drug” delivery device allows fine-tuning of “drug” release kinetics.

Hairy polyelectrolyte brushes-grafted thermosensitive microgels as artificial synovial fluid for simultaneous biomimetic lubrication and arthritis treatment

We report the fabrication of poly(3-sulfopropyl methacrylate potassium salt) (PSPMK) brushes grafted poly(N-isopropylacrylamide) (PNIPAAm) microgels and their potential as artificial synovial fluid for biomimetic aqueous lubrication and arthritis treatment. The negatively charged PSPMK brushes and thermosensitive PNIPAAm microgels play water-based hydration lubrication and temperature-triggered drug release, respectively. Under soft friction pairs, an ultralow coefficient of friction was achieved, while the hairy thermosensitive microgels showed a desirable temperature-triggered drugs release performance. Such a soft charged hairy microgel offers great possibility for designing intelligent synovial fluid. What is more, the combination of lubrication and drug loading capabilities enables the large clinical potential of novel soft hairy nanoparticles as synthetic joint lubricant fluid in arthritis treatment.

Stimuli-responsive polypropylene for the sustained delivery of TPGS and interaction with erythrocytes

Hemocompatibility and oxidative stress are significant for blood-contacting devices. In this study, N-isopropylacrylamide (NIPAAm) and N-(3-aminopropyl)methacrylamide hydrochloride (APMA) were cografted on polypropylene (PP) membrane using ultraviolet grafting to load antioxidative d-α-tocopheryl polyethylene glycol 1000 succinate (TPGS) and control the release of TPGS. The immobilization of NIPAAm and APMA onto PP membrane was confirmed by attenuated total reflectance Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy. Combined with data from platelet adhesion, red blood cell (RBC) attachment, and hemolysis rate, the hemocompatibility of PP was significantly improved. An in-depth characterization using hemolysis rate test, scanning electron microscopy, atomic force microscopy, and confocal laser scanning microscopy was conducted to confirm that the mechanism of the release of TPGS interacted with RBCs was different at different stages. The release of TPGS from the loading PP membranes affected hemolysis at different stages. At the early stage of release, TPGS maintained the tiny (nanometer-sized) tubers on the membrane surface and enhanced the membrane permeabilization by generating nanosized pores on the cell membranes. Afterward, the incorporated TPGS slowed the lipid peroxidation of erythrocytes and filled in the lipid bilayer of erythrocyte to prevent hemolysis. Thus, the approach implemented to graft NIPAAm and APMA and load TPGS was suitable to develop medical device with excellent hemocompatibility and antioxidative property.

Controlled drug release from the aggregation-disaggregation behavior of pH-responsive microgels

In this submission, two independent sets of microgels were synthesized that exhibit pH responsivity over different solution pH ranges. The microgels were synthesized by copolymerizing two different comonomers with poly(N-isopropylacrylamide) (pNIPAm). The microgels copolymerized with acrylic acid exhibit a negative charge above pH 4.25, while the microgels copolymerized with N-[3-(dimethylamino)propyl]methacrylamide exhibit a positive charge below pH 8.4; these microgels are neutral outside of these pH ranges. We show that aggregates form when the two independent sets of microgels are exposed to one another in a solution that renders them both charged. Furthermore, in solutions of pH outside of this range, the microgels disaggregate because one of the microgels becomes neutralized. This behavior was exploited to load (aggregation) and release (disaggregation) a small-molecule model drug, methylene blue. This aggregate-based system is one example of how pNIPAm-based microgels can be used for controlled/triggered drug delivery, which can have implications for therapeutics.

Responsive polymers for biosensing and protein delivery

Responsive polymers have found their way into numerous sensing and drug delivery platforms; some examples of biosensing and protein delivery are highlighted here.