Ultrasoft platelet-like particles stop bleeding in rodent and porcine models of trauma

Uncontrolled bleeding after trauma represents a substantial clinical problem. The current standard of care to treat bleeding after trauma is transfusion of blood products including platelets; however, donated platelets have a short shelf life, are in limited supply, and carry immunogenicity and contamination risks. Consequently, there is a critical need to develop hemostatic platelet alternatives. To this end, we developed synthetic platelet-like particles (PLPs), formulated by functionalizing highly deformable microgel particles composed of ultralow cross-linked poly (N-isopropylacrylamide) with fibrin-binding ligands. The fibrin-binding ligand was designed to target to wound sites, and the cross-linking of fibrin polymers was designed to enhance clot formation. The ultralow cross-linking of the microgels allows the particles to undergo large shape changes that mimic platelet shape change after activation; when coupled to fibrin-binding ligands, this shape change facilitates clot retraction, which in turn can enhance clot stability and contribute to healing. Given these features, we hypothesized that synthetic PLPs could enhance clotting in trauma models and promote healing after clotting. We first assessed PLP activity in vitro and found that PLPs selectively bound fibrin and enhanced clot formation. In murine and porcine models of traumatic injury, PLPs reduced bleeding and facilitated healing of injured tissue in both prophylactic and immediate treatment settings. We determined through biodistribution experiments that PLPs were renally cleared, possibly enabled by ultrasoft particle properties. The performance of synthetic PLPs in the preclinical studies shown here supports future translational investigation of these hemostatic therapeutics in a trauma setting.

Highly swelling pH-responsive microgels for dual mode near infra-red fluorescence reporting and imaging

Near infra-red (NIR) fluorescence is a desirable property for probe particles because such deeply penetrating light enables remote reporting of the local environment in complex surroundings and imaging. Here, two NIR non-radiative energy transfer (NRET) fluorophores (Cy5 and Cy5.5) are coupled to preformed pH-responsive poly(ethylacrylate-methacrylic acid-divinylbenzene) microgel particles (PEA-MAA-5/5.5 MGs) to obtain new NIR fluorescent probes that are cytocompatible and swell strongly. NIR ratiometric photoluminescence (PL) intensity analysis enables reporting of pH-triggered PEA-MAA-5/5.5 MG particle swelling ratios over a very wide range (from 1-90). The dispersions have greatly improved colloidal stability compared to a reference temperature-responsive NIR MG based on poly(N-isopropylacrylamide) (PNP-5/5.5). We also show that the wavelength of maximum PL intensity (λ _max) is a second PL parameter that enables remote reporting of swelling for both PEA-MAA-5/5.5 and PNP-5/5.5 MGs. After internalization the PEA-MAA-5/5.5 MGs are successfully imaged in stem cells using NIR light. They are also imaged after subcutaneous injection into model tissue using NIR light. The new NIR PEA-MAA-5/5.5 MGs have excellent potential for reporting their swelling states (and any changes) within physiological settings as well as very high ionic strength environments (e.g., waste water).

Using green emitting pH-responsive nanogels to report environmental changes within hydrogels: a nanoprobe for versatile sensing

Remotely reporting the local environment within hydrogels using inexpensive laboratory techniques has excellent potential to improve our understanding of the nanometer-scale changes that cause macroscopic swelling or deswelling. Whilst photoluminescence (PL) spectroscopy is a popular method for such studies this approach commonly requires bespoke and time-consuming synthesis to attach fluorophores which may leave toxic residues. A promising and more versatile alternative is to use a pre-formed nanogel probe that contains a donor/acceptor pair and then "dope" that into the gel during gel assembly. Here, we introduce green-emitting methacrylic acid-based nanogel probe particles and use them to report the local environment within four different gels as well as stem cells. As the swelling of the nanogel probe changes within the gels the non-radiative energy transfer efficiency is strongly altered. This efficiency change is sensitively reported using the PL ratiometric intensity from the donor and acceptor. We demonstrate that our new nanoprobes can reversibly report gel swelling changes due to five different environmental stimuli. The latter are divalent cations, gel degradation, pH changes, temperature changes and tensile strain. In the latter case, the nanoprobe rendered a nanocomposite gel mechanochromic. The results not only provide new structural insights for hierarchical natural and synthetic gels, but also demonstrate that our new green-fluorescing nanoprobes provide a viable alternative to custom fluorophore labelling for reporting the internal gel environment and its changes.

Nanogels and Microgels: From Model Colloids to Applications, Recent Developments, and Future Trends

Nanogels and microgels are soft, deformable, and penetrable objects with an internal gel-like structure that is swollen by the dispersing solvent. Their softness and the potential to respond to external stimuli like temperature, pressure, pH, ionic strength, and different analytes make them interesting as soft model systems in fundamental research as well as for a broad range of applications, in particular in the field of biological applications. Recent tremendous developments in their synthesis open access to systems with complex architectures and compositions allowing for tailoring microgels with specific properties. At the same time state-of-the-art theoretical and simulation approaches offer deeper understanding of the behavior and structure of nano- and microgels under external influences and confinement at interfaces or at high volume fractions. Developments in the experimental analysis of nano- and microgels have become particularly important for structural investigations covering a broad range of length scales relevant to the internal structure, the overall size and shape, and interparticle interactions in concentrated samples. Here we provide an overview of the state-of-the-art, recent developments as well as emerging trends in the field of nano- and microgels. The following aspects build the focus of our discussion: tailoring (multi)functionality through synthesis; the role in biological and biomedical applications; the structure and properties as a model system, e.g., for densely packed arrangements in bulk and at interfaces; as well as the theory and computer simulation.

Deswelling Induced Morphological Changes in Dual pH and Temperature Responsive Ultra-Low Crosslinked Poly (N-isopropyl acrylamide)-co-Acrylic Acid Microgels

Poly(N-isopropylacrylamide) microgels prepared without exogenous crosslinker are extremely "soft" as a result of their very low crosslinking density, with network connectivity arising only from the self-crosslinking of pNIPAm chains. As a result of this extreme softness, our group and others have taken interest in using these materials in a variety of bioengineering applications, while also pursuing studies of their fundamental properties. Here, we report deswelling triggered structural changes in poly (N-isopropylacrylamide-co-acrylic acid) (ULC10AAc) microgels prepared by precipitation polymerization. Dynamic light scattering suggests that the deswelling of these particles not only depends on the collapse of the pNIPAm chains but is also influenced by the ionization state of the acrylic acid moieties present in the copolymer. The ULC10AAc microgel behaves like a traditional crosslinked pNIPAm microgel at pH 3.5, showing a sharp decrease in the hydrodynamic diameter around the lower critical solution temperature (LCST) of pNIPAm. As the pH is increased to 4.5 we observe multiple transitions in the deswelling curve, suggesting inhomogeneity in the structure and/or composition of the microgels. At pH 6.5 the microgels cease to be thermoresponsive over the studied temperature range due to increased charge repulsion between the fully deprotonated AAc groups and an increase in gel osmotic pressure due to solvated counterion ingress. Atomic force microscopy images of particles deposited at different temperatures reveal a temperature induced morphological change, with punctate structures forming inside microgels at pH 4.5 and 6.5 and temperature above the gel volume phase transition temperature (VPTT).

Microgel core/shell architectures as targeted agents for fibrinolysis

We demonstrate the utility of microgel core/shell structures conjugated to fibrin-specific peptides as fibrinolytic agents. Poly(N-isopropylmethacrylamide) (pNIPMAm) based microgels conjugated to the peptide GPRPFPAC (GPRP) were observed to bring about fibrin clot erosion, merely through exploitation of the dynamic nature of the clots. These results suggest the potential utility of peptide-microgel hybrids in clot disruption and clotting modulation.

Enhancing clot properties through fibrin-specific self-cross-linked PEG side-chain microgels

Excessive bleeding and resulting complications are a major cause of death in both trauma and surgical settings. Recently, there have been a number of investigations into the design of synthetic hemostatic agents with platelet-mimicking activity to effectively treat patients suffering from severe hemorrhage. We developed platelet-like particles from microgels composed of polymers carrying polyethylene glycol (PEG) side-chains and fibrin-targeting single domain variable fragment antibodies (PEG-PLPs). Comparable to natural platelets, PEG-PLPs were found to enhance the fibrin network formation in vitro through strong adhesion to the emerging fibrin clot and physical, non-covalent cross-linking of nascent fibrin fibers. Furthermore, the mechanical reinforcement of the fibrin mesh through the incorporation of particles into the network leads to a ∼three-fold decrease of the overall clot permeability as compared to control clots. However, transport of biomolecules through the fibrin clots, such as peptides and larger proteins is not hindered by the presence of PEG-PLPs and the altered microstructure. Compared to control clots with an elastic modulus of 460+/-260 Pa, PEG-PLP-reinforced fibrin clots exhibit higher degrees of stiffness as demonstrated by the significantly increased average Younǵs modulus of 1770 +/±720 Pa, as measured by AFM force spectroscopy. Furthermore, in vitro degradation studies with plasmin demonstrate that fibrin clots formed in presence of PEG-PLPs withstand hydrolysis for 24 h, indicating enhanced stabilization against exogenous fibrinolysis. The entire set of data suggests that the designed platelet-like particles have high potential for use as hemostatic agents in emergency medicine and surgical settings.

Responsive Nanogel Probe for Ratiometric Fluorescent Sensing of pH and Strain in Hydrogels

In this study a new pH-responsive nanogel probe containing a complementary nonradiative resonance energy transfer (NRET) fluorophore pair is investigated and its ability to act as a versatile probe of network-related changes in three hydrogels demonstrated. Fluorescent sensing using NRET is a powerful method for studying relationships between Angstrom length-scale structure and macroscopic properties of soft matter. Unfortunately, inclusion of NRET fluorophores into such materials requires material-specific chemistry. Here, low concentrations of preformed nanogel probes were included into hydrogel hosts. Ratiometric photoluminescence (PL) data for the gels labeled with the nanogel probes enabled pH-triggered swelling and deswelling to be studied as well as Ca²⁺-triggered collapse and solute release. PL measurements during compression of a nanogel probe-labeled nanocomposite gel demonstrated mechanochromic behavior and strain sensing. The new nanogel probes have excellent potential for investigating the internal structures of gels and provide a versatile ratiometric fluorescent platform for studying pH and strain.

Phase behavior of binary and polydisperse suspensions of compressible microgels controlled by selective particle deswelling

We investigate the phase behavior of suspensions of poly(N-isopropylacrylamide) (pNIPAM) microgels with either bimodal or polydisperse size distribution. We observe a shift of the fluid-crystal transition to higher concentrations depending on the polydispersity or the fraction of large particles in suspension. Crystallization is observed up to polydispersities as high as 18.5%, and up to a number fraction of large particles of 29% in bidisperse suspensions. The crystal structure is random hexagonal close-packed as in monodisperse pNIPAM microgel suspensions. We explain our experimental results by considering the effect of bound counterions. Above a critical particle concentration, these cause deswelling of the largest microgels, which are the softest, changing the size distribution of the suspension and enabling crystal formation in conditions where incompressible particles would not crystallize.

Oligo(ethylene glycol)-sidechain microgels prepared in absence of cross-linking agent: Polymerization, characterization and variation of particle deformability

We present a systematic study of self-cross-linked microgels formed by precipitation polymerization of oligo ethylene glycol methacrylates. The cross-linking density of these microgels and, thus, the network flexibility can be easily tuned through the modulation of the reaction temperature during polymerization. Microgels prepared in absence of any difunctional monomer, i.e. cross-linker, show enhanced deformability and particle spreading on solid surfaces as compared to microgels cross-linked with varying amounts of poly(ethylene glycol diacrylate) (PEG-DA) in addition to self-crosslinking. Particles prepared at low reaction temperatures exhibit the highest degree of spreading due to the lightly cross-linked and flexible polymer network. Moreover, AFM force spectroscopy studies suggest that cross-linker-free microgels constitute of a more homogeneous polymer network than PEG-DA cross-linked particles and have elastic moduli at the particle apex that are ~5 times smaller than the moduli of 5 mol-% PEG-DA cross-linked microgels. Resistive pulse sensing experiments demonstrate that microgels prepared at 75 and 80°C without PEG-DA are able to deform significantly to pass through nanopores that are smaller than the microgel size. Additionally, we found that polymer network flexibility of microgels is a useful tool to control the formation of particle dewetting patterns. This offers a promising new avenue for build-up of 2D self-assembled particle structures with patterned chemical and mechanical properties.

Platelet-Microcapsule Hybrids Leverage Contractile Force for Targeted Delivery of Hemostatic Agents

We report a cell-mediated, targeted drug delivery system utilizing polyelectrolyte multilayer capsules that hybridize with the patient's own platelets upon intravenous administration. The hybridized platelets function as the sensor and actuator for targeted drug delivery and controlled release in our system. These capsules are biochemically and mechanically tuned to enable platelet adhesion and capsule rupture upon platelet activation and contraction, enabling the targeted and controlled "burst" release of an encapsulated biotherapeutic. As platelets are the "first responders" in the blood clot formation process, this platelet-hybridized system is ideal for the targeted delivery of clot-augmenting biotherapeutics wherein immediate therapeutic efficacy is required. As proof-of-concept, we tailored this system to deliver the pro-clotting biotherapeutic factor VIII for hemophilia A patients that have developed inhibitory antifactor VIII antibodies. The polyelectrolyte multilayer capsules physically shield the encapsulated factor VIII from the patient's inhibitors during circulation, preserving its bioactivity until it is delivered at the target site via platelet contractile force. Using an in vitro microfluidic vascular injury model with factor VIII-inhibited blood, we demonstrate a 3.8× increase in induced fibrin formation using capsules loaded with factor VIII at a concentration an order of magnitude lower than that used in systemic delivery. We further demonstrate that clot formation occurs 18 min faster when factor VIII loaded capsules are used compared to systemic delivery at the same concentration. Because platelets are integral in the pathophysiology of thrombotic disorders, cancer, and innate immunity, this paradigm-shifting smart drug delivery system can be similarly applied to these diseases.

Dynamic assembly of ultrasoft colloidal networks enables cell invasion within restrictive fibrillar polymers

In regenerative medicine, natural protein-based polymers offer enhanced endogenous bioactivity and potential for seamless integration with tissue, yet form weak hydrogels that lack the physical robustness required for surgical manipulation, making them difficult to apply in practice. The use of higher concentrations of protein, exogenous cross-linkers, and blending synthetic polymers has all been applied to form more mechanically robust networks. Each relies on generating a smaller network mesh size, which increases the elastic modulus and robustness, but critically inhibits cell spreading and migration, hampering tissue regeneration. Here we report two unique observations; first, that colloidal suspensions, at sufficiently high volume fraction (ϕ), dynamically assemble into a fully percolated 3D network within high-concentration protein polymers. Second, cells appear capable of leveraging these unique domains for highly efficient cell migration throughout the composite construct. In contrast to porogens, the particles in our system remain embedded within the bulk polymer, creating a network of particle-filled tunnels. Whereas this would normally physically restrict cell motility, when the particulate network is created using ultralow cross-linked microgels, the colloidal suspension displays viscous behavior on the same timescale as cell spreading and migration and thus enables efficient cell infiltration of the construct through the colloidal-filled tunnels.

An oil-in-water nanoemulsion enhances immunogenicity of H5N1 vaccine in mice

To enhance the immunogenicity of the Influenza H5N1 vaccine, we developed an oil-in-water nanoemulsion (NE) adjuvant. NE displayed good temperature stability and maintained particle size. More importantly, it significantly enhanced IL-6 and MCP-1 production to recruit innate cells, including neutrophils, monocytes/macrophages and dendritic cells to the local environment. Furthermore, NE enhanced dendritic cell function to induce robust antigen-specific T and B cell immune responses. NE-adjuvanted H5N1 vaccine not only elicited significantly higher and long-lasting antibody responses, but also conferred enhanced protection against homologous clade 1 as well as heterologous clade 2 H5N1 virus challenge in young as well as in aged mice. The pre-existing immunity to seasonal influenza did not affect the immunogenicity of NE-adjuvanted H5N1 vaccine.

The CONTIN algorithm and its application to determine the size distribution of microgel suspensions

We review a powerful regularization method, known as CONTIN, for obtaining the size distribution of colloidal suspensions from dynamic light scattering data. We show that together with the so-called L-curve criterion for selecting the optimal regularization parameter, the method correctly describes the average size and size distribution of microgel suspensions independently characterized using small-angle neutron scattering. In contrast, we find that when using the default regularization process, where the regularizer is selected via the "probability to reject" method, the results are not as satisfactory.

Design of functional cationic microgels as conjugation scaffolds

We describe the development of primary amine functionalized microgels with the potential as dye scaffolds for bioimaging.

Influence of binary microgel phase behavior on the assembly of multi-functional raspberry-structured microgel heteroaggregates

We investigate the influence of microgel composition on phase behavior of binary microgel dispersions using poly(N-isopropylacrylamide) microgels cross-linked with 5 mol% and 1 mol% N,N'-methylenebis(acrylamide), or poly(N-isopropylmethacrylamide) microgels cross-linked with 5 mol% N,N'-methylenebis(acrylamide). We then explore the dispersion phase behavior in the context of microgel deposition at a planar interface. These results are then compared to the observed assembly of microgels at curved interfaces, in the form of raspberry-like patchy particles (RLPPs) consisting of a polystyrene core surrounded by a (two-component) microgel shell. Results suggest that microgel composition has a large influence on the ability of binary dispersions to coat planar and curved interfaces. In particular, we demonstrate that binary dispersions of microgels containing higher cross-linker content exhibit decreased packing densities that are very pronounced at a curved interface. To enhance packing density we also explore the use of a two-step coating process to fabricate RLPPs with enhanced control over topography. Development of these complex vehicles is potentially beneficial in the modulation of biological systems where spatial and temporal presentation of molecules can have a large influence on cellular behavior.

Segregation of mass at the periphery of N-isopropylacrylamide-co-acrylic-acid microgels at high temperatures

We investigate poly(N-isopropylacrylamide) (pNIPAM) microgels randomly copolymerized with large mol % of protonated acrylic acid (AAc), finding that above the lower critical solution temperature the presence of the acid strongly disrupts pNIPAM's collapse, leading to unexpected new behavior at high temperatures. Specifically, we see a dramatic increase in the ratio between the radius of gyration and the hydrodynamic radius above the theoretical value for homogeneous spheres, and a corresponding increase of the network length scale, which we attribute to the presence of a heterogeneous polymer distribution that forms due to frustration of pNIPAM's coil-to-globule transition by the AAc. We analyze this phenomenon using a Debye-Bueche-like scattering contribution as opposed to the Lorentzian term often used, interpreting the results in terms of mass segregation at the particle periphery.

Thin Films Constructed by Centrifugal Deposition of Highly Deformable, Charged Microgels

Thin films composed entirely of microgel building blocks were fabricated using two kinds of self-cross-linked, oppositely charged microgels, via centrifugal deposition. Atomic force microscopy studies revealed that both microgels form very thin monolayer films due to a large degree of microgel deformation during deposition. Meanwhile, centrifugal deposition from a mixture of these two kinds of microgels resulted in the formation of microgel bilayers with a total thickness of around 20 nm. The film thickness increased linearly with the deposition time. Additionally, isotropic stretching/release by heating/cooling of the dried microgel films generated complicated buckling patterns, while anisotropic (uniaxial) stretching/release resulted in parallel buckling perpendicular to the stretching direction. The damage caused by anisotropic stretching and 100 °C treatment can be healed by addition of water, while damage caused via treatment at 150 °C cannot be healed due to the occurrence of polymer cross-linking, which inhibits the mobility of the microgel building blocks.

Ultrasoft, highly deformable microgels

Microgels are colloidally stable, hydrogel microparticles that have previously been used in a range of (soft) material applications due to their tunable mechanical and chemical properties. Most commonly, thermo and pH-responsive poly(N-isopropylacrylamide) (pNIPAm) microgels can be fabricated by precipitation polymerization in the presence of the co-monomer acrylic acid (AAc). Traditionally pNIPAm microgels are synthesized in the presence of a crosslinking agent, such as N,N'-methylenebisacrylamide (BIS), however, microgels can also be synthesized under 'crosslinker free' conditions. The resulting particles have extremely low (<0.5%), core-localized crosslinking resulting from rare chain transfer reactions. AFM nanoindentation of these ultralow crosslinked (ULC) particles indicate that they are soft relative to crosslinked microgels, with a Young's modulus of ∼10 kPa. Furthermore, ULC microgels are highly deformable as indicated by a high degree of spreading on glass surfaces and the ability to translocate through nanopores significantly smaller than the hydrodynamic diameter of the particles. The size and charge of ULCs can be easily modulated by altering reaction conditions, such as temperature, monomer, surfactant and initiator concentrations, and through the addition of co-monomers. Microgels based on the widely utilized, biocompatible polymer polyethylene glycol (PEG) can also be synthesized under crosslinker free conditions. Due to their softness and deformability, ULC microgels are a unique base material for a wide variety of biomedical applications including biomaterials for drug delivery and regenerative medicine.

Impact of single-particle compressibility on the fluid-solid phase transition for ionic microgel suspensions

We study ionic microgel suspensions composed of swollen particles for various single-particle stiffnesses. We measure the osmotic pressure π of these suspensions and show that it is dominated by the contribution of free ions in solution. As this ionic osmotic pressure depends on the volume fraction of the suspension ϕ, we can determine ϕ from π, even at volume fractions so high that the microgel particles are compressed. We find that the width of the fluid-solid phase coexistence, measured using ϕ, is larger than its hard-sphere value for the stiffer microgels that we study and progressively decreases for softer microgels. For sufficiently soft microgels, the suspensions are fluidlike, irrespective of volume fraction. By calculating the dependence on ϕ of the mean volume of a microgel particle, we show that the behavior of the phase-coexistence width correlates with whether or not the microgel particles are compressed at the volume fractions corresponding to fluid-solid phase coexistence.

Ultrasoft microgels displaying emergent platelet-like behaviours

Efforts to create platelet-like structures for the augmentation of haemostasis have focused solely on recapitulating aspects of platelet adhesion; more complex platelet behaviours such as clot contraction are assumed to be inaccessible to synthetic systems. Here, we report the creation of fully synthetic platelet-like particles (PLPs) that augment clotting in vitro under physiological flow conditions and achieve wound-triggered haemostasis and decreased bleeding times in vivo in a traumatic injury model. PLPs were synthesized by combining highly deformable microgel particles with molecular-recognition motifs identified through directed evolution. In vitro and in silico analyses demonstrate that PLPs actively collapse fibrin networks, an emergent behaviour that mimics in vivo clot contraction. Mechanistically, clot collapse is intimately linked to the unique deformability and affinity of PLPs for fibrin fibres, as evidenced by dissipative particle dynamics simulations. Our findings should inform the future design of a broader class of dynamic, biosynthetic composite materials.

Disposable platform provides visual and color-based point-of-care anemia self-testing

BACKGROUND

Anemia, or low blood hemoglobin (Hgb) levels, afflicts 2 billion people worldwide. Currently, Hgb levels are typically measured from blood samples using hematology analyzers, which are housed in hospitals, clinics, or commercial laboratories and require skilled technicians to operate. A reliable, inexpensive point-of-care (POC) Hgb test would enable cost-effective anemia screening and chronically anemic patients to self-monitor their disease. We present a rapid, stand-alone, and disposable POC anemia test that, via a single drop of blood, outputs color-based visual results that correlate with Hgb levels.

METHODS

We tested blood from 238 pediatric and adult patients with anemia of varying degrees and etiologies and compared hematology analyzer Hgb levels with POC Hgb levels, which were estimated via visual interpretation using a color scale and an optional smartphone app for automated analysis.

RESULTS

POC Hgb levels correlated with hematology analyzer Hgb levels (r = 0.864 and r = 0.856 for visual interpretation and smartphone app, respectively), and both POC test methods yielded comparable sensitivity and specificity for detecting any anemia (n = 178) (<11 g/dl) (sensitivity: 90.2% and 91.1%, specificity: 83.7% and 79.2%, respectively) and severe anemia (n = 10) (<7 g/dl) (sensitivity: 90.0% and 100%, specificity: 94.6% and 93.9%, respectively).

CONCLUSIONS

These results demonstrate the feasibility of this POC color-based diagnostic test for self-screening/self-monitoring of anemia.

TRIAL REGISTRATION

Not applicable.

FUNDING

This work was funded by the FDA-funded Atlantic Pediatric Device Consortium, the Georgia Research Alliance, Children's Healthcare of Atlanta, the Georgia Center of Innovation for Manufacturing, and the InVenture Prize and Ideas to Serve competitions at the Georgia Institute of Technology.

Microgel mechanics in biomaterial design

The field of polymeric biomaterials has received much attention in recent years due to its potential for enhancing the biocompatibility of systems and devices applied to drug delivery and tissue engineering. Such applications continually push the definition of biocompatibility from relatively straightforward issues such as cytotoxicity to significantly more complex processes such as reducing foreign body responses or even promoting/recapitulating natural body functions. Hydrogels and their colloidal analogues, microgels, have been and continue to be heavily investigated as viable materials for biological applications because they offer numerous, facile avenues in tailoring chemical and physical properties to approach biologically harmonious integration. Mechanical properties in particular are recently coming into focus as an important manner in which biological responses can be altered. In this Account, we trace how mechanical properties of microgels have moved into the spotlight of research efforts with the realization of their potential impact in biologically integrative systems. We discuss early experiments in our lab and in others focused on synthetic modulation of particle structure at a rudimentary level for fundamental drug delivery studies. These experiments elucidated that microgel mechanics are a consequence of polymer network distribution, which can be controlled by chemical composition or particle architecture. The degree of deformability designed into the microgel allows for a defined response to an imposed external force. We have studied deformation in packed colloidal phases and in translocation events through confined pores; in all circumstances, microgels exhibit impressive deformability in response to their environmental constraints. Microgels further translate their mechanical properties when assembled in films to the properties of the bulk material. In particular, microgel films have been a large focus in our lab as building blocks for self-healing materials. We have shown that their ability to heal after damage arises from polymer mobility during hydration. Furthermore, we have shown film mobility dictates cell adhesion and spreading in a manner that is fundamentally different from previous work on mechanotransduction. In total, we hope that this Account presents a broad introduction to microgel research that intersects polymer chemistry, physics, and regenerative medicine. We expect that research intersection will continue to expand as we fill the knowledge gaps associated with soft materials in biological milieu.

Tunable swelling and rolling of microgel membranes

The tunable swelling and rolling of films assembled via layer-by-layer (LbL) methods from poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAm-co-AAc) microgels and poly(ethylenimine) (PEI) have been systematically studied. Microgel/PEI films assembled at pH 7.4 display a high degree of in-plane swelling at low pH that dramatically increases the film area and drives self-delamination from the substrate to form a free-standing film. The degree of film swelling can be controlled by the size of microgels used in film fabrication. Taking advantage of this feature, self-rolled scrolls can be easily obtained from microgel/PEI films prepared from microgels of two different sizes. The rolling direction can be controlled by the assembly of different size microgels in different film strata, and the final shape of the scrolls can be controlled by scratching the desired film edges. The present work contributes to a deeper understanding of microgel/PEI film swelling properties and introduces a facile and novel method to prepare free-standing films and self-rolled scrolls.

Dynamic materials from microgel multilayers

Multilayer coatings made from hydrogel microparticles (microgels) are conceptually very simple materials: thin films composed of microgel building blocks held together by polyelectrolyte "glue". However, the apparent simplicity of their fabrication and structure belies extremely complex properties, including those of "dynamic" coatings that display rapid self-healing behavior in the presence of solvent. This contribution covers our work with these materials and highlights some of the key findings regarding damage mechanisms, healing processes, film structure/composition, and how the variation of fabrication parameters can impact self-healing behavior.

Microgel film dynamics modulate cell adhesion behavior

A material's mechanical properties greatly control cell behavior at the cell–substrate interface. In this work, we demonstrate that microgel multilayers have unique elastic and viscoelastic-like properties that can be modulated to produce morphological changes in fibroblasts cultured on the film. Protein adsorption is also examined and the data are contrasted with the number of cells adhered. The dynamic interaction of cell and substrate is only partially explained by conventional understanding of surface–receptor interactions and substrate elasticity. Viscoelasticity, a mechanical property not often considered, plays a significant role at cellular length and time scales for microgel films.

Direct observation of ligand-induced receptor dimerization with a bioresponsive hydrogel

Microgel assay for real-time measurement of protein multimerization, assembly, and disassembly identifies physiologically important dimerization pathway.

Modulation of the deswelling temperature of thermoresponsive microgel films

We demonstrate fine-tuning of the deswelling temperatures of thermoresponsive microgels within a biologically relevant range (30-40 °C). This was achieved by copolymerizing N-isopropylacrylamide and N-isopropylmethacrylamide (NIPAm and NIPMAm, respectively) in varying ratios; the parent homopolymers are well-known thermoresponsive polymers. Polyelectrolyte layer-by-layer (LbL) assemblies of these microgels retain the temperature response properties as demonstrated by temperature-dependent light scattering. Furthermore, films composed of more than one type of microgel building block were shown to have multiple temperature responses similar to those observed for the individual building blocks, permitting further tailoring of the temperature responsive interface. Additional experiments with mixed composition films, investigating multiple assembly processes, show that the location of the microgels within the film does not interfere with the temperature response. This suggests that microgels within the polyelectrolyte assembly behave independently of neighboring microgels with respect to their thermally induced deswelling.

Plastic deformation, wrinkling, and recovery in microgel multilayers

Microgel multi-layer films assembled from anionic particles and linear polycation were prepared on elastomeric substrates and their self-healing properties studied. Dried films were imaged in situ during mechanical deformation and were determined to undergo plastic deformation in response to linear strain, leading to film buckling upon strain relaxation. Hydration leads to rapid reorganization of the film building blocks, permitting recovery of the film to the undamaged state. Additionally, films were determined to heal in the presence of high relative humidity environments, suggesting that film swelling and hydration is a major factor in the restoration of film integrity, and that full immersion in solvent is not required for healing. Films prepared from microgels with lower levels of acid content and/or polycation length, factors strongly connected to the charge density and presumably the connectivity of the film, also display self-healing characteristics.

Development of self-assembling mixed protein micelles with temperature-modulated avidities

Elastin-like polypeptides (ELPs) are polypentapeptides that undergo hydrophobic collapse and aggregation above a specific transition temperature, Tt . ELP diblocks sharing a common "core" block (I60) but varying "outer" blocks (A80, P40) were designed, where Tt,I < Tt,A < Tt,P . The formation of ∼55 nm diameter mixed micelles from these ELP diblocks was verified using dynamic light scattering (DLS), multiangle light scattering (MALS) and fluorescence resonance energy transfer (FRET). To confer affinity to the blood circulating protein fibrinogen, a fibrinogen-binding tetrapeptide sequence (GPRP) was fused to A80-I60, while P40-I60 was fused to a non-binding control (GPSP). The self-assembling, peptide-displaying, mixed micelles exhibit temperature-modulated avidities for immobilized and soluble fibrinogen at 32 °C and 42 °C. In this initial proof-of-concept design, the engineered mixed micelles were shown to disengage fibrinogen at elevated temperatures. The modular nature of this system can be used for developing in vivo depot systems that will only be triggered to release in situ upon specific stimuli.

Host response to microgel coatings on neural electrodes implanted in the brain

The performance of neural electrodes implanted in the brain is often limited by host response in the surrounding brain tissue, including astrocytic scar formation, neuronal cell death, and inflammation around the implant. We applied conformal microgel coatings to silicon neural electrodes and examined host responses to microgel-coated and uncoated electrodes following implantation in the rat brain. In vitro analyses demonstrated significantly reduced astrocyte and microglia adhesion to microgel-coated electrodes compared to uncoated controls. Microgel-coated and uncoated electrodes were implanted in the rat brain cortex and the extent of activated microglia and astrocytes as well as neuron density around the implant were evaluated at 1, 4, and 24 weeks postimplantation. Microgel coatings reduced astrocytic recruitment around the implant at later time points. However, microglial response indicated persistence of inflammation in the area around the electrode. Neuronal density around the implanted electrodes was also lower for both implant groups compared to the uninjured control. These results demonstrate that microgel coatings do not significantly improve host responses to implanted neural electrodes and underscore the need for further improvements in implantable materials.

Packed Colloidal Phases Mediate the Synthesis of Raspberry-Structured Microgel Heteroaggregates

Hybrid nanoparticles with complex architectures combine the properties of two distinct materials and integrate them to synergistically provide new characteristics to the assembly. In this work we demonstrate the ability to decorate the surface of a variety of micrometer-sized "core" particles with responsive microgels, forming raspberry-like particles. We use a templating technique wherein the microgel coating is applied from a high-volume-fraction colloidal phase, leading to high surface coverage and enhanced colloidal stability of the resultant particles. Concentrated colloidal dispersions enable microgel/core combinations driven by both specific and nonspecific interactions and offer improved coverage relative to dilute heteroaggregation. This approach is versatile and allows both the core material and microgel phase to be altered while still remaining effective. Though the recovered particles are highly diluted, recycling the unincorporated microgels following raspberry-like particle isolation and reforming the packed colloidal assembly allow multiple cycles of particle synthesis, which improves overall yield.

Multifunctional nanogels for siRNA delivery

The application of RNA interference to treat disease is an important yet challenging concept in modern medicine. In particular, small interfering RNA (siRNA) have shown tremendous promise in the treatment of cancer. However, siRNA show poor pharmacological properties, which presents a major hurdle for effective disease treatment especially through intravenous delivery routes. In response to these shortcomings, a variety of nanoparticle carriers have emerged, which are designed to encapsulate, protect, and transport siRNA into diseased cells. To be effective as carrier vehicles, nanoparticles must overcome a series of biological hurdles throughout the course of delivery. As a result, one promising approach to siRNA carriers is dynamic, versatile nanoparticles that can perform several in vivo functions. Over the last several years, our research group has investigated hydrogel nanoparticles (nanogels) as candidate delivery vehicles for therapeutics, including siRNA. Throughout the course of our research, we have developed higher order architectures composed entirely of hydrogel components, where several different hydrogel chemistries may be isolated in unique compartments of a single construct. In this Account, we summarize a subset of our experiences in the design and application of nanogels in the context of drug delivery, summarizing the relevant characteristics for these materials as delivery vehicles for siRNA. Through the layering of multiple, orthogonal chemistries in a nanogel structure, we can impart multiple functions to the materials. We consider nanogels as a platform technology, where each functional element of the particle may be independently tuned to optimize the particle for the desired application. For instance, we can modify the shell compartment of a vehicle for cell-specific targeting or evasion of the innate immune system, whereas other compartments may incorporate fluorescent probes or regulate the encapsulation and release of macromolecular therapeutics. Proof-of-principle experiments have demonstrated the utility of multifunctional nanogels. For example, using a simple core/shell nanogel architecture, we have recently reported the delivery of siRNA to chemosensitize drug resistant ovarian cancer cells. Ongoing efforts have resulted in several advanced hydrogel structures, including biodegradable nanogels and multicompartment spheres. In parallel, our research group has studied other properties of the nanogels, including their behavior in confined environments and their ability to translocate through small pores.

Reversible Inter- and Intra-Microgel Cross-Linking using Disulfides

Thermoresponsive hydrogel nanoparticles composed of poly(N-isopropylmethacrylamide) (pNIPMAm) and the disulfide-based cross-linker N,N'-bis(acryloyl)cystamine (BAC) have been prepared using a redox-initiated, aqueous precipitation polymerization approach, leading to improved stability of the disulfide bond compared to traditional thermally-initiated methods. The resultant particles demonstrate complete erosion in response to reducing conditions or thiol competition. This stands in contrast to the behavior of thermally-initiated particles, which retain a cross-linked network following disulfide cleavage due to uncontrolled chain-branching and self-cross-linking side reactions. The synthetic strategy has also been combined with the non-degradable cross-linker N,N-methylenebisacrylamide (BIS) to generate "co-cross-linked" pNIPMAm-BAC-BIS microgels. These particles are redox-responsive, swell upon BAC cross-link scission and present reactive thiols. This pendant thiol functionality was demonstrated to be useful for conjugation of thiol-reactive probes and in reversible network formation by assembling particles cross-linked by disulfide linkages.

The polymer/colloid duality of microgel suspensions

Colloidal dispersions have been studied for decades as a result of their utility in numerous applications and as models for molecular and atomic condensed phases. More recently, a number of groups have exploited in such studies submicrometer-sized hydrogel particles (microgels) that have environmentally tunable sizes. The experimental convenience of tuning the dispersion's colloidal volume fraction while maintaining a constant number density of particles provides a clear advantage over more tedious studies that employ traditional hard-sphere particles. However, as studies delved deeper into the fundamental physics of colloidal dispersions comprising microgel particles, it became abundantly clear that a microgel's utility as a tunable hard sphere was limited and that the impact of softness was more profound than previously appreciated. Herein we review the brief history of microgel-based colloidal dispersions and discuss their transition from tunable hard spheres to a class of soft matter that has revealed a landscape of physics and chemistry notable for its extraordinary richness and diversity.

Tunable Encapsulation of Proteins within Charged Microgels

The binding of cytochrome c to pH and thermoresponsive colloidal hydrogels was investigated using multiangle light scattering, measuring loading through changes in particle molar mass and root mean square radius. Loosely cross-linked microgels [composed of a random copolymer of N-isopropylacrylamide (NIPAm) and acrylic acid (AAc)] demonstrated a high loading capacity for protein. Encapsulation was dependent on both the charge characteristics of the network and the salinity of the medium. Under favorable binding conditions (neutral pH, low ionic strength), microgels containing the highest studied charge density (30 mol% AAc) were capable of encapsulating greater than 9.7 × 10(5) cytochrome c molecules per particle. Binding resulted in the formation of a polymer-protein complex and condensation of the polymer. Anionic microgels demonstrated a change in density ~20-fold in the presence of oppositely charged proteins. These studies of cytochrome c encapsulation represent a significant step towards direct measurement of encapsulation efficiency in complex media as we pursue responsive nanogels and microgels for the delivery of macromolecular therapeutic agents.

Bulk modulus of poly(N-isopropylacrylamide) microgels through the swelling transition

We report measurements of the bulk modulus of individual poly(N-isopropylacrylamide) microgels along their swelling transition. The modulus is determined by measuring the volume deformation of the microgel as a function of osmotic pressure using dextran solutions. We find that the modulus softens through the transition, displaying a nonmonotonous behavior with temperature. This feature is correctly reproduced by the theory of Flory for polymer gels, once the concentration dependence of the solvency parameter is properly incorporated.

Control of poly(N-isopropylacrylamide) microgel network structure by precipitation polymerization near the lower critical solution temperature

Poly(N-isopropylacrylamide) (pNIPAm) microgels were synthesized by precipitation polymerization at temperatures ranging from 37 to 45 °C using redox initiator system ammonium persulfate (APS)/N,N,N',N'-tetramethylethylenediamine (TEMED) or photoinitiator 2,2'-azobis(amidinopropane) dihydrochloride (V50). Photon correlation spectroscopy (PCS) and atomic force microscopy (AFM) studies revealed that spherical microgels with narrow size dispersities can be obtained with these methods and that the resultant microgels have volume phase transition behaviors expected from their compositions. Additionally, the low-temperature redox initiator strategy produces microgels devoid of self-cross-linking, thereby permitting the synthesis of completely degradable microgels when using N,N'-(1,2-dihydroxyethylene)bisacrylamide (DHEA) as a cleavable cross-linker. We also demonstrate the potential utility of the approach in bioconjugate syntheses; in this case, avidin immobilization is demonstrated by one-pot copolymerization at low temperature.

Network deconstruction reveals network structure in responsive microgels

Detailed characterization of hydrogel particle erosion revealed critical physicochemical differences between spheres, where network decomposition was informative of network structure. Real-time, in situ monitoring of the triggered erosion of colloidal hydrogels (microgels) was performed via multiangle light scattering. The solution-average molar mass and root-mean-square radii of eroding particles were measured as a function of time for microgels prepared from N-isopropylacrylamide (NIPAm) or N-isopropylmethacrylamide (NIPMAm), copolymerized with a chemically labile cross-linker (1,2-dihydroxylethylene)bisacrylamide (DHEA). Precipitation polymerization was employed to yield particles of comparable dimensions but with distinct topological features. Heterogeneous cross-linker incorporation resulted in a heterogeneous network structure for pNIPAm microgels. During the erosion reaction, mass loss proceeded from the exterior toward the interior of the polymer. In contrast, pNIPMAm microgels had a more homogeneous network structure, which resulted in a more uniform mass loss throughout the particle during erosion. Although both particle types degraded into low molar mass products, pNIPAm microgels were incapable of complete dissolution due to the presence of nondegradable cross-links arising from chain transfer and branching during particle synthesis. The observations described herein provide insight into key design parameters associated with the synthesis of degradable hydrogel particles, which may be of use in various biotechnological applications.

Resistive Pulse Analysis of Microgel Deformation During Nanopore Translocation

Deformation of 570-nm radius poly(N-isopropylacrylamide-co-acrylic acid) microgels passing through individual 375- to 915-nm radius nanopores in glass has been investigated by the resistive-pulse method. Particle translocation through nanopores of dimensions smaller than the microgel yields electrical signatures reflecting the dynamics of microgel deformation. Translocation rates, and event duration and peak shape, are functions of the conductivities of microgel and electrolyte. Our results demonstrate that nanopore resistive-pulse methods provide new fundamental insights into microgel permeation through porous membranes.

Resistive Pulse Analysis of Microgel Deformation During Nanopore Translocation

Deformation of 570-nm radius poly(N-isopropylacrylamide-co-acrylic acid) microgels passing through individual 375- to 915-nm radius nanopores in glass has been investigated by the resistive-pulse method. Particle translocation through nanopores of dimensions smaller than the microgel yields electrical signatures reflecting the dynamics of microgel deformation. Translocation rates, and event duration and peak shape, are functions of the conductivities of microgel and electrolyte. Our results demonstrate that nanopore resistive-pulse methods provide new fundamental insights into microgel permeation through porous membranes.

Synthesis and Physicochemical Properties of Cationic Microgels Based on Poly(N-isopropylmethacrylamide)

Surfactant-free, radical precipitation co-polymerization of N-isopropylmethacrylamide (NIPMAm) and the cationic co-monomer N-(3-aminopropyl) methacrylamide hydrochloride (APMH) was carried out to prepare microgels functionalized with primary amines. The morphology and hydrodynamic diameter of the microgels were characterized by atomic force microscopy (AFM) and photon correlation spectroscopy (PCS), with the effect of NaCl concentration and initiator type on the microgel size and yield being investigated. When a V50-initiated reaction was carried out in pure water, relatively small microgels (~160 nm diameter) were obtained in low yield (~20%). However, both the yield and size increased if the reaction was carried out in saline or by using APS as initiator instead of V50. Stable amine-laden microgels in the range from 160 nm to 950 nm in diameter with narrow size distributions were thus produced using reaction media with controlled salinity. Microgel swelling and electrophoretic mobility values as a function of pH, ionic strength and temperature were also studied, illustrating the presence of cationic sidechains and their influence on microgel properties. Finally, the availability of the primary amine groups for post-polymerization modification was confirmed via modification with fluorescein-NHS.

Peptide-functionalized nanogels for targeted siRNA delivery

A major bottleneck in the development of siRNA therapies is their delivery to the desired cell type or tissue, followed by effective passage across the cell membrane with subsequent silencing of the targeted mRNA. To address this problem, we describe the synthesis of core/shell hydrogel nanoparticles (nanogels) with surface-localized peptides that specifically target ovarian carcinoma cell lines possessing high expression levels of the Eph2A receptor. These nanogels are also demonstrated to be highly effective in the noncovalent encapsulation of siRNA and enable cell-specific delivery of the oligonucleotides in serum-containing medium. Cell toxicity and viability assays reveal that the nanogel construct is nontoxic under the conditions studied, as no toxicity or decrease in cell proliferation is observed following delivery. Importantly, a preliminary investigation of gene silencing illustrates that nanogel-mediated delivery of siRNA targeted to the EGF receptor results in knockdown of that receptor. Excellent protection of siRNA during endosomal uptake and endosomal escape of the nanogels is suggested by these results since siRNA activity in the cytosol is required for gene silencing.

Chronic inflammatory responses to microgel-based implant coatings

Inflammatory responses to implanted biomedical devices elicit a foreign body fibrotic reaction that limits device integration and performance in various biomedical applications. We examined chronic inflammatory responses to microgel conformal coatings consisting of thin films of poly(N-isopropylacrylamide) hydrogel microparticles cross-linked with poly(ethylene glycol) diacrylate deposited on poly(ethylene terephthalate) (PET). Unmodified and microgel-coated PET disks were implanted subcutaneously in rats for 4 weeks and explants were analyzed by histology and immunohistochemistry. Microgel coatings reduced chronic inflammation and resulted in a more mature/organized fibrous capsule. Microgel-coated samples exhibited 22% thinner fibrous capsules that contained 40% fewer cells compared to unmodified PET disks. Furthermore, microgel-coated samples contained significantly higher levels of macrophages (80%) than unmodified PET controls. These results demonstrate that microgel coatings reduce chronic inflammation to implanted biomaterials. (c) 2010 Wiley Periodicals, Inc. J Biomed Mater Res, 2010.