Study of Poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAM) Microgel Particle Induced Deformations of Tissue-Mimicking Phantom by Ultrasound Stimulation

Microgels react to force: mechanical properties, syntheses, and force-activated functions

Microgels are colloidal polymer networks with high molar mass and properties between rigid particles, flexible macromolecules, and micellar aggregates. Their unique stimuli-responsiveness in conjunction with their colloidal phase behavior render them useful for many applications ranging from engineering to biomedicine. In many scenarios either the microgel's mechanical properties or its interactions with mechanical force play an important role. Here, we firstly explain microgel mechanical properties and how these are measured by atomic force microscopy (AFM), then we equip the reader with the synthetic background to understand how specific architectures and chemical functionalities enable these mechanical properties, and eventually we elucidate how the interaction of force with microgels can lead to the activation of latent functionality. Since the interaction of microgels with force is a multiscale and multidisciplinary subject, we introduce and interconnect the different research areas that contribute to the understanding of this emerging field in this Tutorial Review.

Elucidating the combinatorial effect of substrate stiffness and surface viscoelasticity on cellular phenotype

Cells maintain tensional homeostasis by monitoring the mechanics of their microenvironment. In order to understand this mechanotransduction phenomenon, hydrogel materials have been developed with either controllable linear elastic or viscoelastic properties. Native biological tissues, and biomaterials used for medical purposes, often have complex mechanical properties. However, due to the difficulty in completely decoupling the elastic and viscous components of hydrogel materials, the effect of complex composite materials on cellular responses has largely gone unreported. Here, we characterize a novel composite hydrogel system capable of decoupling and individually controlling both the bulk stiffness and surface viscoelasticity of the material by combining polyacrylamide (PA) gels with microgel thin films. By taking advantage of the high degree of control over stiffness offered by PA gels and viscoelasticity, in terms of surface loss tangent, of microgel thin films, it is possible to study the influence that bulk substrate stiffness and surface loss tangent have on complex fibroblast responses, including cellular and nuclear morphology and gene expression. This material system provides a facile method for investigating cellular responses to complex material mechanics with great precision and allows for a greater understanding of cellular mechanotransduction mechanisms than previously possible through current model material platforms.

Emergence of Non-Hexagonal Crystal Packing of Deswollen and Deformed Ultra-Soft Microgels under Osmotic Pressure Control

Highly solvent swollen poly(N-isopropylacrylamide-co-acrylic acid) microgels are synthesized without exogenous crosslinker, making them extremely soft and deformable. These ultralow crosslinked microgels (ULC) are incubated under controlled osmotic pressure to provide a slow (and presumably thermodynamically controlled) approach to higher packing densities. It is found that ULC microgels show stable colloidal packing over a very wide range of osmotic pressures and thus packing densities. Surprising observation of co-existence between hexagonal and square lattices is also made over the lower range of studied osmotic pressures, with microgels apparently changing shape from spheres to cubes in defects or grain boundaries. It is proposed that the unusual packing behavior observed for ULC microgels is due to the extreme softness of these particles, where deswelling causes deformation and shrinking of the particles that result in unique packing states governed by contributions to the entropy at the colloidal system, single particle and ionic levels. These observations further suggest that more detailed experimental and theoretical studies of ultra-soft microgels are required to obtain a complete understanding of their behavior in packed and confined geometries.

On the Determination of Mechanical Properties of Aqueous Microgels-Towards High-Throughput Characterization

Aqueous microgels are distinct entities of soft matter with mechanical signatures that can be different from their macroscopic counterparts due to confinement effects in the preparation, inherently made to consist of more than one domain (Janus particles) or further processing by coating and change in the extent of crosslinking of the core. Motivated by the importance of the mechanical properties of such microgels from a fundamental point, but also related to numerous applications, we provide a perspective on the experimental strategies currently available and emerging tools being explored. Albeit all techniques in principle exploit enforcing stress and observing strain, the realization differs from directly, as, e.g., by atomic force microscope, to less evident in a fluid field combined with imaging by a high-speed camera in high-throughput strategies. Moreover, the accompanying analysis strategies also reflect such differences, and the level of detail that would be preferred for a comprehensive understanding of the microgel mechanical properties are not always implemented. Overall, the perspective is that current technologies have the capacity to provide detailed, nanoscopic mechanical characterization of microgels over an extended size range, to the high-throughput approaches providing distributions over the mechanical signatures, a feature not readily accessible by atomic force microscopy and micropipette aspiration.

Synthetic platelet microgels containing fibrin knob B mimetic motifs enhance clotting responses

Native platelets are crucial players in wound healing. Key to their role is the ability of their surface receptor GPIIb/IIIa to bind fibrin at injury sites, thereby promoting clotting. When platelet activity is impaired as a result of traumatic injury or certain diseases, uncontrolled bleeding can result. To aid clotting and tissue repair in cases of poor platelet activity, our lab has previously developed synthetic platelet-like particles capable of promoting clotting and improving wound healing responses. These are constructed by functionalizing highly deformable hydrogel microparticles (microgels) with fibrin-binding ligands including a fibrin-specific whole antibody or a single-domain variable fragment. To improve the translational potential of these clotting materials, we explored the use of fibrin-binding peptides as cost-effective, robust, high-specificity alternatives to antibodies. Herein, we present the development and characterization of soft microgels decorated with the peptide AHRPYAAK that mimics fibrin knob 'B' and targets fibrin hole 'b'. These "Fibrin-Affine Microgels with Clotting Yield" (FAMCY) were found to significantly increase clot density in vitro and decrease bleeding in a rodent trauma model in vivo. These results indicate that FAMCYs are capable of recapitulating the platelet-mimetic properties of previous designs while utilizing a less costly, more translational design.

A Spontaneous Membrane-Adsorption Approach to Enhancing Second Near-Infrared Deep-Imaging-Guided Intracranial Tumor Therapy

Herein, a functional class of microenvironment-associated nanomaterials is reported for improving the second near-infrared (NIR-II) imaging and photothermal therapeutic effect on intracranial tumors via a spontaneous membrane-adsorption approach. Specific peptides, photothermal agents, and biological alkylating agents were designed to endow the nanogels with high targeting specificity, photothermal properties, and pharmacological effects. Importantly, the frozen scanning electron microscopy technology (cryo-SEM) was utilized to observe the self-association of nanomaterials on tumor cells. Interestingly, the spontaneous membrane-adsorption behavior of nanomaterials was captured through direct imaging evidence. Histological analysis showed that the cross-linking adhesion in intracranial tumor and monodispersity in normal tissues of the nanogels not only enhanced the retention time but also ensured excellent biocompatibility. Impressively, in vivo data confirmed that the microenvironment-associated nanogels could significantly enhance brain tumor clearance rate within a short treatment timeframe (only two weeks). In short, utilizing the spontaneous membrane-adsorption strategy can significantly improve NIR-II diagnosis and phototherapy in brain diseases while avoiding high-risk complications.

A novel 'smart' PNIPAM-based copolymer for breast cancer targeted therapy: Synthesis, and characterization of dual pH/temperature-responsive lactoferrin-targeted PNIPAM-co-AA

Despite the active research towards introducing novel anticancer agents, the long-term sequelae and side effects of chemotherapy remain the major obstacle to achieving clinical success. Recent cancer research is now utilizing the medicinal chemistry toolbox to tailor novel 'smart' carrier systems that can reduce the major limitations of chemotherapy ranging from non-specificity and ubiquitous biodistribution to systemic toxicity. In this aspect, various stimuli-responsive polymers have gained considerable interest due to their intrinsic tumor targeting properties. Among these polymers, poly(N-isopropylacrylamide (PNIPAM) has been chemically modified to tune its thermoresponsivity or even copolymerized to endow new stimulus responsiveness for enhancing tumor targeting. Herein, we set our design rationale to impart additional active targeting entity to pH/temperature-responsive PNIPAM-based polymer for more efficient controlled payloads accumulation at the tumor through cellular internalization via synthesizing novel "super intelligent" lactoferrin conjugated PNIPAM-acrylic acid (LF-PNIPAM-co-AA) copolymer. The synthesized copolymer was physicochemically characterized and evaluated as a smart nanocarrier for targeting breast cancer. In this regard, Honokiol (HK) was utilized as a model anticancer drug and encapsulated in the nanoparticles to overcome its lipophilic nature and allow its parenteral administration, for achieving sustainable drug release with targeting action. Results showed that the developed HK-loaded LF-PNIPAM-co-AA nanohydrogels displayed high drug loading capacity reaching to 18.65 wt.% with excellent physical and serum stability. Moreover, the prepared HK-loaded nanohydrogels exhibited efficient in vitro and in vivo antitumor activities. In vivo, HK-loaded nanohydrogels demonstrated suppression of VEGF-1 and Ki-67 expression levels, besides inducing apoptosis through upregulating the expression level of active caspase-3 in breast cancer-bearing mice. Overall, the developed nanohydrogels (NGs) with pH and temperature responsivity provide a promising nanocarrier for anticancer treatment.

Ultrasonic Microplotting of Microgel Bioinks

Material scaffolds that mimic the structure, function, and bioactivity of native biological tissues are in constant development. Recently, material scaffolds composed of microgel particles have shown promise for applications ranging from bone regeneration to spheroid cell growth. Previous studies with poly N-isopropylacrylamide microgel scaffolds utilized a layer-by-layer (LBL) technique where individual, uniform microgel layers are built on top of each other resulting in a multilayer scaffold. However, this technique is limited in its applications due to the inability to control microscale deposition or patterning of multiple particle types within a microgel layer. In this study, an ultrasonic microplotting technique is used to address the limitations of LBL fabrication to create patterned microgel films. Printing parameters, such as bioink formulation, surface contact angle, and print head diameter, are optimized to identify the ideal parameters needed to successfully print microgel films. It was found that bioinks composed of 2 mg/mL of microgels and 20% polyethylene glycol by volume (v/v), on bovine serum albumin-coated glass, with a print head diameter of 50 μm resulted in the highest quality prints. Patterned films were created with a maximum resolution of 50 μm with the potential for finer resolutions to be achieved with alternative bioink compositions and printing parameters. Overall, ultrasonic microplotting can be used to create more complex microgel films than is possible with LBL techniques and offers the possibility of greater printing resolution in 3D with further technology development.

Platelet-like particles improve fibrin network properties in a hemophilic model of provisional matrix structural defects

Following injury, a fibrin-rich provisional matrix is formed to stem blood loss and provide a scaffold for infiltrating cells, which rebuild the damaged tissue. Defects in fibrin network formation contribute to impaired healing outcomes, as evidenced in hemophilia. Platelet-fibrin interactions greatly influence fibrin network structure via clot contraction, which increases fibrin density over time. Previously developed hemostatic platelet-like particles (PLPs) are capable of mimicking platelet functions including binding to fibrin fibers, augmenting clotting, and inducing clot retraction. In this study, we aimed to apply PLPs within a plasma-based in vitro hemophilia B model of deficient fibrin network structure to determine the ability of PLPs to improve fibrin structure and wound healing responses within hemophilia-like abnormal fibrin network formation. PLP impact on structurally deficient clot networks was assessed via confocal microscopy, a micropost deflection model, atomic force microscopy and an in vitro wound healing model of early cell migration within a provisional fibrin matrix. PLPs improved clot network density, force generation, and stiffness, and promoted fibroblast migration within an in vitro model of early wound healing under hemophilic conditions, indicating that PLPs could provide a biomimetic platform for improving wound healing events in disease conditions that cause deficient fibrin network formation.

Ultrasound enhanced synthetic platelet therapy for augmented wound repair

Native platelets perform a number of functions within the wound healing process, including interacting with fibrin fibers at the wound site to bring about retraction after clot formation. Clot retraction improves clot stability and enhances the function of the fibrin network as a provisional matrix to support cellular infiltration of the wound site, thus facilitating tissue repair and remodeling after hemostasis. In cases of traumatic injury or disease, platelets can become depleted and this process disrupted. To that end, our lab has developed synthetic platelet-like particles (PLPs) that recapitulate the clot retraction abilities of native platelets through a Brownian-wrench driven mechanism that drives fibrin network densification and clot retraction over time, however, this Brownian-motion driven process occurs on a longer time scale than native active actin/myosin-driven platelet-mediated clot retraction. We hypothesized that a combinatorial therapy comprised of ultrasound stimulation of PLP motion within fibrin clots would facilitate a faster induction of clot retraction on a more platelet-mimetic time scale and at a lower dosage than required for PLPs acting alone. We found that application of ultrasound in combination with a subtherapeutic dosage of PLPs resulted in increased clot density and stiffness, improved fibroblast migration in vitro and increased epidermal thickness and angiogenesis in vivo, indicating that this combination therapy has potential to facilitate multiphase pro-healing outcomes. Additionally, while these particular studies focus on the role of ultrasound in enhancing specific interactions between fibrin-binding synthetic PLPs embedded within fibrin networks, these studies have wide applicability in understanding the role of ultrasound stimulation in enhancing multi-scale colloidal interactions within fibrillar matrices.

Nanosilver composite pNIPAm microgels for the development of antimicrobial platelet-like particles

Platelets crucially facilitate wound healing but can become depleted in traumatic injury or chronic wounds. Previously, our group developed injectable platelet-like particles (PLPs) comprised of highly deformable, ultralow crosslinked pNIPAm microgels (ULCs) coupled to fibrin binding antibodies to treat post-trauma bleeding. PLP fibrin-binding facilitates homing to sites of injury, promotes clot formation, and, due to high particle deformability, induces clot retraction. Clot retraction augments healing by increasing clot stability, enhancing clot stiffness, and promoting cell migration into the wound bed. Because post-traumatic healing is often complicated by infection, the objective of these studies was to develop antimicrobial nanosilver microgel composite PLPs to augment hemostasis, fight infection, and promote healing post-trauma. A key goal was to maintain particle deformability following silver incorporation to preserve PLP-mediated clot retraction. Clot retraction, antimicrobial activity, hemostasis after trauma, and healing after injury were evaluated via confocal microscopy, colony-forming unit assays, a murine liver trauma model, and a murine full-thickness injury model in the absence or presence of infection, respectively. We found that nanosilver incorporation does not affect base PLP performance while bestowing significant antimicrobial activity and enhancing infected wound healing outcomes. Therefore, Ag-PLPs have great promise for treating hemorrhage and improving healing following trauma.

A Way to Predict Gold Nanoparticles/Polymer Hybrid Microgel Agglomeration Based on Rheological Studies

In this work, a detailed rheological study of hybrid poly(acrylamide-co-acrylic acid) P(AAm-co-AAc) aqueous microgel dispersions is performed. Our intention is to understand how the presence of gold nanoparticles, AuNP, embedded within the microgel matrix, affects the viscoelastic properties, the colloidal gel structure formation, and the structure recovery after cessation of the deformation of the aqueous microgel dispersions. Frequency sweep experiments confirmed that hybrid microgel dispersions present a gel-like behavior and that the presence of AuNP content within microgel matrix contributes to the elasticity of the microgel dispersions. Strain sweep test confirmed that hybrid microgels aqueous dispersion also form colloidal gel structures that break upon deformation but that can be recovered when the deformation decreases. The fractal analysis performed to hybrid microgels, by applying Shih et al. and Wu and Morbidelli's scaling theories, evidenced that AuNP significantly affects the colloidal gel structure configuration ending up with the formation of agglomerates or microgel clusters with closer structures in comparison to the reference P(AAm-co-AAc) aqueous microgel dispersions.

Development of biomimetic antimicrobial platelet-like particles comprised of microgel nanogold composites

A blood clot is formed in response to bleeding by platelet aggregation and adherence to fibrin fibers. Platelets contract over time, stabilizing the clot, which contributes to wound healing. We have developed platelet-like particles (PLPs) that augment clotting and induce clot retraction by mimicking the fibrin-binding capabilities and morphology of native platelets. Wound repair following hemostasis can be complicated by infection; therefore, we aim to augment wound healing by combining PLPs with antimicrobial gold to develop nanogold composites (NGCs). PLPs were synthesized with N-isopropylacrylamide (NIPAm)/co-acrylic acid in a precipitation polymerization reaction and conjugated to a fibrin-specific antibody. Two methods were employed to create NGCs: 1) noncovalent swelling with aqueous gold nanospheres, and 2) covalent seeding and growth. Since the ability of PLPs to mimic platelet morphology and clot retraction requires a high degree of particle deformability, we investigated how PLPs created from NGCs affected these properties. Cryogenic Scanning Electron Microscopy (cryoSEM) and atomic force microscopy (AFM) demonstrated that particle deformability, platelet-mimetic morphology and clot retraction were maintained in NGC-based PLPs. The effect of NGCs on bacterial adhesion and growth was assessed with antimicrobial assays. These results demonstrate NGCs fabricated through noncovalent and covalent methods retain deformability necessary for clot collapse and exhibit some antimicrobial potential. Therefore, NGCs are promising materials for preventing hemorrhage and infection following trauma.

Thermoresponsive Poly(N-Isopropylacrylamide-co-Dimethylaminoethyl Methacrylate) Microgel Aqueous Dispersions with Potential Antimicrobial Properties

The work herein describes the preparation of thermoresponsive microgels with potential antimicrobial properties. Most of the work performed so far regarding microgels with antimicrobial activity, deals with the ability of microgels to carry and release antibiotics or antimicrobial agents (antimicrobial peptides). The originality of this work lies in the possibility of developing intrinsic antimicrobial microgels by copolymerization of the well-known thermoresponsive monomer, N-isopropylacrylamide (NIPAM) with dimethylaminoethyl methacrylate (DMAEMA), a water-soluble monomer, to form microgels via precipitation polymerization (radical polymerization). Due to the presence of a tertiary amine in the DMAEMA comonomer, microgels can be modified by N-alkylation reaction with methyl and butyl iodide. This quaternization confers positive charges to the microgel surfaces and thus the potential antimicrobial activity. The effect of DMAEMA content and its quaternization with both, methyl and butyl iodide is evaluated in terms of thermal and surface charge properties, as well as in the microgel size and viscoelastic behavior. Finally, a preliminary study of the antimicrobial activity against different microorganisms is also performed in terms of minimum inhibitory concentration (MIC). From this study we determined that in contrast with butylated microgels, methylated ones show potential antimicrobial activity and good physical properties besides of maintaining microgel thermo-responsiveness.

Platelet-like particles dynamically stiffen fibrin matrices and improve wound healing outcomes

Native platelets perform several critical functions within the context of wound healing, including participating in initial hemostasis and interacting with fibrin at the wound site to induce clot retraction. Platelet depletion or dysfunction due to trauma or disease can inhibit robust wound healing responses. There has been a focus recently on developing synthetic, non-immunogenic platelet mimetic technologies for the purpose of augmenting hemostatic responses in cases of deficient native platelet functionality. Here we describe the application of synthetic platelet-like particles (PLPs), capable of recapitulating the deformable platelet body and fibrin specificity found in native platelets, to enhance healing outcomes. We first demonstrate PLPs mimic activated platelet morphology and induce fibrin clot retraction. During clot retraction, native platelets generate forces within a fibrin network to stiffen the fibrin matrix; therefore, we hypothesized that our PLPs will likewise be able to stiffen provisional fibrin matrices. Due to previous studies indicating that increased matrix stiffness is linked to increased cellular migration, we further hypothesize that PLP-mediated fibrin stiffening will enhance cell migration and improve healing outcomes within in vitro and in vivo models of wound healing. PLPs were found to enhance fibroblast migration in in vitro models of early wound healing and enhance healing outcomes in an in vivo murine model of wound healing. These studies demonstrate the utility of PLPs for enhancing wound repair and also provide insight into the role of native platelet-mediated clot retraction in wound healing.

Synthesis and structure of deuterated ultra-low cross-linked poly(N-isopropylacrylamide) microgels

Partial deuteration of the N-isopropylacrylamide monomer reveals new insights into the self-cross-linking of polymer chains in ultra-low cross-linked microgels.

Biomimetic microgels with controllable deformability improve healing outcomes

Platelets mediate hemostasis by aggregating and binding to fibrin to promote clotting. Over time, platelets contract the fibrin network to induce clot retraction, which contributes to wound healing outcomes by increasing clot stability and improving blood flow to ischemic tissue. In this study, we describe the development of hollow platelet-like particles (PLPs) that mimic the native platelet function of clot retraction in a controlled manner and demonstrate that clot retraction-inducing PLPs promote healing in vivo. PLPs are created by coupling fibrin-binding antibodies to CoreShell (CS) or hollow N-isopropylacrylamide (NIPAm) microgels with varying degrees of shell crosslinking. We demonstrate that hollow microgels with loosely crosslinked shells display a high degree of deformability and mimic activated platelet morphology, while intact CS microgels and hollow microgels with increased crosslinking in the shell do not. When coupled to a fibrin-binding antibody to create PLPs, hollow particles with low degrees of shell crosslinking cause fibrin clot collapse in vitro, recapitulating the clot retraction function of platelets, while other particle types do not. Furthermore, hollow PLPs with low degrees of shell crosslinking improve some wound healing outcomes in vivo.