Steps Toward Recapitulating Endothelium: A Perspective on the Next Generation of Hemocompatible Coatings.

Endothelium, the lining in this blood vessel, orchestrates three main critical functions such as protecting blood components, modulating of hemostasis by secreting various inhibitors, and directing clot digestion (fibrinolysis) by activating tissue plasminogen activator. No other surface can perform these tasks; thus, the contact of blood and blood-contacting medical devices inevitably leads to the activation of coagulation, often causing device failure, and thromboembolic complications. This perspective, first, discusses the biological mechanisms of activation of coagulation and highlights the efforts of advanced coatings to recapitulate one characteristic of endothelium, hereafter single functions of endothelium and noting necessity of the synergistic integration of its three main functions. Subsequently, it is emphasized that to overcome the challenges of blood compatibility an endothelium-mimicking system is needed, proposing a synergy of bottom-up synthetic biology, particularly synthetic cells, with passive- and bioactive surface coatings. Such integration holds promise for developing advanced biomaterials capable of recapitulating endothelial functions, thereby enhancing the hemocompatibility and performance of blood-contacting medical devices.

Tackling the Root Cause of Surface-Induced Coagulation: Inhibition of FXII Activation to Mitigate Coagulation Propagation and Prevent Clotting.

Factor XII (FXII) is a zymogen present in blood that tends to adsorb onto the surfaces of blood-contacting medical devices. Once adsorbed, it becomes activated, initiating a cascade of enzymatic reactions that lead to surface-induced coagulation. This process is characterized by multiple redundancies, making it extremely challenging to prevent clot formation and preserve the properties of the surface. In this study, a novel modulatory coating system based on C1-esterase inhibitor (C1INH) functionalized polymer brushes, which effectively regulates the activation of FXII is proposed. Using surface plasmon resonance it is demonstrated that this coating system effectively repels blood plasma proteins, including FXII, while exhibiting high activity against activated FXII and plasma kallikrein under physiological conditions. This unique property enables the modulation of FXII activation without interfering with the overall hemostasis process. Furthermore, through dynamic Chandler loop studies, it is shown that this coating significantly improves the hemocompatibility of polymeric surfaces commonly used in medical devices. By addressing the root cause of contact activation, the synergistic interplay between the antifouling polymer brushes and the modulatory C1INH is expected to lay the foundation to enhance the hemocompatibility of medical device surfaces.

Brush-Like Coatings Provide a Cloak of Invisibility to Titanium Implants.

Orthopedic implants such as knee and hip implants are one of the most important types of medical devices. Currently, the surface of the most advanced implants consists of titanium or titanium-alloys with high porosity at the bone-contacting surface leading to superior mechanical properties, excellent biocompatibility, and the capability of inducing osseointegration. However, the increased surface area of porous titanium provides a nidus for bacteria colonization leading to implant-related infections, one of the main reasons for implant failure. Here, two readily applicable titanium-coatings based on hydrophilic carboxybetaine polymers that turn the surface stealth thereby preventing bacterial adhesion and colonization are developed. These coatings are biocompatible, do not affect cell functionality, exhibit great antifouling properties, and do not cause additional inflammation during the healing process. In this way, the coatings can prevent implant-related infections, while at the same time being completely innocuous to its biological environment. Thus, these coating strategies are a promising route to enhance the biocompatibility of orthopedic implants and have a high potential for clinical use, while being easy to implement in the implant manufacturing process.

Glycan-Driven Formation of Raft-Like Domains with Hierarchical Periodic Nanoarrays on Dendrimersome Synthetic Cells.

The accurate spatial segregation into distinct phases within cell membranes coordinates vital biochemical processes and functionalities in living organisms. One of nature's strategies to localize reactivity is the formation of dynamic raft domains. Most raft models rely on liquid-ordered L0 phases in a liquid-disordered Ld phase lacking correlation and remaining static, often necessitating external agents for phase separation. Here, we introduce a synthetic system of bicomponent glycodendrimersomes coassembled from Janus dendrimers and Janus glycodendrimers (JGDs), where lactose-lactose interactions exclusively drive lateral organization. This mechanism results in modulated phases across two length scales, yielding raft-like microdomains featuring nanoarrays at the nanoscale. By varying the density of lactose and molecular architecture of JGDs, the nanoarray type and size, shape, and spacing of the domains were controlled. Our findings offer insight into the potential primordial origins of rudimentary raft domains and highlight the crucial role of glycans within the glycocalyx.

Green Solvent-Based Antifouling Polymer Brushes Demonstrate Excellent Hemocompatibility.

Medical devices are crucial for patient care, yet even the best biomaterials lead to infections and unwanted activation of blood coagulation, potentially being life-threatening. While hydrophilic polymer brushes are the best coatings to mitigate these issues, their reliance on fossil raw materials underscores the urgency of bio-based alternatives. In this work, we introduce polymer brushes of a green solvent-based monomer, prohibiting protein adsorption, bacterial colonization, and blood clot formation at the same level as fossil-based polymer brushes. The polymer brushes are composed of N,N-dimethyl lactamide acrylate (DMLA), can be polymerized in a controlled manner, and show strong hydrophilicity as determined by thermodynamic analysis of the surface tension components. The contact of various challenging protein solutions results in repellency on the poly(DMLA) brushes. Furthermore, the poly(DMLA) brushes completely prevent the adhesion and colonization of Escherichia coli. Remarkably, upon blood contact, the poly(DMLA) brushes successfully prevent the formation of a fibrin network and leukocyte adhesion on the surface. While showcasing excellent antifouling properties similar to those of N-hydroxypropyl methacrylamide (HPMA) polymer brushes as one of the best antifouling coatings, the absence of hydroxyl groups prevents activation of the complement system in blood. We envision the polymer brushes to contribute to the future of hemocompatible coatings.

Self-Assembly of Glycerol-Amphiphilic Janus Dendrimers Amplifies and Indicates Principles for the Selection of Stereochemistry by Biological Membranes.

The principles for the selection of the stereochemistry of phospholipids of biological membranes remain unclear and continue to be debated. Therefore, any new experiments on this topic may help progress in this field. To address this question, three libraries of constitutional isomeric glycerol-amphiphilic Janus dendrimers (JDs) with nonsymmetric homochiral, racemic, and symmetric achiral branching points were synthesized by an orthogonal-modular-convergent methodology. These JDs amplify self-assembly, and therefore, monodisperse vesicles known as dendrimersomes (DSs) with predictable dimensions programmed by JD concentration were assembled by rapid injection of their ethanol solution into water. DSs of homochiral JD enantiomers, racemic, including mixtures of different enantiomers, and achiral exhibited similar DS size-concentration dependence. However, the number of bilayers of DSs assembled from homochiral, achiral, and racemic JDs determined by cryo-TEM were different. Statistical analysis of the number of bilayers and coarse-grained molecular dynamics simulations demonstrated that homochiral JDs formed predominantly unilamellar DSs. Symmetric achiral JDs assembled only unilamellar DSs while racemic JDs favored multilamellar DSs. Since cell membranes are unilamellar, these results indicate a new rationale for nonsymmetric homochiral vs racemic selection. Simultaneously, these experiments imply that the symmetric achiral lipids forming more stable membrane, probably had been the preferable assemblies of prebiotic cell membranes.

Zwitterionic Dendrimersomes: A Closer Xenobiotic Mimic of Cell Membranes.

Building functional mimics of cell membranes is an important task toward the development of synthetic cells. So far, lipid and amphiphilic block copolymers are the most widely used amphiphiles with the bilayers by the former lacking stability while membranes by the latter are typically characterized by very slow dynamics. Herein, a new type of Janus dendrimer containing a zwitterionic phosphocholine hydrophilic headgroup (JDPC ) and a 3,5-substituted dihydrobenzoate-based hydrophobic dendron is introduced. JDPC self-assembles in water into zwitterionic dendrimersomes (z-DSs) that faithfully recapitulate the cell membrane in thickness, flexibility, and fluidity, while being resilient to harsh conditions and displaying faster pore closing dynamics in the event of membrane rupture. This enables the fabrication of hybrid DSs with components of natural membranes, including pore-forming peptides, structure-directing lipids, and glycans to create raft-like domains or onion vesicles. Moreover, z-DSs can be used to create active synthetic cells with life-like features that mimic vesicle fusion and motility as well as environmental sensing. Despite their fully synthetic nature, z-DSs are minimal cell mimics that can integrate and interact with living matter with the programmability to imitate life-like features and beyond.

The Relation Between Protein Adsorption and Hemocompatibility of Antifouling Polymer Brushes.

Whenever an artificial surface comes into contact with blood, proteins are rapidly adsorbed onto its surface. This phenomenon, termed fouling, is then followed by a series of undesired reactions involving activation of complement or the coagulation cascade and adhesion of leukocytes and platelets leading to thrombus formation. Thus, considerable efforts are directed towards the preparation of fouling-resistant surfaces with the best possible hemocompatibility. Herein, a comprehensive hemocompatibility study after heparinized blood contact with seven polymer brushes prepared by surface-initiated atom transfer radical polymerization is reported. The resistance to fouling is quantified and thrombus formation and deposition of blood cellular components on the coatings are analyzed. Moreover, identification of the remaining adsorbed proteins is performed via mass spectroscopy to elucidate their influence on the surface hemocompatibility. Compared with an unmodified glass surface, the grafting of polymer brushes minimizes the adhesion of platelets and leukocytes and prevents the thrombus formation. The fouling from undiluted blood plasma is reduced by up to 99%. Most of the identified proteins are connected with the initial events of foreign body reaction towards biomaterial (coagulation cascade proteins, complement component, and inflammatory proteins). In addition, several proteins that are not previously linked with blood-biomaterial interaction are presented and discussed.

Dendrimersome Synthetic Cells Harbor Cell Division Machinery of Bacteria.

The integration of active cell machinery with synthetic building blocks is the bridge toward developing synthetic cells with biological functions and beyond. Self-replication is one of the most important tasks of living systems, and various complex machineries exist to execute it. In Escherichia coli, a contractile division ring is positioned to mid-cell by concentration oscillations of self-organizing proteins (MinCDE), where it severs membrane and cell wall. So far, the reconstitution of any cell division machinery has exclusively been tied to liposomes. Here, the reconstitution of a rudimentary bacterial divisome in fully synthetic bicomponent dendrimersomes is shown. By tuning the membrane composition, the interaction of biological machinery with synthetic membranes can be tailored to reproduce its dynamic behavior. This constitutes an important breakthrough in the assembly of synthetic cells with biological elements, as tuning of membrane-divisome interactions is the key to engineering emergent biological behavior from the bottom-up.

Ionic Combisomes: A New Class of Biomimetic Vesicles to Fuse with Life.

The construction of biomembranes that faithfully capture the properties and dynamic functions of cell membranes remains a challenge in the development of synthetic cells and their application. Here a new concept for synthetic cell membranes based on the self-assembly of amphiphilic comb polymers into vesicles, termed ionic combisomes (i-combisomes) is introduced. These combs consist of a polyzwitterionic backbone to which hydrophobic tails are linked by electrostatic interactions. Using a range of microscopies and molecular simulations, the self-assembly of a library of combs in water is screened. It is discovered that the hydrophobic tails form the membrane's core and force the backbone into a rod conformation with nematic-like ordering confined to the interface with water. This particular organization resulted in membranes that combine the stability of classic polymersomes with the biomimetic thickness, flexibility, and lateral mobility of liposomes. Such unparalleled matching of biophysical properties and the ability to locally reconfigure the molecular topology of its constituents enable the harboring of functional components of natural membranes and fusion with living bacteria to "hijack" their periphery. This provides an almost inexhaustible palette to design the chemical and biological makeup of the i-combisomes membrane resulting in a powerful platform for fundamental studies and technological applications.

Brush-Like Interface on Surface-Attached Hydrogels Repels Proteins and Bacteria.

Interfacing artificial materials with biological tissues remains a challenge. The direct contact of their surface with the biological milieu results in multiscale interactions, in which biomacromolecules adsorb and act as transducers mediating the interactions with cells and tissues. So far, only antifouling polymer brushes have been able to conceal the surface of synthetic materials. However, their complex synthesis has precluded their translation to applications. Here, it is shown that ultrathin surface-attached hydrogel coatings of N-(2-hydroxypropyl) methacrylamide (HPMA) and carboxybetaine methacrylamide (CBMAA) provide the same level of protection as brushes. In spite of being readily applicable, these coatings prevent the fouling from whole blood plasma and provide a barrier to the adhesion of Gram positive and negative bacteria. The analysis of the components of the surface free energy and nanoindentation experiments reveals that the excellent antifouling properties stem from the strong surface hydrophilicity and the presence of a brush-like structure at the water interface. Moreover, these coatings can be functionalized to achieve antimicrobial activity while remaining stealth and non-cytotoxic to eukaryotic cells. Such level of performance is previously only achieved with brushes. Thus, it is anticipated that this readily applicable strategy is a promising route to enhance the biocompatibility of real biomedical devices.

Complement Activation Dramatically Accelerates Blood Plasma Fouling On Antifouling Poly(2-hydroxyethyl methacrylate) Brush Surfaces.

Non-specific protein adsorption (fouling) triggers a number of deleterious events in the application of biomaterials. Antifouling polymer brushes successfully suppress fouling, however for some coatings an extremely high variability of fouling for different donors remains unexplained. The authors report that in the case of poly(2-hydroxyethyl methacrylate) (poly(HEMA)) this variability is due to the complement system activation that causes massive acceleration in the fouling kinetics of blood plasma. Using plasma from various donors, the fouling kinetics on poly(HEMA) is analyzed and correlated with proteins identified in the deposits on the surface and with the biochemical compositions of the plasma. The presence of complement components in fouling deposits and concentrations of C3a in different plasmas indicate that the alternative complement pathway plays a significant role in the fouling on poly(HEMA) through the "tick-over" mechanism of spontaneous C3 activation. The generated C3b binds to the poly(HEMA) surface and amplifies complement activation locally. Heat-inactivated plasma prevents accelerated fouling kinetics, confirming the central role of complement activation. The results highlight the need to take into account the variability between individuals when assessing interactions between biomaterials and blood plasma, as well as the importance of the mechanistic insight that can be gained from protein identification.

Unraveling the Mechanism and Kinetics of Binding of an LCI-eGFP-Polymer for Antifouling Coatings.

The ability of proteins to adsorb irreversibly onto surfaces opens new possibilities to functionalize biological interfaces. Herein, the mechanism and kinetics of adsorption of protein-polymer macromolecules with the ability to equip surfaces with antifouling properties are investigated. These macromolecules consist of the liquid chromatography peak I peptide from which antifouling polymer brushes are grafted using single electron transfer-living radical polymerization. Surface plasmon resonance spectroscopy reveals an adsorption mechanism that follows a Langmuir-type of binding with a strong binding affinity to gold. X-ray reflectivity supports this by proving that the binding occurs exclusively by the peptide. However, the lateral organization at the surface is directed by the cylindrical eGFP. The antifouling functionality of the unimolecular coatings is confirmed by contact with blood plasma. All coatings reduce the fouling from blood plasma by 8894% with only minor effect of the degree of polymerization for the studied range (DP between 101 and 932). The excellent antifouling properties, combined with the ease of polymerization and the straightforward coating procedure make this a very promising antifouling concept for a multiplicity of applications.

Improving Hemocompatibility: How Can Smart Surfaces Direct Blood To Fight against Thrombi.

Nature utilizes endothelium as a blood interface that perfectly controls hemostasis, preventing the uncontrolled formation of thrombi. The management of positive and negative feedback that finely tunes thrombosis and fibrinolysis is essential for human life, especially for patients who undergo extracorporeal circulation (ECC) after a severe respiratory or cardiac failure. The exposure of blood to a surface different from healthy endothelium inevitably initiates coagulation, drastically increasing the mortality rate by thromboembolic complications. In the present study, an ultrathin antifouling fibrinolytic coating capable of disintegrating thrombi in a self-regulated manner is reported. The coating system is composed of a polymer brush layer that can prevent any unspecific interaction with blood. The brushes are functionalized with a tissue plasminogen activator (tPA) to establish localized fibrinolysis that solely and exclusively is active when it is required. This interactive switching between the dormant and active state is realized through an amplification mechanism that increases (positive feedback) or restores (negative feedback) the activity of tPA depending on whether a thrombus is detected and captured or not. Thus, only a low surface density of tPA is necessary to lyse real thrombi. Our work demonstrates the first report of a coating that self-regulates its fibrinolytic activity depending on the conditions of blood.

Enhanced Concanavalin†A Binding to Preorganized Mannose Nanoarrays in Glycodendrimersomes Revealed Multivalent Interactions.

The effect of the two-dimensional glycan display on glycan-lectin recognition remains poorly understood despite the importance of these interactions in a plethora of cellular processes, in (patho)physiology, as well as its potential for advanced therapeutics. Faced with this challenge we utilized glycodendrimersomes, a type of synthetic vesicles whose membrane mimics the surface of a cell and offers a means to probe the carbohydrate biological activity. These single-component vesicles were formed by the self-assembly of sequence-defined mannose-Janus dendrimers, which serve as surrogates for glycolipids. Using atomic force microscopy and molecular modeling we demonstrated that even mannose, a monosaccharide, was capable of organizing the sugar moieties into periodic nanoarrays without the need of the formation of liquid-ordered phases as assumed necessary for rafts. Kinetics studies of Concanavalin A binding revealed that those nanoarrays resulted in a new effective ligand yielding a ten-fold increase in the kinetic and thermodynamic constant of association.

Unraveling topology-induced shape transformations in dendrimersomes.

The vital functions of cell membranes require their ability to quickly change shape to perform complex tasks such as motion, division, endocytosis, and apoptosis. Membrane curvature in cells is modulated by very complex processes such as changes in lipid composition, the oligomerization of curvature-scaffolding proteins, and the reversible insertion of protein regions that act like wedges in the membrane. But, could much simpler mechanisms support membrane shape transformation? In this work, we demonstrate how the change of amphiphile topology in the bilayer can drive shape transformations of cell membrane models. To tackle this, we have designed and synthesized new types of amphiphiles-Janus dendrimers-that self-assemble into uni-, multilamellar, or smectic-ordered vesicles, named dendrimersomes. We synthesized Janus dendrimers containing a photo-labile bond that upon UV-Vis irradiation cleavage lose a part of the hydrophilic dendron. This leads to a change from a cylindrically to a wedge-shaped amphiphile. The high mobility of these dendrimers allows for the concentration of the wedge-shaped amphiphiles and the generation of transmembrane asymmetries. The concentration of the wedges and their rate of segregation allowed control of the budding and generation of structures such as tubules and high genus vesicles.

Efficacy of A Poly(MeOEGMA) Brush on the Prevention of Escherichia coli Biofilm Formation and Susceptibility.

Urinary tract infections are one of the most common hospital-acquired infections, and they are often associated with biofilm formation in indwelling medical devices such as catheters and stents. This study aims to investigate the antibiofilm performance of a polymer brush-poly[oligo(ethylene glycol) methyl ether methacrylate], poly(MeOEGMA)-and evaluate its effect on the antimicrobial susceptibility of Escherichia coli biofilms formed on that surface. Biofilms were formed in a parallel plate flow chamber (PPFC) for 24 h under the hydrodynamic conditions prevailing in urinary catheters and stents and challenged with ampicillin. Results obtained with the brush were compared to those obtained with two control surfaces, polydimethylsiloxane (PDMS) and glass. The polymer brush reduced by 57% the surface area covered by E. coli after 24 h, as well as the number of total adhered cells. The antibiotic treatment potentiated cell death and removal, and the total cell number was reduced by 88%. Biofilms adapted their architecture, and cell morphology changed to a more elongated form during that period. This work suggests that the poly(MeOEGMA) brush has potential to prevent bacterial adhesion in urinary tract devices like ureteral stents and catheters, as well as in eradicating biofilms developed in these biomedical devices.

Direct Visualization of Vesicle Disassembly and Reassembly Using Photocleavable Dendrimers Elucidates Cargo Release Mechanisms.

Release of cargo molecules from cell-like nanocarriers can be achieved by chemical perturbations, including changes to pH and redox state and via optical modulation of membrane properties. However, little is known about the kinetics or products of vesicle breakdown due to limitations in real-time imaging at nanometer length scales. Using a library of 12 single-single type photocleavable amphiphilic Janus dendrimers, we developed a self-assembling light-responsive dendrimersome vesicle platform. A photocleavable ortho-nitrobenzyl inserted between the hydrophobic and hydrophilic dendrons of amphiphilic Janus dendrimers allowed for photocleavage and disassembly of their supramolecular assemblies. Distinct methods used to self-assemble amphiphilic Janus dendrimers produced either nanometer size small unilamellar vesicles or micron size giant multilamellar and onion-like dendrimersomes. In situ observation of giant photosensitive dendrimersomes via confocal microscopy elucidated rapid morphological transitions that accompany vesicle breakdown upon 405 nm laser illumination. Giant dendrimersomes displayed light-induced cleavage, disassembling and reassembling into much smaller vesicles at millisecond time scales. Additionally, photocleavable vesicles demonstrated rapid release of molecular and macromolecular cargos. These results guided our design of multilamellar particles to photorelease surface-attached proteins, photoinduce cargo recruitment, and photoconvert vesicle morphology. Real-time characterization of the breakdown and reassembly of lamellar structures provides insights on partial cargo retention and informs the design of versatile, optically regulated carriers for applications in nanoscience and synthetic biology.

Nanovesicles displaying functional linear and branched oligomannose self-assembled from sequence-defined Janus glycodendrimers.

Cell surfaces are often decorated with glycoconjugates that contain linear and more complex symmetrically and asymmetrically branched carbohydrates essential for cellular recognition and communication processes. Mannose is one of the fundamental building blocks of glycans in many biological membranes. Moreover, oligomannoses are commonly found on the surface of pathogens such as bacteria and viruses as both glycolipids and glycoproteins. However, their mechanism of action is not well understood, even though this is of great potential interest for translational medicine. Sequence-defined amphiphilic Janus glycodendrimers containing simple mono- and disaccharides that mimic glycolipids are known to self-assemble into glycodendrimersomes, which in turn resemble the surface of a cell by encoding carbohydrate activity via supramolecular multivalency. The synthetic challenge of preparing Janus glycodendrimers containing more complex linear and branched glycans has so far prevented access to more realistic cell mimics. However, the present work reports the use of an isothiocyanate-amine "click"-like reaction between isothiocyanate-containing sequence-defined amphiphilic Janus dendrimers and either linear or branched oligosaccharides containing up to six monosaccharide units attached to a hydrophobic amino-pentyl linker, a construct not expected to assemble into glycodendrimersomes. Unexpectedly, these oligoMan-containing dendrimers, which have their hydrophobic linker connected via a thiourea group to the amphiphilic part of Janus glycodendrimers, self-organize into nanoscale glycodendrimersomes. Specifically, the mannose-binding lectins that best agglutinate glycodendrimersomes are those displaying hexamannose. Lamellar "raft-like" nanomorphologies on the surface of glycodendrimersomes, self-organized from these sequence-defined glycans, endow these membrane mimics with high biological activity.