Combinatorial synthesis with high throughput discovery of protein-resistant membrane surfaces

Hemocompatibility challenge of membrane oxygenator for artificial lung technology

The artificial lung (AL) technology is one of the membrane-based artificial organs that partly augments lung functions, i.e. blood oxygenation and CO₂ removal. It is generally employed as an extracorporeal membrane oxygenation (ECMO) device to treat acute and chronic lung-failure patients, and the recent outbreak of the COVID-19 pandemic has re-emphasized the importance of this technology. The principal component in AL is the polymeric membrane oxygenator that facilitates the O₂/CO₂ exchange with the blood. Despite the considerable improvement in anti-thrombogenic biomaterials in other applications (e.g., stents), AL research has not advanced at the same rate. This is partly because AL research requires interdisciplinary knowledge in biomaterials and membrane technology. Some of the promising biomaterials with reasonable hemocompatibility - such as emerging fluoropolymers of extremely low surface energy - must first be fabricated into membranes to exhibit effective gas exchange performance. As AL membranes must also demonstrate high hemocompatibility in tandem, it is essential to test the membranes using in-vitro hemocompatibility experiments before in-vivo test. Hence, it is vital to have a reliable in-vitro experimental protocol that can be reasonably correlated with the in-vivo results. However, current in-vitro AL studies are unsystematic to allow a consistent comparison with in-vivo results. More specifically, current literature on AL biomaterial in-vitro hemocompatibility data are not quantitatively comparable due to the use of unstandardized and unreliable protocols. Such a wide gap has been the main bottleneck in the improvement of AL research, preventing promising biomaterials from reaching clinical trials. This review summarizes the current state-of-the-art and status of AL technology from membrane researcher perspectives. Particularly, most of the reported in-vitro experiments to assess AL membrane hemocompatibility are compiled and critically compared to suggest the most reliable method suitable for AL biomaterial research. Also, a brief review of current approaches to improve AL hemocompatibility is summarized. STATEMENT OF SIGNIFICANCE: The importance of Artificial Lung (AL) technology has been re-emphasized in the time of the COVID-19 pandemic. The utmost bottleneck in the current AL technology is the poor hemocompatibility of the polymer membrane used for O₂/CO₂ gas exchange, limiting its use in the long-term. Unfortunately, most of the in-vitro AL experiments are unsystematic, irreproducible, and unreliable. There are no standardized in-vitro hemocompatibility characterization protocols for quantitative comparison between AL biomaterials. In this review, we tackled this bottleneck by compiling the scattered in-vitro data and suggesting the most suitable experimental protocol to obtain reliable and comparable hemocompatibility results. To the best of our knowledge, this is the first review paper focusing on the hemocompatibility challenge of AL technology.

Combinatorial Polyacrylamide Hydrogels for Preventing Biofouling on Implantable Biosensors

Biofouling on the surface of implanted medical devices and biosensors severely hinders device functionality and drastically shortens device lifetime. Poly(ethylene glycol) and zwitterionic polymers are currently considered "gold-standard" device coatings to reduce biofouling. To discover novel anti-biofouling materials, a combinatorial library of polyacrylamide-based copolymer hydrogels is created, and their ability is screened to prevent fouling from serum and platelet-rich plasma in a high-throughput parallel assay. It is found that certain nonintuitive copolymer compositions exhibit superior anti-biofouling properties over current gold-standard materials, and machine learning is used to identify key molecular features underpinning their performance. For validation, the surfaces of electrochemical biosensors are coated with hydrogels and their anti-biofouling performance in vitro and in vivo in rodent models is evaluated. The copolymer hydrogels preserve device function and enable continuous measurements of a small-molecule drug in vivo better than gold-standard coatings. The novel methodology described enables the discovery of anti-biofouling materials that can extend the lifetime of real-time in vivo sensing devices.

Combinatorial Polyacrylamide Hydrogels for Preventing Biofouling on Implantable Biosensors

Biofouling on the surface of implanted medical devices and biosensors severely hinders device functionality and drastically shortens device lifetime. Poly(ethylene glycol) and zwitterionic polymers are currently considered "gold-standard" device coatings to reduce biofouling. To discover novel anti-biofouling materials, a combinatorial library of polyacrylamide-based copolymer hydrogels is created, and their ability is screened to prevent fouling from serum and platelet-rich plasma in a high-throughput parallel assay. It is found that certain nonintuitive copolymer compositions exhibit superior anti-biofouling properties over current gold-standard materials, and machine learning is used to identify key molecular features underpinning their performance. For validation, the surfaces of electrochemical biosensors are coated with hydrogels and their anti-biofouling performance in vitro and in vivo in rodent models is evaluated. The copolymer hydrogels preserve device function and enable continuous measurements of a small-molecule drug in vivo better than gold-standard coatings. The novel methodology described enables the discovery of anti-biofouling materials that can extend the lifetime of real-time in vivo sensing devices.

Use of Protein Repellents to Enhance the Antimicrobial Functionality of Quaternary Ammonium Containing Dental Materials

An advancement in preventing secondary caries has been the incorporation of quaternary ammonium containing (QAC) compounds into a composite resin mixture. The permanent positive charge on the monomers allows for electrostatic-based killing of bacteria. Spontaneous adsorption of salivary proteins onto restorations dampens the antimicrobial capabilities of QAC compounds. Protein-repellent monomers can work with QAC restorations to achieve the technology's full potential. We discuss the theory behind macromolecular adsorption, direct and indirect characterization methods, and advances of protein repellent dental materials. The translation of protein adsorption to microbial colonization is covered, and the concerns and fallbacks of the state-of-the-art protein-resistant monomers are addressed. Last, we present new and exciting avenues for protein repellent monomer design that have yet to be explored in dental materials.

Polarized, Cobblestone, Human Retinal Pigment Epithelial Cell Maturation on a Synthetic PEG Matrix

Cell attachment is essential for the growth and polarization of retinal pigment epithelial (RPE) cells. Currently, surface coatings derived from biological proteins are used as the gold standard for cell culture. However, downstream processing and purification of these biological products can be cumbersome and expensive. In this study, we constructed a library of chemically modified nanofibers to mimic the Bruch's membrane of the retinal pigment epithelium. Using atmospheric-pressure plasma-induced graft polymerization with a high-throughput screening platform to modify the nanofibers, we identified three polyethylene glycol (PEG)-grafted nanofiber surfaces (PEG methyl ether methacrylate, n = 4, 8, and 45) from a library of 62 different surfaces as favorable for RPE cell attachment, proliferation, and maturation in vitro with cobblestone morphology. Compared with the biologically derived culture matrices such as vitronectin-based peptide Synthemax, our newly discovered synthetic PEG surfaces exhibit similar growth and polarization of retinal pigment epithelial (RPE) cells. However, they are chemically defined, are easy to synthesize on a large scale, are cost-effective, are stable with long-term storage capability, and provide a more physiologically accurate environment for RPE cell culture. To our knowledge, no one has reported that PEG derivatives directly support attachment and growth of RPE cells with cobblestone morphology. This study offers a unique PEG-modified 3D cell culture system that supports RPE proliferation, differentiation, and maturation with cobblestone morphology, providing a new avenue for RPE cell culture, disease modeling, and cell replacement therapy.

Printing-Assisted Surface Modifications of Patterned Ultrafiltration Membranes

Understanding and restricting microbial surface attachment will enhance wastewater treatment with membranes. We report a maskless lithographic patterning technique for the generation of patterned polymer coatings on ultrafiltration membranes. Polyethylene glycol, zwitterionic, or negatively charged hydrophilic polymer compositions in parallel- or perpendicular-striped patterns with respect to feed flow were evaluated using wastewater. Membrane fouling was dependent on the orientation and chemical composition of the coatings. Modifications reduced alpha diversity in the attached microbial community (Shannon indices decreased from 2.63 to 1.89) which nevertheless increased with filtration time. Sphingomonas species, which condition membrane surfaces and facilitate cellular adhesion, were depleted in all modified membranes. Microbial community structure was significantly different between control, different patterns, and different chemistries. This study broadens the tools for surface modification of membranes with polymer coatings and for understanding and optimization of antifouling surfaces.

Phosphate Ions Affect the Water Structure at Functionalized Membrane Surfaces

Antifouling surfaces improve function, efficiency, and safety in products such as water filtration membranes, marine vehicle coatings, and medical implants by resisting protein and biofilm adhesion. Understanding the role of water structure at these materials in preventing protein adhesion and biofilm formation is critical to designing more effective coatings. Such fouling experiments are typically performed under biological conditions using isotonic aqueous buffers. Previous studies have explored the structure of pure water at a few different antifouling surfaces, but the effect of electrolytes and ionic strength (I) on the water structure at antifouling surfaces is not well studied. Here sum frequency generation (SFG) spectroscopy is used to characterize the interfacial water structure at poly(ether sulfone) (PES) and two surface-modified PES films in contact with 0.01 M phosphate buffer with high and low salt (Ionic strength, I= 0.166 and 0.025 M, respectively). Unmodified PES, commonly used as a filtration membrane, and modified PES with a hydrophobic alkane (C18) and with a poly(ethylene glycol) (PEG) were used. In the low ionic strength phosphate buffer, water was strongly ordered near the surface of the PEG-modified PES film due to exclusion of phosphate ions and the creation of a surface potential resulting from charge separation between phosphate anions and sodium cations. However, in the high ionic strength phosphate buffer, the sodium and potassium chloride (138 and 3 mM, respectively) in the phosphate buffered saline screened this charge and substantially reduced water ordering. A much smaller water ordering and subsequent reduction upon salt addition was observed for the C18-modified PES, and little water structure change was seen for the unmodified PES. The large difference in water structuring with increasing ionic strength between widely used phosphate buffer and phosphate buffered saline at the PEG interface demonstrates the importance of studying antifouling coatings in the same aqueous environment for which they are designed. These results further suggest that strong long-range water structuring is limited in high ionic strength environments, such as within cells, facilitating chemical and biological reactions and processes.

Sequence-Defined Oligomers from Hydroxyproline Building Blocks for Parallel Synthesis Applications

The functionality of natural biopolymers has inspired significant effort to develop sequence-defined synthetic polymers for applications including molecular recognition, self-assembly, and catalysis. Conjugation of synthetic materials to biomacromolecules has played an increasingly important role in drug delivery and biomaterials. We developed a controlled synthesis of novel oligomers from hydroxyproline-based building blocks and conjugated these materials to siRNA. Hydroxyproline-based monomers enable the incorporation of broad structural diversity into defined polymer chains. Using a perfluorocarbon purification handle, we were able to purify diverse oligomers through a single solid-phase extraction method. The efficiency of synthesis was demonstrated by building 14 unique trimers and 4 hexamers from 6 diverse building blocks. We then adapted this method to the parallel synthesis of hundreds of materials in 96-well plates. This strategy provides a platform for the screening of libraries of modified biomolecules.

The Carbon-Water Interface: Modeling Challenges and Opportunities for the Water-Energy Nexus

Providing clean water and sufficient affordable energy to all without compromising the environment is a key priority in the scientific community. Many recent studies have focused on carbon-based devices in the hope of addressing this grand challenge, justifying and motivating detailed studies of water in contact with carbonaceous materials. Such studies are becoming increasingly important because of the miniaturization of newly proposed devices, with ubiquitous nanopores, large surface-to-volume ratio, and many, perhaps most of the water molecules in contact with a carbon-based surface. In this brief review, we discuss some recent advances obtained via simulations and experiments in the development of carbon-based materials for applications in water desalination. We suggest possible ways forward, with particular emphasis on the synergistic combination of experiments and simulations, with simulations now sometimes offering sufficient accuracy to provide fundamental insights. We also point the interested reader to recent works that complement our short summary on the state of the art of this important and fascinating field.

Combinatorial hydrogel library enables identification of materials that mitigate the foreign body response in primates

The foreign body response is an immune-mediated reaction that can lead to the failure of implanted medical devices and discomfort for the recipient^1–6. There is a critical need for biomaterials that overcome this key challenge in the development of medical devices. Here we use a combinatorial approach for covalent chemical modification to generate a large library of variants of one of the most widely used hydrogel biomaterials, alginate. We evaluated the materials in vivo and identified three triazole-containing analogs that substantially reduce foreign body reactions in both rodents and, for at least 6 months, in non-human primates. The distribution of the triazole modification creates a unique hydrogel surface that inhibits recognition by macrophages and fibrous deposition. In addition to the utility of the compounds reported here, our approach may enable the discovery of other materials that mitigate the foreign body response.

A new combinatorial method for synthesizing, screening, and discovering antifouling surface chemistries

A set of diverse monomers were synthesized using combinatorial chemistry and tested using our unique high-throughput screening platform. The versatility of our platform is exemplified by possible applications in reducing biological fouling on ship hulls, filtration membranes, and surgical instruments, to name a few. To demonstrate its efficacy, the novel monomers were graft-polymerized onto light sensitive poly(ether sulfone) (PES) membranes via atmospheric-pressure plasma polymerization. A diverse library was synthesized by reacting a common vinyl ester linker with a library of maleimides containing various different functional groups. This allowed us to produce a library of many different surfaces and graft them all using the same linker chemistry. The modified surfaces were then tested and screened for the best antiprotein adsorption (nonfouling) properties. Membranes, functionalized with carboxylic acid, zwitterionic, and ester groups, had the lowest protein adhesion compared with that of an unmodified control PES membrane after a static fouling test. After dynamic fouling, these same functionalities as well as a hydroxyl group exhibited the highest permeability. These monomers performed better than our best previously synthesized amide monomers as well as our best poly(ethylene glycol) monomers, which are known to have very high protein resistance. Hansen solubility parameters qualitatively predicted which monomers performed best, indicating favorable interactions with water molecules.

Crosslinked copolyazoles with a zwitterionic structure for organic solvent resistant membranes

Cross-linking of hydroxyl modified polytriazoles.

An osmolyte-based micro-volume ultrafiltration technique

This paper discusses a novel, simple, and inexpensive micro-volume ultrafiltration technique for protein concentration, desalting, buffer exchange, and size-based protein purification. The technique is suitable for processing protein samples in a high-throughput mode. It utilizes a combination of capillary action, and osmosis for drawing water and other permeable species from a micro-volume sample droplet applied on the surface of an ultrafiltration membrane. A macromolecule coated on the permeate side of the membrane functions as the osmolyte. The action of the osmolyte could, if required, be augmented by adding a supersorbent polymer layer over the osmolyte. The mildly hydrophobic surface of the polymeric ultrafiltration membrane used in this study minimized sample droplet spreading, thus making it easy to recover the retained material after separation, without sample interference and cross-contamination. High protein recoveries were observed in the micro-volume ultrafiltration experiments described in the paper.

Macromolecular prodrugs of ribavirin: concerted efforts of the carrier and the drug

Polymers in tune. Automated parallel polymer synthesis is developed to obtain libraries of macromolecular prodrugs of ribavirin, a broad-spectrum antiviral agent. As many as 10 identified lead polymer conjugates exhibit therapeutic efficacy matching that of the pristine drug and at the same time suppressed the origin of the main side effect of ribavirin.

Enhanced stem cell pluripotency in surface-modified electrospun fibrous matrices

A previously screened "hit chemistry" (N-[3-(dimethylamino)propyl] methacrylamide) that supports strong attachment and long-term self-renewal of ES cells is selected and grafted to poly(ether sulfone) (PES) fibrous matrices through plasma-induced graft polymerization. The 3D modified fibers exhibit higher cell proliferation and greater expression of pluripotency markers of mouse ES cells than 2D membranes. It is the first demonstration of scaling up an optimal synthetic surface chemistry in 2D using a high throughput synthesis, screening, and selection method to 3D that strongly influences pluripotent stem cell growth.