Louisa R. Carr

Zwitterionic hydrogels implanted in mice resist the foreign-body reaction

The performance of implantable biomedical devices is impeded by the foreign-body reaction, which results in formation of a dense collagenous capsule that blocks mass transport and/or electric communication between the implant and the body. No known materials or coatings can completely prevent capsule formation. Here we demonstrate that ultra-low-fouling zwitterionic hydrogels can resist the formation of a capsule for at least 3 months after subcutaneous implantation in mice. Zwitterionic hydrogels also promote angiogenesis in surrounding tissue, perhaps owing to the presence of macrophages exhibiting phenotypes associated with anti-inflammatory, pro-healing functions. Thus, zwitterionic hydrogels may be useful in a broad range of applications, including generation of biocompatible implantable medical devices and tissue scaffolds.

Imaging and cell targeting characteristics of magnetic nanoparticles modified by a functionalizable zwitterionic polymer with adhesive 3,4-dihydroxyphenyl-l-alanine linkages

Multifunctional magnetic nanoparticles (MNPs) modified by a zwitterionic polymer (pCBMA-DOPA(2)) containing one poly(carboxybetaine methacrylate) (pCBMA) chain and two 3,4-dihydroxyphenyl-L-alanine (DOPA) residue groups were developed. Results showed that MNPs modified by pCBMA were not only stable in complex media, but also provided abundant functional groups for ligand immobilization. The pCBMA-DOPA(2) MNPs had a hydrodynamic particle size of about 130 nm, a strong saturation magnetization of 110.2 emu/g Fe and a high transverse relaxivity of 428 mM(-1)s(-1). Long-term stability in phosphate-buffered saline (PBS) and 10% NaCl solution was achieved for over six months. Compared to MNPs coated with dextran, pCBMA-DOPA(2) MNPs presented better stability in 100% human blood serum at 37 degrees C. Macrophage cell uptake studies revealed that the uptake ratio of pCBMA-DOPA(2) MNPs was much lower than that of dextran MNPs. Furthermore, quantitative analysis results showed that after pCBMA-DOPA(2) MNPs were conjugated with a targeting RGD peptide, uptake by human umbilical vein endothelial cell (HUVEC) was notably increased, which was further visualized by magnetic resonance imaging (MRI).

Zwitterionic poly(carboxybetaine) hydrogels for glucose biosensors in complex media

Zwitterionic hydrogels based on poly(carboxybetaine) methacrylate (polyCBMA) were developed to protect implantable electrochemical glucose biosensors from biofouling in complex media. To enhance the linearity and sensitivity of the sensing profile, both physical and chemical adsorption methods were developed. Results show that glucose sensors coated with polyCBMA hydrogels via the chemical method achieve very high sensitivity and good linearity in response to glucose in PBS, 10%, 50%, and 100% human blood serum. Essentially identical glucose signals were observed even after prolonged exposure to blood samples for over 12 days. The excellent performance of polyCBMA hydrogel coating offers great promise for designing biocompatible implantable glucose biosensors in biological medium.

The hydrolysis of cationic polycarboxybetaine esters to zwitterionic polycarboxybetaines with controlled properties

In this work, we report a new class of materials, cationic polycarboxybetaine esters, which have unique properties when they interact with proteins, DNAs, and bacteria. These cationic polymers can be converted to nontoxic and nonfouling zwitterionic polymers upon their hydrolysis. Due to their unique properties, they are very promising for a wide range of applications, such as highly effective gene delivery carriers and environmentally friendly antimicrobial coatings. Three positively charged polyacrylamides, of which the pedant groups bear carboxybetaine ester groups, were synthesized. These three polymers have different spacer groups between the quaternary ammonium and the ester groups. Their hydrolysis behaviors were studied using proton NMR under different NaOH concentrations. Their interactions with biomolecules and microorganisms before and after hydrolysis were demonstrated by protein adsorption/resistance, DNA condensation/release, and antimicrobial properties. The polymers were grafted onto a gold-coated surface covered with initiators using surface-initiated atom transfer radical polymerization (ATRP). Fibrinogen adsorption was measured by surface plasmon resonance (SPR) sensors. While the polymer-grafted surfaces have high protein adsorption, the surfaces became nonfouling after hydrolysis. Linear polymers were also synthesized and DNA/polymer complexes were evaluated. Agarose gel electrophoresis shows that DNA can be condensed into nanoparticles by the cationic polymers before hydrolysis and released from the DNA/polymer complexes upon the hydrolysis of the cationic polymers into zwitterionic polymers. The complexes formed were characterized by dynamic light scattering measurements. In addition, the interactions of linear polymers with bacteria were also evaluated. The polycarboxybetaine ester with a pentene spacer exhibits evident antimicrobial properties when they are incubated with Gram negative bacteria (Escherichia Coli). The polymer can be converted to a nontoxic polycarboxybetaine after hydrolysis. This work shows that the biological properties of polycarboxybetaine esters can be dramatically changed via controlled hydrolysis.

Engineering the Polymer Backbone To Strengthen Nonfouling Sulfobetaine Hydrogels

We have demonstrated that molecularly engineering the chemical structure of a monomer can lead to hydrogels with improved mechanical strength. In this case, hydrogels from zwitterionic sulfobetaine methacrylate monomers were compared to sulfobetaine vinylimidazole (pSBVI) hydrogels. We show that the introduction of the vinylimidazole backbone improves the tensile and compressive mechanical properties of the sulfobetaine hydrogel by an order of magnitude over the same properties of a methacrylate hydrogel. Zwitterionic groups have been shown to create surface coating materials with ultralow fouling properties, and we demonstrate here that the presence of the imidazole group does not compromise the nonfouling properties attributed to the zwitterionic sulfobetaine: surfaces coated with pSBVI exhibited exceptionally low nonspecific protein adsorption, and cell adhesion was reduced by 97% relative to low-fouling poly(2-hydroxyethyl methacrylate) (pHEMA) hydrogels.

Uniform zwitterionic polymer hydrogels with a nonfouling and functionalizable crosslinker using photopolymerization

We reported previously the design and synthesis of a zwitterionic carboxybetaine dimethacrylate crosslinker, and we showed that its use with zwitterionic carboxybetaine methacrylate led to nonfouling hydrogels with high mechanical properties and high hydration. Now, we use photopolymerization to improve the uniformity of the polymer network, resulting in drastically improved mechanical properties (compressive modulus up to 90 MPa). Furthermore, we designed and synthesized a new functionalizable carboxybetaine dimethacrylate crosslinker, enabling functionalization of the higher strength hydrogels against a nonfouling background. Additionally, the biostability of the carboxybetaine hydrogel systems was tested, and it was found that these hydrogels are stable in oxidative, acidic, and basic environments.

Functionalizable and Ultrastable Zwitterionic Nanogels

Multifunctional biomimetic nanogels based on zwitterionic poly(carboxybetaine methacrylate) (pCBMA) were synthesized by inverse microemulsion free-radical polymerization. pCBMA nanogels exhibited excellent stability in 100% fetal bovine serum. Dextran labeled with fluorescein isothiocyanate (FITC-dextran) was encapsulated in nanogels as a model drug. Encapsulated FITC-dextran exhibited controlled release from the pCBMA nanogel. Additionally, pCBMA nanogels carry abundant carboxylate groups as functional groups used to conjugate ligands to the nanogels for targeted drug delivery. Flow cytometry results obtained showed that pCBMA nanogels conjugated with cyclo[Arg-Gly-Asp-D-Tyr-Lys] ligands dramatically improved the nanogel uptake by primary human umbilical vein endothelium cells. Functionalizable zwitterionic pCBMA nanogels hold great potential as targeted drug-delivery vectors for biomedical applications.

The effect of lightly crosslinked poly(carboxybetaine) hydrogel coating on the performance of sensors in whole blood

Surface coatings of high packing densities have been routinely used to prevent nonspecific biomolecular and microorganism attachment. Hydrogels are another class of low fouling materials used to create three-dimensional matrixes for the free diffusion of small analytes or drugs and the high-loading of bio-recognition elements. However, biomolecules are subject to being entrapped within hydrogel matrixes or adhered onto hydrogel surfaces, making them questionable for use in whole blood. Here, we demonstrate the feasibility of a lightly crosslinked poly(carboxybetaine) hydrogel for use in whole blood, as opposite to the conventional wisdom of high packing density in surface coatings. Proteins are able to diffuse in and out of the matrix freely without being altered from their native conformations. In order to demonstrate its long-term performance in whole blood, this hydrogel was used as the surface coating of a glucose sensor. This work paves a new way for the development of surface coatings and sensors to achieve long-term stability and high performance in whole blood.

Engineering buffering and hydrolytic or photolabile charge shifting in a polycarboxybetaine ester gene delivery platform.

Polycarboxybetaine esters (PCB-esters) can condense plasmid DNA into nanosized polyplexes for highly effective gene delivery with low toxicity. The design and characterization of tertiary CB-ester monomers and PCB-ester polymers are presented here to study the effects of molecular variation on functions important to nonviral gene transfer. Both buffering capacity and charge-shifting behavior can be tuned by modifying the distance between the charged groups and the ester size or type. A carbon spacer length (CSL) of one was found to bring the pKa of the tertiary amine into the optimal range for proton buffering. Ester hydrolytic degradation switches this polymer from cationic (DNA binding) to zwitterionic (DNA releasing) form while conferring nontoxicity. To allow rapid and externally controlled degradation, the effect of this charge-switching behavior on DNA release from polyplexes was directly studied with a novel photolabile PCB-nitrobenzyl ester (PCB-NBE). Photoinitiated ester degradation precipitated the rapid release of 72±5% of complexed DNA from PCB-NBE polyplexes. These insights reveal the key parameters important for the PCB-ester platform and the significance of charge switching to an effective and nontoxic nonviral gene delivery platform.

Single nonfouling hydrogels with mechanical and chemical functionality gradients.

Hydrogels are widely studied as tissue engineering scaffolds, but the biological tissues they are designed to mimic are often complex tissues with non-uniform chemical and mechanical profiles. This work reports a new strategy to create hydrogels composed of a continuous sheet of a single nonfouling but functionalizable material with mechanical and/or chemical functionality gradients. By using different combinations of functionalizable or nonfunctionalizable versions of nonfouling carboxybetaine methacrylate (CBMA) and carboxybetaine dimethacrylate crosslinker (CBMAX), various hydrogels with gradients of crosslinking densities and/or functionalizable groups can be created. In this work, we demonstrate this concept with two nonfouling hydrogels, both with a mechanical gradient: one with uniform functionalizability and the other with a gradient in chemical functionalizability. With this versatile system, hydrogels with built-in gradient profiles of various types can be controlled at will for a given application.