(354i) Functionalizing Surfaces with Zwitterionic Polymers to Control Cell Adhesion and Direct Neurite Growth

The most successful neural prosthesis to date is the cochlear implant (CI), which restores hearing to patients with profound hearing loss. To perceive sound in a CI, a wearable external device processes and transmits encoded wavelength information to a subcutaneous receptor-stimulation unit. The internal component decodes the information and transmits the signal, in the form of electrical current, to an electrode array positioned in the scala tympani of the cochlea. This electrical signal effectively bypasses the dysfunctional residual hair cells of the cochlea and directly stimulates the spiral ganglion neurons (SGNs), the primary neural receptors in the ear. Tonal stimulation is achieved through positioning electrode signaling along the length of the array, thereby mimicking the tonotopic organization of the cochlea. While this hearing restoration for profoundly deaf individuals is quite a remarkable feat, CI patients have difficulty distinguishing complex tonal information such as voice inflection. This limited resolution is due, in large part, to the distance between the CI electrode array and target neurons. Electrical signals must be strong enough to reach target neurons positioned hundreds to thousands of micrometers from electrode arrays, which causes significant current spread and consequent stimulation of a broad range of SGNs. Thus, even devices with advanced electrical design suffer from low resolution information. Additionally, dense fibrous tissue forms on the surface of implanted electrodes which is common to all implantable devices. This dense tissue attenuates applied currents and increases stimulation thresholds, which further decreases the functional outcomes of CIs. Nonspecific protein adsorption to implant surfaces may trigger the foreign body response: the inflammatory response evoked by the presence of a foreign object in host tissue. Macrophages recognize these nonspecifically-protein labeled implants as foreign, attach to the surface, and attempt to ingest them. Because the implant is large and not biodegradable, these macrophages fuse together and form foreign body giant cells, which secrete inflammatory cytokines and induce fibroblast adhesion. This process permanently isolates the device and renders the implant nonfunctional or less effective.

Eliminating nonspecific protein adsorption to device surfaces could serve as one approach to mitigate the biological response to a material. Recently, zwitterionic materials such as carboxybetaine methacrylate (CBMA) and sulfobetaine methacrylate (SBMA) have been developed as robust ultra-low fouling biomaterials. Zwitterions are inherently net neutral molecules containing an equal number of positively and negatively charged atoms. When used as monomers, the resulting highly polar zwitterionic polymers maintain high wettability, while their zero-charge nature prevents attraction of negatively- or positively-charged molecules to material surfaces. The high energy of hydration is theorized to give zwitterionic materials their antifouling characteristics.

Surface properties are a key component when engineering low fouling materials because the surface comes into direct contact with host fluid, blood protein, and other immunogenic factors. These biomolecules and their nonspecific adsorption to implant surfaces play an integral role in fibrous tissue formation. A common approach to achieve nonfouling properties involves introducing functional groups onto the surface, which then act as anchors for polymer growth. In this work, glass was used as a proof of concept substrate because it can be easily modified with a variety of functional groups using silane coupling chemistry. Glass samples were first exposed to oxygen plasma to generate hydroxyl surface groups followed by reaction with a silane-containing coupling agent incorporating a reactive methacrylate group. Zwitterionic methacrylates were then covalently attached to glass surface methacrylates by illuminating a monomer- and photoinitiator-containing solution in contact with the substrate. UV-light was used to induce the polymerization reaction and covalently build a grafted zwitterionic polymer from the substrate. For our experiments, SBMA and CBMA were used as zwitterionic monomers to alter the surface properties of functionalized glass substrates. The concentration of zwitterionic monomer in solution was varied to change surface coverage and probe the physicochemical properties of the antifouling polymer-grafted surface.

Grafting of the methacrylate silane coupling agent and zwitterionic SBMA and CBMA polymers was characterized through X-ray photoelectron spectroscopy (XPS) analysis, which provides elemental composition of the first 5-10 nm of a surface. XPS measurements showed an increase in carbon content upon addition of the grafting agent. Grafting of the SBMA and CBMA to the glass surface was characterized through nitrogen content. Both SBMA and CBMA showed appearance of the characteristic nitrogen peak in the spectrum. SBMA also contained sulfur further corroborating the grafting of SBMA. To understand how the elemental composition of the surface changes as a function of SBMA and CBMA solution concentration, percent nitrogen was quantified. At low SBMA and CBMA solution concentrations, the atomic nitrogen content is less than 1%, indicating low levels of grafting and surface coverage with the polymer. As the zwitterion solution concentration increases, the atomic composition increases and approaches the theoretical maximum of a pure zwitterionic polymer, calculated using atomic percent. These observations imply that the surface coverage of the SBMA and CBMA polymers is strongly dependent on the concentration of monomer in the solution. Further, for both the SBMA- and CBMA-coated samples, the percent silicon decreased with increasing solution concentration of monomer and showed very little, if any, signal at higher concentrations. These observations are consistent with increases in thickness with higher monomer solution concentrations. On the basis of the XPS data collected, it is estimated that the thickness of the polymer layers measured using all monomer solution concentrations is less than the penetration depth of XPS data collection. XPS only examines the atomic composition the first 5-10 nm of the surface, and thus it is possible that changes (a few nanometers) in film thickness occur giving rise to changes in percent nitrogen.

Surface grafting of a polymer or other molecule is accompanied by a corresponding change in surface properties including surface energy that can be evaluated using water contact angle measurements. The silanized glass was relatively hydrophobic with a contact angle of over 57 degrees as would be expected given the nonpolar nature of the coupling agent used to functionalize the glass substrates. Conversely, zwitterionic polymers are hydrophilic, due to the presence of charged atoms along the polymer side chains and are expected to decrease contact angles. The contact angles of the SBMA- and CBMA-coated samples decreased sharply with increasing wt% monomer in solution. At higher concentrations, the contact angles remained unchanged at approximately 15 degrees. The marked decrease in contact angle for zwitterion-coated surfaces indicates increases in hydrophilicity and further corroborates that the surface was successfully functionalized with the zwitterionic polymer. With large changes in surface properties, it is reasonable to believe that the zwitterionic polymer will affect adsorption of molecules including proteins. As mentioned previously, the first step in foreign body response is nonspecific protein adsorption, such as would occur when a surface comes into contact with serum during implantation. Hydrophilic, net neutral surfaces such as zwitterionic polymers have been shown to effectively prevent the nonspecific adsorption of protein. Fibrinogen adsorption was quantified using immunofluorescence. Adsorption of fibrinogen to SBMA- and CBMA-coated surfaces correlates to water contact angles with sharp decreases up to solution concentrations of 1 wt%. These observations suggest that, similar to the contact angle, a maximum in graft density is reached at low solution concentrations. The protein adsorption data, in combination with the XPS data, suggest that only a very thin film (a few nanometers) of the zwitterionic polymer brushes is necessary in order to achieve a significant decrease in protein adsorption and water contact angle.

Fibroblasts, the primary cell type that contributes to fibrosis on implanted devices, were seeded onto SBMA- and CBMA-coated substrates. An almost 50% decrease in cell density was observed on SBMA-coated surfaces with a remarkable 99% reduction in cell density for CBMA-coated samples. Additionally, astrocytes and Schwann cells both exhibited similar repulsion by SBMA- and CBMA-coated surfaces. To determine the ability of zwitterionic-functionalized glass to spatially control and direct cell growth, micropatterned substrates were fabricated with alternating zwitterion-coated and uncoated bands. Due to the inherent spatial control afforded by photopolymerization, areas of functionalization can be precisely defined through use of a photomask. Fibroblasts, astrocytes, and Schwann cells all elongated along uncoated regions avoiding interactions with zwitterion-coated substrates. Quantification of cell density in uncoated compared to zwitterion-coated regions revealed significant decreases in cell density for all cell types with CBMA exhibiting lower cell density than SBMA. Zwitterionic micropatterns were also generated to be used in directing the growth of Schwann cells and SGN neurites. Neurite extensions grew directly between zwitterion-coated bands avoiding interaction with the functionalized regions. While SGN neurites extend along SBMA stripes, CBMA patterns significantly improve alignment compared to SBMA patterns. Therefore, CBMA-coated surfaces demonstrated a superior ability to resist cell adhesion and to direct cell growth. The described photopolymerizable surfaces enabled investigation of cell-material interactions that could enable engineering of surfaces that resist fibrotic tissue formation. Additionally, generated micropatterns provide potential in directing regeneration of neurites for neural prosthetic development.