AVS2004 Session BI-WeA: "Passive" and Non-Fouling Surfaces

Wednesday, November 17, 2004 2:00 PM in Room 210D

Wednesday Afternoon

Start	Invited?	Item
2:00 PM		BI-WeA-1 PEG Modified Trichlorosilanes as Protein Repellent Coatings for Oxide Surfaces R. De Palma (IMEC, Belgium); K. Jans (KULeuven, Belgium); K. Bonroy, W. Laureyn (IMEC, Belgium); G. Maes (KULeuven, Belgium); C. Van Hoof (IMEC, Belgium) The construction of oxide based microelectronic devices interfaced with biological components requires methods for assembling biomolecules on their surfaces in a controlled manner. Examples include biosensors, chip-based diagnostic assays and biomaterials used for implants and tissue engineering. A key issue in the design of analytical devices which contact biomolecules is that non-specific adsorption of biological species, particularly proteins, can limit their performance. Surface-bound poly(ethyleneglycol) is a powerful reagent to construct protein repellent surfaces on various substrates. Most procedures reported to yield PEG layers on oxides require several steps and thus decrease the surface controllability. To overcome the problems encountered during PEG surface modification, we have synthesized novel reagents which combine the silane surface modification properties and the protein resistant properties of short PEG (≤ 6) units to generate robust coatings for glass and metal oxides. Tantalum was used as a substrate because of the high chemical stability of its thin passivating oxide and was found to play an important role in the silane SAM formation. The molecular architecture of the deposited silane layers and the PEG chain conformation was studied using contact angle measurements, XPS, AFM, RAIRS, LDI-TOF-MS and ellipsometry. The non-specific adsorption of human serum and its 4 most abundant proteins were elaborated using quartz crystal microbalance with dissipation monitoring (QCM-D) and confocal fluorescence microscopy. The protein repellent properties of the PEG silane SAMs were shown to be strongly correlated to the PEG chain length and their molecular architecture. The correlation between the PEG length and the viscoelastic properties of the adsorbed protein film have led to a better insight into the phenomenon of protein repellence. Future work will involve the deposition of mixed PEG silane SAMs to further improve the protein resistant properties.
2:20 PM		BI-WeA-2 The Effect of Cloud-Point Grafting of Sulfonated Poly(ethylene glycol) on Albumin Adsorption L.G. Britcher, H.J. Griesser (University of South Australia); Y.H. Kim (Korea Institute of Science and Technology) Sulfonated poly(ethylene glycol) (PEG-SO₃) has shown promise as a thromboresistant material, although its activity does reduce when grafted onto a polyurethane surface. It is thought that a synergistic effect exists between the PEG and terminal SO3 groups, improving its blood compatibility¹. The flexible hydrophilic PEG chains cause protein rejection by a steric barrier mechanism, while the negative charge of the SO₃ terminal groups causes electrical repulsion of negatively charged proteins and platelets¹ ^,². Though platelet adhesion is decreased on PEG-SO₃ grafted surfaces compared with PEG grafted surfaces, protein adsorption is not suppressed completely. A negative cilia adsorption model has been proposed for these surfaces, however, it does not explain why albumin adsorption is enhanced. Therefore, further work is required in order to understand what conformation of the PEG-SO₃ on the surface along with the distribution of SO₃ groups will lead to improved anti-fouling properties. One method for changing the PEG conformation is to use cloud point conditions for grafting the PEG. Surfaces grafted under these conditions have shown to decrease protein adsorption significantly, as the coating is very dense due to the brush conformation of the PEG³. However, charge repulsion effects may arise with densely packed sulfonated PEGs. In this study, we aim to investigate whether cloud point conditions can be used to graft PEG-SO₃ onto surfaces and if the coating thus obtained suppresses albumin adsorption. This should lead to further understanding of the protein adsorption model on PEG-SO₃ surfaces. ¹ Y. Hann et al., Biomaterials 24, 2213 (2003). ² H. Lee et al., Colloid Surf B: Biointerfaces 18, 355 (2000). ³ P Kingshott et al., Biomaterials 23, 2043, (2002).
3:00 PM	Invited	BI-WeA-4 Biologically Inspired Peptide-Mimetic Polymers for Prevention of Cell and Protein Fouling P.B. Messersmith, A.E. Barron, J.L. Dalsin, A. Statz, R.J. Meagher (Northwestern University) The minimization of nonspecific interactions between biomolecules, cells and material surfaces is integral to refining the biological response biomaterials, and therefore is important to the success of numerous emerging healthcare technologies. A primary motivation for this study is the significant need for new nonfouling strategies capable of functioning effectively for long periods of time in-vivo, and which can be readily applied to a variety of material or device surfaces. In this talk, I will describe our ongoing research efforts aimed at developing new macromolecules that meet these criteria. Specifically, we are focusing on two key aspects of biomaterial surface modification related to prevention of protein and cell fouling: 1) the design and synthesis of new polymers capable of minimizing nonspecific protein and cell attachment to biomaterials; and 2) the development of robust and versatile approaches for anchoring these polymers onto biomaterial surfaces. We have synthesized new peptidomimetic polymers designed to be both fouling resistant and adhesive to surfaces. The anchoring component of the polymers is inspired by the adhesive proteins secreted by mussels for attachment to marine surfaces, whereas the nonfouling polymer is either poly(ethylene glycol) (PEG), or a poly(N-substituted glycine) (polypeptoid). Polypeptoids offer the advantages of resistance to enzymatic degradation, low immunogenicity, and with proper design, the ability to prevent protein and cell attachment at surfaces. The synthesis and characterization of these peptidomimetic polymers will be described, along with evidence for their surface immobilization and performance as antifouling coatings.
3:40 PM		BI-WeA-6 Stable Protein-resistant Surfaces: Covalent Immobilization of Poly(L-Lysine)-g-Poly(Ethylene Glycol) onto Plasma-modified, Aldehyde-activated Substrate Surfaces T.M. Blättler, S. Pasche, M. Textor (Swiss Federal Institute of Technology, Switzerland); H.J. Griesser (University of South Australia) The fabrication of protein resistant surfaces is of considerable interest for a number of applications. Electrostatically adsorbed PEGylated graft copolymers, such as poly(L-lysine)-g-poly(ethylene glycol) (PLL-g-PEG), have been very successful in reducing protein adsorption on negatively charged metal oxide surfaces. The drawback is their instability under extreme conditions (extreme pH or high ionic strength). We have overcome this limitation in the present work by covalently immobilizing PLL-g-PEG onto aldehyde plasma modified substrates. PLL-g-PEG was immobilized on silicon wafers in two consecutive steps: First the silicon wafer was coated with a propionaldehyde plasma polymer layer (AHPP); secondly, the PLL-g-PEG was immobilized covalently by reacting part of the amine groups of the PLL backbone with the aldehyde groups present on the plasma-deposited polymer layer (reductive amination). The stability and the protein resistance of different architectures of PLL-g-PEG were quantitatively investigated by XPS, OWLS and ToF-SIMS. Protein resistance of the polymer-modified surfaces was tested against bovine serum albumin (BSA). Adsorption of BSA was below the detection limit (below 2 ng/cm²), similarly to the electrostatically adsorbed PLL-g-PEG. However, after 24 h exposure of the covalently immobilized PLL-g-PEG to high ionic strength buffer (2400 mM NaCl) no significant change in the protein resistance was observed, while under the same conditions electrostatically adsorbed PLL-g-PEG coatings lost their protein resistant properties. These findings provide good evidence for the covalent nature of the PLL-g-PEG binding to the surface. This work has created a general platform for the covalent immobilization of PLL-g-PEG onto a wide variety of substrates provided that they are compatible with the AHPP coating process.
4:00 PM		BI-WeA-7 Control of Protein Adsorption Using Poly(propylene sulfide)-block- poly(ethylene glycol) Adlayers: New Potential Candidate for the Modification of Biosensor Chip Surfaces L. Feller, S. Tosatti (Swiss Federal Institute of Technology (ETHZ), Switzerland); S. Cerritelli, S. Terrettaz (Swiss Federal Institute of Technology (EPFL), Switzerland); M. Textor (Swiss Federal Institute of Technology (ETHZ), Switzerland); J.A. Hubbell (Swiss Federal Institute of Technology (EPFL), Switzerland) Poly(ethylene glycol) (PEG) has been used in numerous biomedically relevant systems to aid in the minimization of protein adsorption and cell adhesion. PEG can be attached to surfaces through a variety of different approaches including silanization, self-assembly of thiols, and plasma polymerization. In our approach a block copolymer containing one (di- block) or two (tri-block) PEG chains separated by a poly(propylene sulfide) (PPS) part was used. Adsorbed to gold surfaces, a stable linkage between the sulfur atoms of the PPS thioether and the metal surface was observed. The hydrophilic PEG part formed a dense layer of biocompatible PEG chains, which is exposed to the aqueous environment. Various architectures of di- and tri-block PPS-PEG copolymers were synthesized, characterized, and deposited on gold substrates. While the PPS part was kept constant (MW 4000), the PEG part was varied between 1100 and 5000 Da molecular weight. Adsorption of the polymer to the gold surface was characterized by ex situ ellipsometry, X-ray photoelectron spectroscopy (XPS), and in situ surface plasmon resonance (SPR). The resistance of the surfaces to protein adsorption was evaluated using SPR. PPS-PEG readily chemisorbed on gold surfaces after a simple dip-and-rinse process in 1mg/ml methanol solution. We compared different architectures of PPS-PEG and correlated the PEG/PPS ratio with adsorbed mass values and resistance to protein (HSA) adsorption with the aim to find the optimum architecture regarding surface adhesion, stability, polymer conformation and protein resistance.
4:20 PM		BI-WeA-8 Surface Segregation of Pluronic® P104 in Poly(_L-lactic acid) Characterized by XPS and ToF-SIMS J.-X. Yu (State University of New York at Buffalo); C.M. Mahoney (National Institute of Standards and Technology); J. Gardella (State University of New York at Buffalo) This study reports results of the surface and in-depth characterization of two component blend films of poly(L-lactic acid) (PLLA) and Pluronic® surfactant [poly(ethylene oxide) (A) poly(propylene oxide) (B) ABA block copolymer]. The bulk properties of these polymers have been well studied by many groups due to their biomedical applications. Angle dependant X-ray photoelectron spectroscopy (XPS) and Time-of-flight secondary ion mass spectrometer (ToF-SIMS) depth profiling were used for monitoring the surfactant’s surface concentration at different sampling depths. We found a critical saturation concentration of the surfactant, a depletion region beneath the topmost surfactant enriched zone, and the existence of the surfactant’s segregation in the whole film with different intensities. We conclude that the surfactant’s surface segregation increases and then stays stable when increasing its bulk concentration. These results suggest that the selection of the surfactant bulk concentration of the thin film matrices for drugs/proteins delivery should achieve a relatively homogeneous distribution of stabilizer/protein in the PLLA matrix.
4:40 PM	Invited	BI-WeA-9 Heterobifunctional PEG Tethered Chains Surface -Preparation, Physicochemical and Biochemical Properties Y. Nagasaki, K. Uchida (Tokyo University of Science, Japan); H. Otsuka, K. Kataoka (The University of Tokyo, Japan) In the case of microanalysis in a crude sample such as serum, nonspecific adsorption of various proteins and lipids to the surface is an important consideration to achieve specific biosensing with high S/N ratio. In order to avoid the nonspecific adsorption, many types of modification on the sensor surface have been considered. Modification by poly(ethylene glycol) (PEG) tethered chains leads to reduce the nonspecific interaction of biomolecules such as proteins and cells with biomedical devices because PEG is a nontoxic and hydrophilic polymer with low interfacial free energy in water and high-chain mobility inducing excluded volume effects. In this paper, we are focusing on preparation of complete non-fouling surface by mixed PEG tethered chain, which denotes the introduction of short under-brushed PEG layer to the surface pre-modified with comparatively long PEG chain resulted. By using our original heterotelechelic PEG, which means PEG having a functional group at one end and another functional group at the other chain end quantitatively, ligand-installed non-fouling surface was constructed. In the case of dextran gel as a control, non-specific adsorption was avoided to some extent in the case of high molecular weight protein. With decreasing the size of the protein, the non-specific adsorption increased significantly. The conventional PEG tethered chain surface suppressed the non-specific adsorption of the proteins possessing the molecular weight higher than 10kD. However, it is not enough performance for the protein lower than 10kD. In the case of the mixed PEG tethered chain surface, complete non-fouling character was observed. Especially, the mixed PEG tethered chain avoided tetrapeptide (RGDS, MW=450), which is anticipated as ideal biomateials surface.