Non-synthetic polymer biomodification using gold nanoparticles

Tissue engineering, the creation of replacement tissue using natural and synthetic components, requires the ability to manipulate the local chemical environment of polymeric biomaterials, which are materials designed to augment or replace natural functions. Many polymeric biomaterials display excellent mechanical characteristics and compatibility to native tissue, but they do not readily support cell adhesion. Unfortunately, modification of these materials can be difficult. For example, agarose and poly (ethylene glycol) diacrylate (PEGDA) hydrogels only weakly support cell growth, and cell adhesion molecules must be added to improve the cell-material interface. Methods to chemically modify agarose and PEGDA hydrogels have been developed, but these methods tend to be difficult and time consuming. A new technique for modification, using gold nanoparticles embedded within a hydrogel matrix, offers a solution to these problems. The particles serve as attachment points for cell adhesion peptides to facilitate bioconjugation. These methods can be applied to many types of hydrogels with different pore sizes simply by changing the nanoparticle size, as opposed to developing novel synthetic chemistry. Several sizes of gold nanoparticles have been synthesized, entrained in agarose hydrogels, and tested to show that the bulk of particles remain in the gel for a substantial length of time. Mechanical properties of the gold nanoparticle composite hydrogels are similar to the unmodified hydrogels, retaining the native material characteristics. A cell-binding peptide has successfully been conjugated to gold nanoparticles, and the effect of this binding peptide on cell growth and adhesion is being studied by culturing cells on the unmodified and composite hydrogels. Although the initial results are promising, more testing is necessary to quantify the extent of adhesion in each case. The composite gels being examined offer many advantages over the previous methods of polymeric bioconjugation. The chemistry is simple and robust, the gel’s polymeric backbone and mechanical properties are preserved, and the modification technique can be applied to a wide range of biomaterials. Because of this flexibility, this technology is not limited to a single component or tissue type, but can be applied to all areas of tissue engineering, providing novel methods of non-synthetic bioconjugation. In addition to biomodification, these materials offer the opportunity for integrated sensing, due to the well recognized optical properties of gold nanoparticles. Biosensor detection is based on the absorbance shift resulting from surface plasmon resonance (SPR) experienced by aggregated gold nanoparticles. For example, two bound gold nanoparticles experience a SPR-induced absorbance shift as a result of proximity. When the particles are separated, the absorbance returns to its original value. In a proof-of-concept device, particle aggregation is achieved using a modified cell binding peptide (CGGGRGDSGGGC), whereas cleavage is produced by an enzyme that promotes cell detachment (trypsin), returning particles to their initial unaggregated state. Particles are also modified with tri(ethylene glycol) mono-11-mercaptoundecyl ether, a stabilizing agent that protects the particles from unwanted aggregation. Although this proof-of-concept system examines cell adhesion using the RGD peptide/trypsin protease system, the biosensor could be customized to almost any enzyme-substrate combination. Any substrate with thiol ends (which can be added through cysteine termination) has the ability to bind the gold nanoparticles together, and any substrate specific enzyme can cleave the peptide bond activating the sensor. Thus, analyte sensing can be directly built into a modified hydrogel by integrating the prepared gold nanoparticles during gel synthesis. The general modification method described here has numerous advantages. Both the increased biocompatibility and sensing applications of gold nanoparticle-biomaterial composites are improvements over systems based solely on hydrogels and polymers or just nanoparticles alone. The combined system provides the hydrogel biomaterials with increased functionality without the requirement of complicated syntheses. In addition, the nanoparticles are provided with a supportive framework. Some of the most promising biosensor models employ aqueous nanoparticles, which are not inherently portable and operate only in the liquid phase. A hydrogel support permits the development of portable devices with potential for gas phase operation. The methods described here are also very flexible as a result of the ability to functionalize the gold nanoparticles with a wide array of biomolecules, providing a composite system with a variety of features. Advisor: Jessica O. WinterNo embarg