BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 Lecture 10: Bioengineering applications of hydrogels: Molecular Imprinting and Drug Delivery Last Day: polyelectrolyte gels Polyelectrolyte complexes and multilayers Applications in bioengineering Theory of ionic gel swelling Today: Molecular imprinting Hydrogels in drug delivery Supplementary Reading: S.R. Lustig and N.A. Peppas, ‘Solute diffusion in swollen membranes. IX. Scaling laws for solute diffusion in gels,’ J. Appl. Polym. Sci. 36, 735-747 (1988) T. Canal and N.A. Peppas, ‘Correlation between mesh size and equilibrium degree of swelling of polymeric networks,’ J. Biomed. Mater. Res. 23, 1183-1193 (1989) Molecular Imprinting 1,2 Concepts of molecular imprinting ? Molecular imprinting is the design of polymer networks that can recognize a given target molecule and bind it preferentially in the presence of an excess of irrelevant molecules, some of which may have very similar molecular structures o Seeks to mimic specificity in biological recognition obtained through protein-protein interactions ? Steps to the preparation of molecularly-imprinted networks: 1. mixing of binding monomers and target molecule o target can be mixed directly with liquid monomers in bulk or co-dissolved in a non-interfering solvent o monomers bind target null covalent interactions null non-covalent bonding null metal coordination o mixture usually at high concentration (e.g. 50% w/vol solutions): enforces close interactions of target with binding monomers and leads to a tight network that holds the position of functional groups in position of template binding 2. polymerization of monomers in place o usually photopolymerization (rapidly ‘trap’ structure) 3. washing for removal of target molecule from network pockets Lecture 10 – Bioengineering Applications of Hydrogels 1 of 12 BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 ? types of target molecules: 1 o small-molecule drugs o steroids o nucleic acids o amino acids o metal ions o proteins Structure of Molecularly-Imprinted Networks ? structure of molecularly-imprinted networks o imprinted networks can be confined to a thin surface layer or prepared in bulk o surface networks usually perform better for capture of large molecules like proteins ? simple synthetic components for recognition networks o monomers: o methacrylic acid o itaconic acid o acrylamides o 4-vinyl pyrrolidone o β-cyclodextrin o other designed monomers o cross-linkers o ethylene glycol dimethacrylate o PEG dimethacrylate o ‘chain effect’ 3 o binding of monomers to macromolecular templates causes a reduction in chain termination and thus an overall increase in reaction rate ? Example of molecular recognition: molecular imprinting of D-glucose (Peppas) o Monomers chosen as analogs of the amino acid residues that bind to glucose in vivo: o WHAT RECEPTORS BIND GLUCOSE? ? Aspartate Lecture 10 – Bioengineering Applications of Hydrogels 2 of 12 BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 ? Glutamate ? Asparagines ? Serine o Draw structures on board o Simple synthetic monomers chosen to mimic the bonding interactions of these amino acids: QuickTime? and a Graphics decompressor are needed to see this picture. TARGET: D-glucose Hydroxyethyl methacrylate Acrylic acid acrylamide Specificity of binding: Issues: Tightly cross-linked networks hold functional group positions for better recognition but restrict entry of target into network Limited complexity in recognition units ? Improving recognition by surface templating (Ratner 4,5 ) o Protein adsorbed to mica surface, coated with disaccharide, then coated with C 3 F 6 film by radiofrequency glow-discharge plasma treatment o Sugar coating protects protein from denaturation on dehydration Lecture 10 – Bioengineering Applications of Hydrogels 3 of 12 BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 Trehalose (disaccharide) o Resulting recognition: LSZ RNase LSZ in solution can exchange with LSZ = lysozyme imprinted LSZ, but Rnase cannot displace LSZ on surface o Utilizing in-situ formability of photopolymerized hydrogels for lab-on-a-chip applications o Photopolymerized Bulk templates (Peppas): Lecture 10 – Bioengineering Applications of Hydrogels 4 of 12 BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 o Plasma-deposited surface templates patterned by microcontact printing (Ratner): Lecture 10 – Bioengineering Applications of Hydrogels 5 of 12 BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 Hydrogels in drug delivery o What Control of drug release kinetics by hydrogel structure 6,7 o Release from stable hydrogels is controlled by diffusion of solute through the network o Diffusion is described by Fick’s second law: ?C ? 2 C Eqn 1 ?t = D gel ?x 2 o Recall the solution to Fick’s second law for a semi-infinite slab contacting a perfect sink: Eqn 2 c 0 ? c(x) = 1 ? erf ? ? ? 2 tD x ? ? ? c 0 c(x) c 0 Increasing time erf(z) solution x o Diffusion of drugs through a network is controlled by the mesh size (ζ) F r ee s u rf ac e Lecture 10 – Bioengineering Applications of Hydrogels 6 of 12 BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 o The mesh size is related to the network swelling Q and the end-to-end distance between cross-links: (<r 0 2 >) 1/2 =N c 1/2 a statistical segment length Number of segments between cross-links Eqn 3 r 0 2 1/ 2 ? 2M c ? 1/ 2 C 1/ 2 l() = ? ? ? M 0 ? ? ? n o …assuming a polymer chain that has 2 carbon-carbon bonds per repeat unit o derived from random walk chain statistics null Where l is the bond length in the polymer backbone null M c is the molecular weight between cross-links null M 0 is the molecular weight per repeat unit null Where C n is the characteristic ratio for the polymer chain () 2 1/2 Eqn 4 ξ = r 0 1/3 = Q 1/3 () r 0 2 1/2 = C n 1/2 Q 1/3 N 1/2 l φ 2,s null Q is the degree of swelling = V dry polymer /V swollen polymer null N is the degree of polymerization between cross-links null The mesh size is related to the diffusion constant of a solute in the network null Eyring theory of diffusion: ? ?G * ? ?H * ?S * Eqn 5 D = Tνe kT = Tνe kT e k Lecture 10 – Bioengineering Applications of Hydrogels 7 of 12 BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 o Where ?G* is the activation energy, ?H* is activation enthalpy, and ?S* is activation entropy o N = translational oscillating frequency of solute molecule (jump rate!) o T = temperature o k = Boltzman constant null The ratio of diffusion constant in the gel to that in solution is: * ?S gel k ? Eqn 6 D = D gel = e ?S 0 * D 0 e k o Where ?S* gel is the activation entropy for diffusion in the gel and ?S* 0 is the activation entropy in for diffusion in the solvent o This assumes the activation enthalpy and oscillation frequencies for diffusion are approximately the same in the gel and pure solvent (reasonable for dilute and chemically inert systems) null The activation entropies are: Eqn 7 ?S* gel = k ln P* - k ln P 0 Eqn 8 ?S* 0 = k ln P* 0 – k ln P 0 * * * Eqn 9 D = P gel = P gel,opening P gel,volume ? * * P 0 P 0,volume o Where P*volume is the probability that a solute-sized volume of free space exists to jump into o P*opening is the probability that the network has a solute-sized gap to jump through P* gel,volume P* gel,opening drug drug * ξ? r = 1 ? r Eqn 10 P gel,opening = ξ ξ o Where r is the radius of the solute (drug) and ξ is the network mesh size null The probability of a volume to jump into is an exponential of the ratio of the solute size to the available free volume per mole: v* ? * Eqn 11 P gel,volume ~ e v free,gel v* ? * Eqn 12 P 0,volume ~ e v free,1 o Where vfree is the specific free volume and v* is the volume of the solute (drug) o Refs for free volume theory applied here: Lecture 10 – Bioengineering Applications of Hydrogels 8 of 12 ? ? BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 null Yasuda et al. Makromol. Chem. 26, 177 (1969) null Peppas and Reinhart, J. Membrane Sci. 15, 275 (1983) null Now: * ? v* v* ? Eqn 13 P gel,volume = e ? ? ? v free,gel ? v free,1 ? ? * P 0,volume null The free volume in a swollen gel is approximately vfree,1 since the free volume contribution from polymer is extremely low (2.5% even in solid polymers at 25°C) Eqn 14 v free,gel = φ 1vfree,1 + φ 2 v free,2 null Therefore: Eqn 15 v free , gel ~ φ 1 v free,1 = (1-φ 2 )v free,1 = (1-1/Q)v free,1 o Where Q is the swelling degree = V swollen gel /V dry gel = 1/φ 2 null Therefore: ? ? ? ? ? v* ? v* ? ? * ? Q )v free,1 v free,1 ? ? ? v* ? ? 1 ? ? 1 ? Eqn 16 P gel,volume = e ? ? (1? 1 ? = e v free,1 ? Q?1? ? ≈ e ? ? ? Q?1? ? * P 0,volume o v*/v free,1 ~ 1 for most polymers, experimentally null Therefore: ? ? ? ? ?1 ? ? ? Eqn 17 D ? ? ? 1? r ? ? e ?(Q?1)? ξ null And thus finally: ? ? ? ? ?1 ? ? Eqn 18 D gel ? D 0 ? ? 1? r ? ? e ?(Q?1)? ξ o Insulin: MW – 5900 g/mole; hydrodynamic radius = 16 ? Design of glucose-responsive drug delivery microgels for treatment of diabetes 8-10 null Work by Podual and Peppas null Immobilized glucose oxidase enzyme within pH-responsive polyelectrolyte gel network along with encapsulated insulin o Network composed of DEAEM, PEGMA, and TEGDMA o GOD covalently tethered to network o Insulin entrapped in network o Polymerized gels as microspheres Lecture 10 – Bioengineering Applications of Hydrogels 9 of 12 BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 null Synthesis of microgels? o Fast variation in swelling due to microgel dimensions o Mesh size responds in a similar manner, using theory described above: ? The gels thus designed respond to concentrations of glucose in the surrounding medium, dynamically: Gluc Gluc Gluc Gluc Gluc Glucose oxidase insulin Lecture 10 – Bioengineering Applications of Hydrogels 10 of 12 BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 Lecture 10 – Bioengineering Applications of Hydrogels 11 of 12 BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 References 1. Byrne, M. E., Oral, E., Hilt, J. Z. & Peppas, N. A. Networks for recognition of biomolecules: Molecular imprinting and micropatterning poly(ethylene glycol)-containing films. Polymers for Advanced Technologies 13, 798-816 (2002). 2. Hart, B. R. & Shea, K. J. Molecular imprinting for the recognition of N-terminal histidine peptides in aqueous solution. Macromolecules 35, 6192-6201 (2002). 3. Tan, Y. Y. & Vanekenstein, G. O. R. A. A Generalized Kinetic-Model for Radical-Initiated Template Polymerizations in Dilute Template Systems. Macromolecules 24, 1641-1647 (1991). 4. Shi, H. Q., Tsai, W. B., Garrison, M. D., Ferrari, S. & Ratner, B. D. Template-imprinted nanostructured surfaces for protein recognition. Nature 398, 593-597 (1999). 5. Shi, H. Q. & Ratner, B. D. Template recognition of protein-imprinted polymer surfaces. Journal of Biomedical Materials Research 49, 1-11 (2000). 6. Lustig, S. R. & Peppas, N. A. Solute Diffusion in Swollen Membranes .9. Scaling Laws for Solute Diffusion in Gels. Journal of Applied Polymer Science 36, 735-747 (1988). 7. Canal, T. & Peppas, N. A. Correlation between Mesh Size and Equilibrium Degree of Swelling of Polymeric Networks. Journal of Biomedical Materials Research 23, 1183-1193 (1989). 8. Podual, K., Doyle, F. J. & Peppas, N. A. Dynamic behavior of glucose oxidase-containing microparticles of poly(ethylene glycol)-grafted cationic hydrogels in an environment of changing pH. Biomaterials 21, 1439-1450 (2000). 9. Podual, K., Doyle, F. J. & Peppas, N. A. Preparation and dynamic response of cationic copolymer hydrogels containing glucose oxidase. Polymer 41, 3975-3983 (2000). 10. Podual, K., Doyle, F. J. & Peppas, N. A. Glucose-sensitivity of glucose oxidase-containing cationic copolymer hydrogels having poly(ethylene glycol) grafts. Journal of Controlled Release 67, 9-17 (2000). Lecture 10 – Bioengineering Applications of Hydrogels 12 of 12