exam1_solns

# exam1_solns - Exam 1 Model Solutions 80 points total...

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Exam 1 Model Solutions 20.462J/3.962J 03/21/06 Spring 2006 80 points total Useful Equations: Expressions for the molecular weight (M) of degrading polymers vs. time: 1 = 1 + kt M = e M kt o M M o Charlier model for controlled release; amount of drug released at time t and depth of diffusion front: kt 1 ) 1/ 2 kt 1 Q ( t ) = A ˜ ( e k = A 2 C C D o ( e ) s o k kt 1 1/ 2 o s h ( t ) = 2 D C ( e 1 ) kC o π RT ( c ) Osmotic pressure developing across a membrane and resulting flux of water: L p () = J water δ Merrill-Peppas master equation for swelling of nonionic hydrogels: 2 1 = 2 v sp 2 , ( 1 ln( φ ) + φ , 2 s + χφ , 2 s ) , 2 s 1/ 3 M c M V 1 , 2 r , 2 s 1 , 2 s ⎢⎜ ⎟⎥ , 2 r 2 , 2 r 1 5/ 3 χ ) 2, s reduced form for highly swollen gels: 1 v sp ,2 ( 2 M 2/ 3 c V 1 2, r diffusion coefficient for a drug molecule (hydrodynamic radius r ) in a hydrogel with swelling ratio Q : 1 D = D H 2 O 1 r e ( Q 1 ) (cm 2 /sec) ξ 20.462J/3.962J S06 EXAM 1 solutions 1 of 10 9/5/06

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1. (20 points total) Choosing materials for in vivo applications . Answer the questions below with concise responses: 1-3 sentences or fragments, or qualitative sketches as appropriate. a. (5 points) Materials used in vivo can be placed into one of three classes, based on the ultimate fate of the material. What are these 3 classes, and what is the characteristic fate in vivo of materials of each type? Permanent/retrievable : These materials are not degraded/eroded in vivo. They will remain permanently in place until surgically removed from a patient. Bioeliminable : Biomaterials that do not degrade to completely resorbable chemical constituents but which are water soluble and can be excreted through the kidneys or faeces are termed bioeliminable. Biodegradable : Materials that break down to completely metabolized byproducts are biodegradable. These materials are typically reduced to basic byproducts of metabolism, e.g., CO 2 and water. b. (5 points) Draw the chemical structure and name 3 examples of bonds typically sensitive to hydrolysis under physiologic conditions; show the chemical structure of the immediately resulting products that are created by hydrolysis of each. 20.462J/3.962J S06 EXAM 1 solutions 2 of 10 9/5/06
c. (5 points) Biodegradable solid polymers undergo two distinct modes of breakdown, surface erosion or bulk erosion. Fill in the graphs shown below with qualitative representations/annotation to show what behavior you would expect for a material undergoing surface vs. bulk erosion. BULK EROSION SURFACE EROSION Molecular Molecular weight weight time time Mass Mass remaining remaining time time d. (5 points) Why are physical hydrogels most commonly formed by block copolymers (copolymers where two chemically different repeat units occur in strings, e.g., AAAABBBBAAAABBBB), rather than random copolymers (random monomer sequences, e.g., ABAABBABBBAAAB)? (Answer in 1-3 sentences).

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exam1_solns - Exam 1 Model Solutions 80 points total...

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