Smart Polymers: Hydrogels and Shape Memory Polymers

1. Smart Polymers:Hydrogels and Shape Memory Polymers Dr. Jenny Amos February 19, 2009 1

2. Overview Hydrogels Structure Swelling properties Biomedical Uses Shape Memory Polymers Structure Triggers Biomedical Uses

3. Common Hydrogels? Can you think of hydrogels in your everyday life?

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5. Definition Water insoluble, three dimensional network of polymeric chains that are crosslinked by chemical or physical bonding; Polymers capable of swelling substantially in aqueous conditions (eg hydrophilic) Polymeric network in which water is dispersed throughout the structure 4/29/2012 5

6. Behavior of Hydrogels No flow when in the steady-state By weight, gels are mostly liquid, yet they behave like solids They can absorb large quantities of water May absorb 1-20% up to 1000 times their dry weight Cross linkers within the fluid give a gel its structure (hardness) and contribute to stickiness (tack).

7. Classes of Hydrogels Hydrogels can also be classified by the crosslinkers Chemical Covalently crosslinked Absorb water until they reach equilibrium swelling (crosslink density dependent) High stability in harsh environments (high temp, acidic/basic and high stress) Physical Non-covalently crosslinked Disordered networks are held together by associative forces capable of forming non-covalent crosslinks (molecular entanglements, electrostatic interactions, hydrogen bonding, and hydrophobic interactions) Weaker and more reversible forms of chain-chain interaction Respond to physical changes (temperature, pH, ionic strength and stress)

8. Based on ionic charges, hydrogels can be classified as... Neutral hydrogels No charge Anionic hydrogels Negatively charged Cationic hydrogels Positively charged Ampholytic hydrogels Capable of behaving either positively or negatively

9. Classes of Hydrogels

10. Based on structural features, hydrogels can be classified as... Amorphous hydrogels Randomly arranged macromolecular chains Semicrystalline hydrogels Dense regions of ordered macromolecular chains (crystallites) Hydrogen bonded hydrogels 3-D network held together by hydrogen bonds Strong hydrophobic/hydrophillic interactions

11. Chemical HydrogelsMethods Co-polymerization of monomer and crosslinker HEMA and EGDMA (Ethylene glycol dimethacrylate) Crosslinking water soluble polymers Conversion of hydrophobic polymers to hydrophilic polymers plus crosslinking

12. Hydrogel Swelling One or more highly electronegative atoms which results in charge asymmetry favoring hydrogen bonding with water Because of their hydrophilic nature dry materials absorb water By definition, water must constitute at least 10% of the total weight (or volume) for a materials to be a hydrogel When the content of water exceeds 95% of the total weight (or volume), the hydrogel is said to be superabsorbant 4/29/2012 12

13. Important features of hydrogels Usually comprised of highly polyionic polymers Often exhibit large volumetric changes eg. Highly compressed in secretory vessicle and expand rapidly and dramatically on release Can undergo volumetric phase transitions in response to ionic concentrations (Ca++, H+), temperature, .. Volume is determined by combination of attractive and repulsive forces: repulsive electrostatic, hydrophobic attractive, hydrogen binding, cross-linking 4/29/2012 13

14. Swelling...Thermodynamically Speaking Network starts to swell due to the thermodynamic compatibility of the polymer chains and water Swelling in chemical (crosslinked) polymers is dependent on the solvent Swelling force is counterbalanced by the retractive force induced by the crosslinks of the network Swelling equilibrium is reached when these two forces are equal Degree of swelling can be quantified by: ratio of sample volume in the swollen state to volume in the dry state weight degree of swelling: ratio of the weight of swollen sample to that of the dry sample

15. Swelling Properties Gibbs Free Energy DG=DGelastic+DGmix Chemical Potential m1-m1,0=Dmelastic+Dmmix Dmmix=RT(ln(1-2v2,s)+v2,s+c1v^22,s) G=Gibbs Free Energy work exchanged by the system with its surroundings minus the work of the pressure forces during a reversible transformation of the system from the same initial state to the same final state DG<0 Spontaneous DG=0 Equilibrium DG>0 Non-spontaneous

16. Swelling Properties Gibbs Free Energy DG=DGelastic+DGmix Chemical Potential m1-m1,0=Dmelastic+Dmmix Dmmix=RT(ln(1-2v2,s)+v2,s+c1v^22,s) G=Gibbs Free Energy work exchanged by the system with its surroundings minus the work of the pressure forces during a reversible transformation of the system from the same initial state to the same final state DG<0 Spontaneous DG=0 Equilibrium DG>0 Non-spontaneous Electrophoresis: For example, consider charged particles in a fluid. A concentration gradient in a fluid may promote movement of particles in one direction, and the electric potential gradient may promote movement of the particles in another. The chemical potential would account for both concentration and electric components and describe a potential distribution that determines net particle movement. In hydrogels: accounts for water on outside and inside of polymer network, can be altered by heat formed during mixing and entropy changes during mixing Electrophoresis: For example, consider charged particles in a fluid. A concentration gradient in a fluid may promote movement of particles in one direction, and the electric potential gradient may promote movement of the particles in another. The chemical potential would account for both concentration and electric components and describe a potential distribution that determines net particle movement. In hydrogels: accounts for water on outside and inside of polymer network, can be altered by heat formed during mixing and entropy changes during mixing

17. Swelling Properties Gibbs Free Energy DG=DGelastic+DGmix Chemical Potential m1-m1,0=Dmelastic+Dmmix Dmmix=RT(ln(1-2v2,s)+v2,s+c1v^22,s) v2,s=polymer volume fraction of the gel v2,s= Volume of polymer = vp = 1 __________________ __ __ Volume of swollen gel vgel Q Q= volume degree of swelling c1= polymer-water interaction parameter (look up in a table) R= Universal Gas Constant= 8.314?472(15) J?K-1?mol-1

18. Hydrogels Highly swollen hydrogels: cellulose derivatives poly(vinyl alcohol) poly(ethylene glycol) What do these all have in common? Lots of OH (or =O) groups to interact with acidic environments ? hydrophillic ? swelling Moderately or poorly swollen hydrogels: poly(hydroxyethyl methacrylate), PHEMA and derivatives One may copolymerize a highly hydrophilic monomer with other less hydrophilic monomers to achieve desired swelling properties 18

19. Contact Angle (wetability) Hydrophobic – “water hating” Hydrophilic – “water loving” Why is this important to materials selection? Do you want your eyes to be dried out by your contact lenses? Probably not…

20. Electrowetting of the surface Top: charge neutral (grounded) surface with high contact angle Bottom: Allowing voltage between the drop and the electrode changes the distribution of electric charge within the drop and significantly decreases the contact angle. The polarity of voltage in the drawing is arbitrary, and in both directions electrowetting will occur.

22. Other Important Properties Solute diffusion coefficient through the hydrogel Optical properties Mechanical properties Hydrophilic hydrogel surfaces are poor substrates for Protein adsorption Cell adsorption

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24. How biological hydrogels grow 4/29/2012 24

25. pH Change Induced Drug Release Enzymatic pH change with a cationic hydrogel, which is weakly basic Results in ionization, swelling, and release of drug, peptide, or protein

26. Environmentally Responsive Hydrogels Hydrogels that exhibit swelling changes due to the external pH Temperature Ionic Concentration

27. Environmentally Responsive Hydrogels pH

28. Environmentally Responsive Hydrogels Temperature

29. Environmentally Responsive Hydrogels Ionic concentration

30. Hydrogel Advantages Non-thrombogenic Non-ionic hydrogels used for blood contacting applications Heparinized hydrogels show promise Biocompatible Good transport of nutrients to cells and products from cells May be easily modified with cell adhesion ligands Can be injected in vivo as a liquid that gels at body temperature

31. Hydrogel Disadvantages Can be hard to handle Usually mechanically weak Difficult to load with drugs and cells and crosslink in vitro as a prefabricated matrix May be difficult to sterilize

32. Biomedical Uses for Hydrogels Common Scaffolds in tissue engineering. Sustained-release delivery systems Provide absorption, desloughing and debriding capacities of necrotics and fibrotic tissue. Hydrogels that are responsive to specific molecules, such as glucose or antigens can be used as biosensors as well as in DDS. Disposable diapers where they "capture" urine, or in sanitary napkins Contact lenses (silicone hydrogels, polyacrylamides) Medical electrodes using hydrogels composed of cross linked polymers (polyethylene oxide, polyAMPS and polyvinylpyrrolidone) Lubricating surface coating used with catheters, drainage tubes and gloves Hydrogels are non-adherent and can be removed without trauma to the wound.Hydrogels are non-adherent and can be removed without trauma to the wound.

33. Biomedical Uses for Hydrogels Less common uses include Breast implants Dressings for healing of burn or other hard-to-heal wounds. Wound gels are excellent for helping to create or maintain a moist environment. Reservoirs in topical drug delivery; particularly ionic drugs, delivered by iontophoresis Artificial tendon and cartilage Wound healing dressings (Vigilon®, Hydron®, Gelperm®) non-antigenic, flexible wound cover permeable to water and metabolites Artificial kidney membranes Artificial skin Maxillofacial and sexual organ reconstruction materials Vocal cord replacement Butt injections (no I’m not kidding…Google it) 33

34. Headlines

35. Smart Polymers Shape Memory Polymers (SMP) “Memorize” a macroscopic (permanent) shape Manipulated and fixed to a temporary and shape under specific conditions of temperature and stress Relax to the original, stress-free condition under thermal, electrical, or environmental command. This relaxation is associated with elastic deformation stored during prior manipulation

36. SMPs Definition- materials capable of reversible change in length at operating temperatures, that is, once a load is removed the material returns to its original dimensions A rubbery compound (elastomer) Can be amorphous thermosets or thermoplastics (covalently cross-linked) with Tg below room temperature to allow full chain mobility- the restoring force in entropic Shape memory polymers morph by the glass transition or melting transition from a hard to a soft phase which is responsible for the shape memory effect. In shape memory alloys Martensitic/Austenitic transitions are responsible for the shape memory effect. 36

38. Elastomers have properties that can be explained by a model that shares characteristics of thermosets and thermoplastics -have all of the domains shown below 38

39. Heating/Cooling Cycle

40. Shape Memory Cycle Steps for Shape Memory Cycle Deformation under constant loading rate at constant temperature Cooling under constant load Load is removed and shape fixing was observed for the SMP (but an instant recovery was seen for natural rubber) Shape recovery of the primary equilibrium shape was obtained by heating the SMP The star indicates the start of the experiment (initial sample dimensions, temperature, and load). The star indicates the start of the experiment (initial sample dimensions, temperature, and load).

41. Recovery Cycle Strain recovery of a cross-linked, castable shape-memory polymer upon rapid exposure to a water bath at T = 80 °C

42. More Recovery Fun!! http://www.youtube.com/v/vWlRcazeSnUhttp://www.youtube.com/v/vWlRcazeSnU

43. Uses for Shape Memory Polymers Intravenous cannula Self-adjusting orthodontic wires Pliable tools for small scale surgical procedures where currently metal-based shape memory alloys such as Nitinol are widely used. Minimally invasive implantation of a device in its small temporary shape which after activating the shape memory by e.g. temperature increase assumes its permanent (and mostly bulkier) shape.

44. Smart Sutures

45. Intravenous cannula Pictures of the [shape memory] foam deploying in in vitro aneurysm model.  Foam starts in compressed form (upper left) and expands to fill 60% the aneurysm (lower right).  The time from the laser initiation to the final image was approximately 10 seconds. http://cbst.ucdavis.edu/research/aneurysm-treatment

46. References http://www.sciencedirect.com/science?_ob=MiamiCaptionURL&_method=retrieve&_udi=B6TWB-4PDKB0F-4&_image=fig1&_ba=1&_user=10&_rdoc=1&_fmt=full&_orig=search&_cdi=5558&view=c&_acct=C000029040&_version=1&_urlVersion=0&_userid=571676&md5=1195e73686f064d9bab5ecb93f9fec5a http://ocw.mit.edu/NR/rdonlyres/Biological-Engineering/20-462JSpring-2006/C4A62521-CDC0-44AC-8D25-B270C4761C73/0/lec6_clean.pdf http://www.rsc.org/delivery/_ArticleLinking/ArticleLinking.cfm?JournalCode=JM&Year=2007&ManuscriptID=b615954k&Iss=16 http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6T3R-44GKY25-2&_user=571676&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000029040&_version=1&_urlVersion=0&_userid=571676&md5=17349817854a13b6ef0151877e9dc3b9 Ratner B, Hoffman A, Schoen F, Lemons J. Biomaterials Science: An Introduction to Materials in Medicine. Elsevier Academic Press, 2004. http://books.google.com/books?id=2kDLx3mKm1EC&pg=PA7&lpg=PA7&dq=Biological+Performance+of+Materials+Jonathan+Black&source=web&ots=w08ycKzIyH&sig=97LcJQTMsmthUcq5kZLv9A2BAkY&hl=en&ei=Py-bScSaOKSoNdPIvPgL&sa=X&oi=book_result&resnum=1&ct=result#PPA24,M1

47. Quiz time!