Mechanical Reinforcement of Hydrogels via Bio-Inspired Mineralization by Olivia Saouaf Submitted to the Department of Material Science and Engineering in Partial Fulfillment of the requirements of the Degree of Bachelor of Science at the Massachusetts Institute of Technology June 2019 C 2019 Olivia Saouaf All rights reserved The author hereby grants to MIT permission to reproduce and to reproduce and distribute publicly paper and electronic copies of this thesis document in whole or in part in any medium known or hereafter created. Signature redacted Signature of Author................................. ............................... Olivia Saouaf Department of Material Science and Engineering / 47 May 6, 2019 C ertified b y ..................................................................... ............ Signature redacted Niels Holten-Andersen Associate Professor of Materials Science and Engineering Thesis Supervisor A ccepted by............................................... redacted.Signature 4 --- Juejun H u MASSACHUSETS INSTITUTE Professor of Materials Science and Engineering OF TECHNOWOGY Chair, Undergraduate Committee JUN 1 1 2019 IBRARIES ARCHIVES 1 Mechanical Reinforcement of Hydrogels via Bio-Inspired Mineralization By Olivia Saouaf Submitted to the Department of Materials Science and Engineering on May 6, 2019 in Partial Fulfillment of the Requirements for the Degree of Bachelor of Science in Material Science and Engineering Abstract Nanocomposites made of polymer networks and mineral particles lend great mechanical integrity to biological materials. This study aims to imitate these natural materials by creating a hydrogel mineralized with magnetite particles. We create a hydrogel of polyallylamine crosslinked via tannic acid molecules. Crosslinking is dependent upon pH as well as amounts of periodate and tannic acid. The addition of greater amounts of tannic acid and periodate at higher pH creates a more strongly crosslinked network, shown through rheological measurements as the gel's shear modulus increases. Upon mineralization, a 102-103 order of magnitude increase in shear modulus occurs. This work elucidates a method for nanocomposite hydrogel synthesis that creates a mechanically strong biocompatible material for future applications in bio- interfacing technology and drug delivery. Thesis Supervisor: Niels Holten-Andersen Title: Associate Professor of Materials Science and Engineering Acknowledgements I would like to thank Professor Niels Holten-Andersen for his guidance and during this work. I would also like to thank the Laboratory for Bio-Inspired Interfaces members for their support of my research, especially Jake Song for his mentorship and Sungjin Kim for mineralization of gels. 2 Table of Contents L ist o f F ig ures.......................................................................................4 L ist of T ables................................................................................... .. 5 1. In tro du ction ..................................................................................... 6 2 . B ackground ..................................................................................... 8 2.1. H ydrogel N etw ork......................................................................8 2.2. Polyallylam ine.......................................................................... 8 2 .3. T annic A cid ................................................................................ 9 2.4. P eriodate................................................................................. 10 2.5. M agnetite M ineralization ............................................................... 11 2 .6 . R h eo logy .................................................................................. 12 3. M aterials and M ethods........................................................................13 3.1. M aterials ............................................................................... 13 3.2. E quipm ent............................................................................... 13 3 .3 . P rocedures.................................................................................13 3.4. T estin g ..................................................................................... 14 4. R esults and D iscussion ....................................................................... 16 4.1. Effects of pH on Gel Formation.....................................................17 4.2. Periodate Amount Variation..........................................................18 4.3. Tannic Acid Amount Variation.........................................................20 4.4. Mineralization With and Without Post Treatment..................................21 5. C onclusion ................................................................................. .. 23 6 . F uture W ork s...................................................................................24 R eferences...................................................................................... .. 25 A ppendix .......................................................................................... 27 3 Figure List Figure 1: A mineralized hydrogel network ..................................................... 6 Figure 2: Molecular structure of polyallylamine.............................................8 Figure 3: Structure of tannic acid molecule......................................................9 Figure 4: Diagram of pH influenced tannic acid bonding and conjugation...............10 Figure 5: Sodium periodate structure..........................................................10 Figure 6: Gel formation and mineralization process...........................................11 Figure 7: Example rheological data...............................................................16 Figure 8: Samples made with different pH......................................................17 Figure 9: A pH variation frequency sweep......................................................18 Figure 10: Gels formed with 2x and 4x original sodium periodate amount.................18 Figure 11: Periodate variation.....................................................................19 Figure 12: Tannic acid variation...............................................................20 Figure 13: A post treated 4x NalO4 2x TA gel..............................................21 Figure 14: Post Treatment and Mineralization, 4x Periodate, 2x Tannic Acid.............22 Figure A 1: pH V ariation........................................................................ 27 Figure A 2: Periodate Variation.................................................................27 Figure A 3: Tannic Acid Variation............................................................28 Figure A 4: Post Treatment......................................................................28 Figure A 4: Mineralization......................................................................29 Figure B 1: SEM image of mineralized hydrogel, no post treatment.......................30 4 List of Tables Table 1: Component ratios for hydrogel synthesis .......................................... 14 Table C. 1: Optimized hydrogel recipe.........................................................31 5 1. Introduction Biology has created numerous nanocomposite materials consisting of mineral particles bound in a polymer matrix which exhibit great strength and toughness [1-3]. Materials like nacre [4] are extremely tough and are also made of cheap, readily available materials. Mimicking these materials gives the potential for new low cost synthetic materials with exceptional mechanical properties as well as biocompatibility. Of particular interest are mineralized hydrogels. Hydrogels consist of a crosslinked hydrophilic polymer network swollen with water [5]. Inorganic nanoparticles are then grown within the network, as in Wang et al. [6]. This study aims to imitate these natural materials by creating a hydrogel mineralized with magnetite particles. We fabricate a hydrogel of polyallylamine hydrochloride crosslinked via tannic acid molecules (Figure 1). Polyallylamine is a polymer known to be biocompatible [7] and iron, the metallic component of magnetite, is found naturally within the body. Tannic acid is a low cost, naturally derived crosslinker [8]. With this system, we propose a low cost, biocompatible, and magnetically responsive mineralized hydrogel for use in bio-interfacing technology and drug delivery. Figure 1. A mineralized hydrogel network consisting ofpolyallylamine chains (blue), tannic acidcrosslinks (green) and magnetite particles (red) 6 The system proposed can be manipulated in crosslinking density to allow for greater network stability, but must remain open enough for the growth of magnetite nanoparticles to be possible. Crosslinking is dependent upon pH, and amounts of periodate and tannic acid. The manipulation of pH and amounts of tannic acid and periodate will change the crosslinking density, which will be measureable by changes in the gel's shear modulus. After a gel is created, it will be mineralized in a bath of iron (III) ions and its shear modulus will be evaluated. Through this process of network adjustment and mineralization, this thesis will show an increase in mechanical strength of PAA hydrogels through the addition of magnetite nanoparticles. 7 2. Background 2.1 Hydrogel Network A hydrogel network consists hydrophilic polymer chains, making up the bulk of the system, which are held together by