Photo-induced Gel Growth with Visible Light Photocatalysts

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Comparison of traditional light-induced additive manufacturing and proposed living additive manufacturing. (a) Free radical polymerization produces materials with chains that cannot be reactivated for insertion of new monomers. Though extremely well developed, these methods cannot achieve homogeneous modification of already printed objects. (b) Herein, we develop a living additive manufacturing approach wherein parent objects comprised of dynamic covalent polymer networks are reactivated via a photoinduced living radical polymerization to generate daughter objects with homogeneous network modifications. These daughter objects of varying composition can be welded together to form increasingly complex progeny with spatially defined stimuli-responsive behaviors. Photoredox catalyzed growth (PRCG) of cross-linked polymer gels. (a) General schematic for controlling polymer network structure by PRCG. Monomers (M) and cross-linkers (CL) can be directly incorporated into the network strands via a living photoredox catalyzed process. (b) Proposed mechanism of PRCG polymerization using a suitable photocatalyst (10-phenylphenothiazine, PTH) and a network comprised of strands bearing trithiocarbonate (TTC) iniferters. Blue LED light excites PTH molecules in solution and leads to photoinduced electron transfer from PTH* to the network-bound TTCs. Strand cleavage produces a propagating strand (green), a strand with a stable TTC anion (blue), and the PTH radical cation in solution (purple). In the presence of monomers (M) or cross-linkers (CL), the propagating strand can grow; it can also undergo degenerative chain transfer (“RAFT process”) with adjacent nonactivated network strands. Turning the light source “OFF” leads to recombination of the cleaved strands via back electron transfer to the PTH radical cation, thereby resulting in net photocontrolled insertion of new M and CL into the network in a living fashion.Living additive manufacturing via PRCG insertion of NIPAAM into a parent PEG-TTC network. (a) Parent network design. Gelation is achieved by SPAAC click chemistry. PRCG is conducted in the presence of monomer and PTH under blue LED irradiation. (b) Experimental storage modulus G′ (blue curve) and simulated results of inverse of average chain length (1/Mc) between junctions (red curve) as a function of monomer conversion in PRCG. (c) Swelling ratio (blue curve). Defined as Ww/Wd, where Ww is the weight of a sample swollen in pure water at 20 °C; Wd is the weight of this sample in the dry state) and simulated average chain length between network junctions, Mc, (red curve) as a function of monomer conversion. (d) Optical images of parent gel I and daughter gels II-a to II-e swollen in pure water at 20 °C.
Professor Jeremiah Johnson
Department of Chemistry, MIT
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Mao Chen
Department of Chemistry, MIT
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Jon Gilbert
MIT Technology Licensing Officer
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Photo-induced Gel Growth with Visible Light Photocatalysts

Provisional Patent Application Filed
Living Additive Manufacturing: Transformation of Parent Gels into Diversely Functionalized Daughter Gels Made Possible by Visible Light Photoredox Catalysis
ACS Central Science, Jan. 13, 2017, p. 124-134


A class of gels whose composition and mechanics can be externally regulated using a living, visible light photoredox catalyzed polymerization reaction can be used for 3D printing, tissue adhesion, scaffolding, and engineering, as well as for cell culture.


To date, the only reports of living light-induced growth in polymer gels have required the use of ultraviolet (UV) light to induce changes in gel microstructure via polymerization. However, because the gel networks can absorb a high amount of UV light, it is difficult for UV to penetrate inner layers of the gel. In addition, side reactions in the course of polymerization can lead to undesirable products within the gel microstructure, and loss of living behavior. In addition, UV light may harm biological systems embedded within or growing on the gel. A method that uses visible-light could overcome all of these deficiencies.


This invention involves an organic photocatalyst, 10-phenylphenothiazine (PTH), which can catalyze vinyl polymerization reactions from monomers already embedded within a gel matrix. Only a small amount of photocatalyst is needed, and a simple household compact fluorescent light bulb or blue LED is sufficient to trigger these reactions. The polymerization has been shown to proceed with excellent control to produce polymers with low dispersity indices. Different monomers and crosslinkers can be embedded into the parent gel at different ratios, influencing the average chain length resulting from polymerization. This in turn effects the material mechanics and composition only in irradiated areas. The process can be repeated many time in patterned regions to produce complex objects. This reaction can give gels the ability to photo-heal and photo-weld, and can also increase or decrease their shear strength and ability to swell or deswell upon aqueous immersion. In addition to these qualities, tuning the reaction creates daughter gels with a variety of mechanical properties and chemical compositions, hydrophilicity, and temperature responsiveness.


  • Enables the preparation of diversified daughter gels with tunable mechanical properties and various chemical compositions from a parent gel
  • Use of a fluorescent or LED light source instead of UV to catalyze reaction
  • New platform to design and synthesize polymer networks with photo-healing, photo-welding, or site-specific responsiveness properties
  • Controlled polymerization reaction with low dispersity and excellent control