Engineered Living Hydrogels

Abstract

Living biological systems, ranging from single cells to whole organisms, can sense, process information, and actuate in response to changing environmental conditions. Inspired by living biological systems, engineered living cells and nonliving matrices are brought together, which gives rise to the technology of engineered living materials. By designing the functionalities of living cells and the structures of nonliving matrices, engineered living materials can be created to detect variability in the surrounding environment and to adjust their functions accordingly, thereby enabling applications in health monitoring, disease treatment, and environmental remediation. Hydrogels, a class of soft, wet, and biocompatible materials, have been widely used as matrices for engineered living cells, leading to the nascent field of engineered living hydrogels. Here, the interactions between hydrogel matrices and engineered living cells are described, focusing on how hydrogels influence cell behaviors and how cells affect hydrogel properties. The interactions between engineered living hydrogels and their environments, and how these interactions enable versatile applications, are also discussed. Finally, current challenges facing the field of engineered living hydrogels for their applications in clinical and environmental settings are highlighted.

Keywords: engineered living hydrogels; microbe-material interactions; real-world applications; synthetic biology.

Figure 1.

The convergence of engineered living cells and hydrogels gives rise to the technology of engineered living hydrogels. In the engineered living hydrogels, cells and hydrogels interact with each other. The living cells can be programmed with diverse functions, including sensing, chemical production, and electricity generation. The hydrogels can also be programmed with various functions, which create chemical gradients, mechanical confinement and forces, and spatial distribution for the engineered living cells.

Figure 2.

Representative hydrogel matrices in engineered living hydrogels. a-c) Living microbial cells dispersed in cell-generated hydrogels. Examples include an E. coli-produced curli fibril biofilm used as an electrical switch (b) and an E. coli-generated curli hydrogel used as a mucoadhesive patch in the gut (c). (b) Reproduced with permission^[41]. Copyright 2014, Springer Nature. (c) Reproduced with permission^[42]. Copyright 2019, Wiley-VCH. d-f) Living microbial cells dispersed in synthetic hydrogels. Examples include microbial cell-laden hydrogel beads used as a heavy-metal detector in the environment (e) and a 3D-printed, cell-laden hydrogel pattern used as a biosensor on the skin (f). (e) Reproduced with permission^[24]. Copyright 2021, Springer Nature. (f) Reproduced with permission^[22]. Copyright 2018, Wiley-VCH. g-i) Living microbial cells enclosed in hydrogel chambers. Examples include a stretchable hydrogel-elastomer hybrid containing microbial cells in channels (h) and a 3D-printed, core-shell hydrogel structure containing microbial cells in cavities (i). (h) Reproduced with permission^[46]. Copyright 2017, National Academy of Sciences. (i) Reproduced with permission^[53]. Copyright 2013, National Academy of Sciences.

Figure 3.

The chemical composition of polymer networks in hydrogels affects cell dynamics. a) Effects on cell viability: the antimicrobial side groups in the polymer network and antimicrobial precursors to prepare the polymer network can induce defects in the cell membrane and cause microbial cell death. b) Effects on cell motility: microbial cells can adhere to the hydrogel surface through non-specific or specific adhesion, which reduces cell motility.

Figure 4.

The chemical composition of aqueous solutions in hydrogels affects cell dynamics. a) In non-porous hydrogels the diffusion of chemical species is regulated by the nanoscale mesh of polymer networks. Non-porous hydrogels allow the diffusion of small molecules, but macromolecules are immobilized. b) Biochemical reactions in hydrogels may interfere with the diffusion of chemical species. For example, the chemicals may be consumed if they interact with the polymer network or cells. c) In porous hydrogels, the convection of chemical species is regulated by the macroscale pores. In contrast to the slow diffusion observed in non-porous hydrogels, porous hydrogels allow fast convection of chemicals. d) The chemical composition of aqueous solutions in hydrogels affects cell distribution. For example, the chemical gradient of nutrients sets up the gradient of cell-population densities: A sufficient nutrient supply leads to a high cell density, while an insufficient nutrient supply leads to a low cell density. e) The chemical composition of aqueous solutions in hydrogels affects the cell phenotype. For example, the chemical gradient of signaling molecules causes the population-density gradient of activated cells: A high concentration of signaling molecules leads to cell activation, while a low concentration does not affect the cell phenotypes.

Figure 5.

The mechanical constraints imposed by hydrogel structures affect cells dynamics. a) Effects on cell motility when cells pass through a spatially constrained structure. For example, in a porous matrix with the pore dimension larger than the cells, the cells exhibit hop-and-trap dynamics (a, top); in a narrow channel with the channel diameter smaller than the cells, the cells “move” via cell growth with shape change (a, bottom). Note that the microfluidic channels shown here are made of silicon and polydimethylsiloxane (PDMS) instead of hydrogels. b) Effects on cell growth when the hydrogel structure constitutes a confined space. For example, in a closed chamber, cell growth is limited by the chamber size (b, top); in a narrow chamber, filamentous microbial cells grow into the structure of the chamber (b, bottom).

Figure 6.

The mechanical forces in hydrogels affect cell growth, adhesion, and escape. a) The compression forces limit cell growth at the single-cell level and population level. b) The adhesion forces between the cell and substrate surface promote cell adhesion and biofilm formation. c) The shear forces caused by the fluid flow induce the deformation and dispersion of the natural hydrogel matrix, which allow the encapsulated microbial cells to escape.

Figure 7.

Cell engineering to regulate the generation of biopolymers. a) Metabolic engineering involves adding (a-1) or removing (a-2) biochemical pathways to optimize the production of the desired biopolymers. b) Genetic engineering involves plasmid engineering (b-1) and transformation (b-2), resulting in the expression of desired genes in microbial cells (b-3). Genetic engineering allows the constitutive and inducible gene expression of biopolymer production by microbial cells. RBS, ribosome binding site; ori, origin of replication.

Figure 8.

Living cells generate biopolymers with diverse chemistries and forming diverse structures. a) Microbial cells can produce diverse biopolymers, including polysaccharides (alginate, cellulose, hyaluronate) and polyamides (poly-γ-glutamate and protein). b) Microbial cells can produce biopolymers that form diverse structures and microstructures, including flat sheets, solid particles, hollow particles, and aligned fibrils.

Figure 9.

Living cells produce biomass for hydrogel generation, repair, reinforcement, and degradation. a) Microbial cells in liquid culture can generate new solid matrices by producing biopolymers. b) Microbial cells can repair damaged materials by sealing cracks in the matrix. c) Microbial cells can reinforce existing architectures by introducing another polymer network into the matrix. d) Microbial cells can induce degradation of a polymer network by secreting depolymerases in the matrix.

Figure 10.

Living cells generate patterns on hydrogels. a) Non-uniform distribution of external stimuli (e.g., chemical gradient and light exposure) triggers the variation of gene expression of microbial cells, so that gradually or suddenly varied patterns are displayed on the material. b) Synthetic genetic circuits in single-strain and multi-strain systems can produce spatially or temporally varied patterns on the material.

Figure 11.

Engineered living hydrogels can sense environmental signals. a) Sensing by engineered living hydrogels in two coupled steps: signal transportation in hydrogels and biochemical reactions in cells. b) Sensing functions and examples by engineered living hydrogels. (b, top) Spatial identification of light intensity. Reproduced with permission^[170]. Copyright 2021 Springer Nature. (b, middle) High-throughput detection of toxic chemicals. Reproduced with permission^[193]. Copyright 2014 Royal Society of Chemistry. (b, bottom) Logic gate sensing and computation. Reproduced with permission^[22]. Copyright 2018 Wiley-VCH.

Figure 12.

Engineered living hydrogels can treat diseases or alleviate environmental pollution. a) Molecular production: synthesis of the molecules in cells and transportation of the synthesized molecules in hydrogels, which can be used to treat diseases (e.g., therapeutics) or to remediate the environment (e.g., pollutant-degrading enzymes). b) Molecular depletion: transportation of the molecules in hydrogels and consumption of the molecules by cells, which can be used to treat diseases (e.g., by removing body wastes or disease by-products) or to remediate the environment (i.e., by removing pollutants). c) Production functions and examples by engineered living hydrogels. (c, top) Continuous production of alcohol. Reproduced with permission^[206]. Copyright 2018 Frontiers Media. (c, middle) On-demand production of penicillin. Reproduced with permission^[60]. Copyright 2012 Wiley-VCH. (c, bottom) Localized generation of therapeutics. Reproduced with permission^[42]. Copyright 2019 Wiley-VCH.

Figure 13.

Engineered living hydrogels that can generate chemical, electrical, or mechanical energy. a) Chemical energy is generated by engineered living hydrogels in three coupled steps: sunlight absorption in hydrogels, chemical production in cells, and molecule transportation in hydrogels. b) Electrical energy is generated by engineered living hydrogels in three coupled steps: nutrient transportation in hydrogels, nutrient consumption and electron generation in cells, and electron transfer in hydrogels. c) Mechanical energy is generated by engineered living hydrogels in two coupled steps: morphological change of the cells and mechanical deformation of hydrogels. d) Representative applications of engineered living hydrogels used for energy conversion. (d, top) Solar to chemical energy conversion. Representative curves of hydrogen production by cyanobacteria entrapped in different hydrogels in the sunlight. Reproduced with permission^[215]. Copyright 2018, Royal Society of Chemistry. (d, middle) Chemical to electrical energy conversion. Representative curves of power density when the cellulose-polyaniline hydrogel is used as an anode in microbial fuel cells. Reproduced with permission^[225]. Copyright 2016, Elsevier. (d, bottom) Chemical to mechanial energy conversion. Representative curves of plane stress at the interface and energy density of the biofilm layer, when the bilayer of the biofilm and substrate are at different relative humidity. Reproduced with permission^[229]. Copyright 2014, Springer Nature.