Engineering liver

Abstract

Interest in "engineering liver" arises from multiple communities: therapeutic replacement; mechanistic models of human processes; and drug safety and efficacy studies. An explosion of micro- and nanofabrication, biomaterials, microfluidic, and other technologies potentially affords unprecedented opportunity to create microphysiological models of the human liver, but engineering design principles for how to deploy these tools effectively toward specific applications, including how to define the essential constraints of any given application (available sources of cells, acceptable cost, and user-friendliness), are still emerging. Arguably less appreciated is the parallel growth in computational systems biology approaches toward these same problems-particularly in parsing complex disease processes from clinical material, building models of response networks, and in how to interpret the growing compendium of data on drug efficacy and toxicology in patient populations. Here, we provide insight into how the complementary paths of engineering liver-experimental and computational-are beginning to interplay toward greater illumination of human disease states and technologies for drug development.

Figure 1

Designer Synthetic Extracellular Matrix (ECM) Materials Allow Control of Matrix Stiffness, Permeability, and Selective Adhesion and Growth Factor Interactions. (A) ECM-modified polyacrylamide gels tuned to soft, moderately stiff, and stiff bulk elastic moduli reveal the role of ECM mechanical properties on activation of hepatic stellate cells [from (22)]; (B) Matrix-tethered EGF, compared to soluble EGF, dramatically alters phenotypic responses of hepatocytes on self-assembling peptide hydrogels with tuned bulk elastic moduli [from (23)]; (C) Schematic illustrating modular design of synthetic 3D hydrogels for cell encapsulation or invasion. Gels comprise a structural water-soluble polymer (such as polyethylene oxide, dextran, hyaluronic acid, etc) crosslinked by Michael addition, photo-polymerization, temperature- or ion-induced phase change, or enzymatically, and with modules including selective protease cleavage sites and tethered adhesion or growth factor ligands or motifs that bind to matrix or growth factors. The crosslink density and choice of polymer influence the permeability (characterized by a “mesh size” typically on the scale of nm) and the bulk mechanical properties, while local mechanical properties sensed by receptors are also influenced by the tether length, stiffness, and ligand orientation or clustering. (D) Example of directed angiogenesis by endothelial cells encapsulated in a synthetic RGD-modified PEG gel. Cells encapsulated in an isotropic gel exhibit isotropic orientation of capillary tubes, while cells encapsulated in a gel with a stiffness gradient show oriented tube formation. [from (27)].

Figure 2

Microscale bioreactors to control hepatic tissue organization and flow. (A) Microfluidic design to mimic flow and gradients along an hepatic sinusoid, with dual flow networks to enable easy loading with a precise number of hepatocytes [from (33)]. Shown is the research-scale precursor to the commercially available multi-reactor chip version. (B) Microfluidic reactor design for examining how mechanical compaction of cell aggregates during tissue formation influences tissue morphogenesis, including formation of biliary networks [from (35)] (C) Multi-well plate bioreactor system that fosters 3D tissue-like formation in an array of channels of a “chip” scaffold seeded with isolated liver cells, where microscale flow is maintained by a microfluidic pump; images at the top show individual 300 um-diameter channels containing tissues formed from co-cultures of hepatocytes with non-parenchymal cells, and treated with either drug alone (left), inflammatory cue alone (middle) or a combination, then stained with live (green) /dead (red) dyes, illustrating synergy between inflammation and drug metabolism in hepatotoxicity [from (52)].