All About Cells

This chapter takes the reader on an imaginary scuba diving tour of the watery world of the cell and its surroundings, pointing out features such as the cytoskeleton (that forms the equivalent of the bones and muscles of our cells), the cell membrane (the outer skin of the cell), and the cell membrane’s embedded proteins that provide selective access to the interior of the cell—organelles (elfin versions our own organs). The chapter stresses the tumultuous action that occurs non-stop within the cells as proteins are assembled for use within and outside the cells. The chapter discusses stem cells, including the discovery of induced pluripotent stem cells. The chapter relates how cells differentiate to become dissimilar cell types, stresses the importance of three-dimensional study of cells (rather than two-dimensional study), and explains the different ways in which cells talk to each other.

Skin and Bones—and Muscle Too

This chapter recounts bioprinting studies of skin, bone, skeletal muscle, and neuromuscular junctions. The chapter begins with a study of bioprinted skin designed to enable the creation of skin with a uniform pigmentation. The chapter relates two very different approaches to bioprinted bone: a synthetic bone called hyperelastic bone and a strategy that prints cartilage precursors to bone and then induces the conversion of the cartilage to bone by judicious choice of bioinks. Muscles move bone, and the chapter discusses an investigation of bioprinted skeletal muscle. Finally, the chapter considers an attempt to bioprint a neuromuscular junction, a synapse—a minute gap—of about 20 billionths of a meter between a motor neuron and the cell membrane of a skeletal muscle cell. A motor neuron is a nerve in the central nervous system that sends signals to the muscles of the body.

Organs-on-a-Chip

This chapter explores organs-on-a-chip, miniaturized bioprinted organ tissues enclosed in a microfluidic housing (microfluidics refers to very small-scale plumbing) that can mimic functions of human physiology or disease and are particularly effective when multiple tissue types—for example, lung, heart, and liver—can interact on the same chip. The chapter sets forth the historical evolution of organs-on-a-chip and instances several studies. In one investigation, experimenters found a totally unexpected result in which a drug produced an inflammation of lung tissue that in turn led to toxic results in nearby heart tissue. In another inquiry, researchers focused on a bioprinted, functional, airway-on-a-chip to characterize inflammatory diseases such as asthma and chronic obstructive lung disease and vet potential medications for their treatment. Their work included quantitative comparisons of normal lung tissue and asthmatic lung tissue to a variety of insults, including household dust mites.

Epilogue

The book you’ve been reading can only be a vignette, a brief description of an evolving field; life goes on. Most happily, so too has Nancy’s life. Her kidney transplant was in May 2016, and she was able to come back quickly to her old job as full-time office manager at a thriving physical therapy clinic where she’s highly esteemed by both staff and patients. She told me,...

The Liver

This chapter reports on efforts to bioprint liver tissue, including the important types of liver cells and also the liver’s cytoarchitecture—the typical pattern of cellular arrangement within liver tissue. The chapter gives an account of the liver’s remarkable regenerative ability, its over 500 vital functions, its unusual blood supply, and the difficulty of growing liver cells in vitro (in the laboratory). The chapter includes a description of a hybrid printing/casting method employing human hepatocytes (liver cells) encapsulated in a hydrogel called a “liver tissue seed.” Implanted into mice with a liver injury, the seed tissue provided functional support to the failing liver and expanded in size by 50-fold over the course of 11 weeks. The chapter also mentions Organovo, the first commercial bioprinting company and a pioneer in bioprinting commercially available human tissues, notably their lead product, liver tissue.

The Kidney

This chapter puts forward a series of experiments in which scientists bioprinted one of the critical components of a kidney’s nephron (the filtering unit of the kidney), namely the proximal convoluted tubule where the majority of nutrient absorption back into the bloodstream takes place (and where most drug-induced toxicities of the kidney occur). The same team of researchers bioprinted colocalized (printed very close together) proximal tubules and blood vessels and, with the use of fluorescence microscopy, were able to observe vectorial transport, the process in which valuable nutrients such as serum albumin are selectively reabsorbed into the bloodstream. They also induced a state of hyperglycemia and administered a countermeasure drug, thus demonstrating the ability of their bioprinted kidney tissue to functionally respond as native kidney tissue does to an overdose of glucose and to a drug designed to mitigate this undesirable condition.

What’s in the Offing?

This chapter attempts to peer into the possible future of bioprinting to consider two conceivable directions that bioprinting might take while also contemplating what we may be able to learn about bioprinting’s trajectory by reflecting on another biomedical quest—the twentieth-century’s attempt to conquer polio. In one study that might offer a route for bioprinting, a team created bioconstructs with cell densities approaching that of native tissue (about 108 cells/gram). The group used embedded 3D printing to create a branched, hierarchical network of vascular channels within a large, high cell density bioconstruct and perfused media through the channels that they created using fugitive ink. This was to provide nutrient support for the cells. They also built a high-density cardiac construct in which the cells beat synchronously and showed functional contractility. They quantitatively measured the deformation of the cardiac tissue during contraction.

Bioprinted Cartilage

This chapter chronicles the difficulties in bioprinting any of the three types of cartilage, with emphasis on the articular cartilage, which is so often damaged in our knees and hips. The chapter sets out the disparity between the apparent simplicity of cartilage (no blood vessels, no nerves, and composed of a single, sparse cell type) and the complexity of its zonal architecture (disparate cell structure and orientation in different regions of the same cartilage tissue). The chapter presents examples of the bioprinting of all three cartilage types: articular cartilage (belonging to the hyaline family of cartilage), elastic cartilage (such as is found in our ears), and fibrocartilage (such as the meniscus in our knees). The chapter discusses the promise of decellularized extracellular matrix (which is extracellular matrix—the cells’ immediate environment—with the cells removed) as a bioink.

Introduction

Bioprinting: To Make Ourselves Anew describes how bioprinting emerged from 3D printing and details the accomplishments and challenges in bioprinting tissues of cartilage, skin, bone, muscle, neuromuscular junctions, liver, heart, lung, and kidney. It explains how scientists are attempting to provide these bioprinted tissues with a blood supply and the ability to carry nerve signals so that the tissues might be used for transplantation into persons with diseased or damaged organs. The book presents all the common terms in the bioprinting field and clarifies their meaning using plain language. The reader will learn about bioink—a bioprinting material containing living cells and supportive biomaterials. Additionally, readers will become at ease with concepts such as fugitive inks (sacrificial inks used to make channels for blood flow), extracellular matrices (the biological environment surrounding cells), decellularization (the process of isolating cells from their native environment), hydrogels (water-based substances that can substitute for the extracellular matrix), rheology (the flow properties of a bioink), bioreactors (containers to provide the environment cells need to thrive and multiply). Further vocabulary that will become familiar includes diffusion (passive movement of oxygen and nutrients from regions of high concentration to regions of low concentration), stem cells (cells with the potential to develop into different bodily cell types), progenitor cells (early descendants of stem cells), gene expression (the process by which proteins develop from instructions in our DNA), and growth factors (substances—often proteins—that stimulate cell growth, proliferation, and differentiation). The book contains an extensive glossary for quick reference.

Printing Paradigms

This chapter introduces the invention of 3D printing in the mid-1980s and discusses how bioprinting emerged from that technology some 15 years later, the significantly more formidable requirements that bioprinting presents compared to 3D printing, and the scientists who were the trailblazers of bioprinting. Mention is made of several medical successes of 3D printing prior to the advent of bioprinting. The chapter describes three of the principal bioprinting techniques and the steps that comprise the fundamental process regardless of the specific technique employed. It provides an introduction to some of the basic lexicon of bioprinting—words such as bioinks, bioreactors, extracellular matrices, rheology, and scaffolds. The chapter acquaints the reader with new and novel developments such as the Kenzan method style of bioprinting as well as the innovation of rotational 3D printing and its possible use in bioprinting.