Bioprinting for Tissue Engineering and Regenerative Medicine

Chi-Chun Pan^1,2, Arnaud Bruyas¹, Yunzhi Peter Yang^3,4

¹Departments of Orthopedic Surgery, ²Mechanical Engineering, ³Materials Science and Engineering, ⁴Bioengineering Stanford University, 300 Pasteur Drive, Stanford, CA 94305

Material Matters, 2016, 11.2

ypyang@stanford.edu

Introduction

In the past two decades, tissue engineering and regenerative medicine have become important interdisciplinary fields that span biology, chemistry, engineering, and medicine.^1,2 These new fields promote the healing and restoration of lost function in damaged or diseased tissues and organs by combining scaffolds, cells, and biological signaling molecules to recreate functional biological substitutes and mimic native tissues and functions.³ One objective of tissue engineering and regenerative medicine is the fabrication of viable tissues and organs for transplantation, but with the exceptions of thin skin and avascular cartilage,⁴ limited success in human patients has been achieved due to the complexity of tissue biology. The traditional tissue engineering approach includes loading cells onto a solid porous biomaterial, called a scaffold, in the presence or absence of growth factors that encourage cells to form the desired tissues with biomimetic complexity.⁵ However, the desired result is rarely achieved because the three component mixture does not adequately promote formation of a well-defined spatial distribution of cells, growth factors, and biomaterials at the microscale level that is characteristic of a tissue-like structure. Three-dimensional (3D) printing, also known as additive manufacturing (AM), holds great promise to overcome this limitation in tissue engineering. Because it is a layer-bylayer process, 3D printing enables the formation of complex geometries using multiple materials (Figure 1). 3D printing for tissue engineering has evolved into a new technology, called bioprinting, defined as “the use of material transfer processes for patterning and assembling biologically relevant materials, molecules, cells, tissues, and biodegradable biomaterials with a prescribed organization to accomplish one or more biological functions.”⁶ In particular, bioprinting enables personalizable and precision medicine by engineering anatomically shaped implants with tissue-like complexity using a patient’s own cells. Currently, 3D bioprinting technologies can be classified into two categories: acellular and cellular constructs.⁷ Acellular bioprinting is used to manufacture the scaffold and biomaterial itself in the absence of cells during the printing process. Acellular bioprinting offers higher accuracy and greater shape complexity than cellular constructs because the fabrication conditions are less restrictive than methods that require maintenance of cell viability. For cellular bioprinting, cells and other biological agents are integrated into the material during manufacturing in order to fabricate living tissue constructs. It is clear that the printing parameters, biomaterials, and properties of the 3D-printed constructs are, therefore, different in each category because of the presence or absence of cells and biological substances. Here we briefly introduce and discuss these two approaches based on the suitable materials for these constructs and the fabrication processes used to manufacture them. We also discuss current limitations, potential solutions, and future directions in bioprinting.

Figure 1. Overview of the 3D printing process.

Manufacturing of Acellular Scaffolds

An acellular scaffold consists of a porous structure that mimics the mechanical and biochemical properties of the extracellular matrix (ECM) and provides mechanical integrity as well as a template for cell attachment in order to stimulate tissue formation.⁸ Acellular scaffolds must present biocompatible and bioresorbable properties as well as biochemical, biophysical, biomechanical, bioelectrical, and biomagnetic signals.⁹ Since pores provide room for cell migration and tissue ingrowth, facilitate vasculature formation, and improve cell viability,¹⁰ porosity and porous structures are other key features for the scaffold. Thus, the use of AM is highly beneficial, allowing very accurate and repeatable control of the scaffold geometry (and, thus, porosity) while allowing for the potential assembly of tissue-like spatial complexity. A wide range of applications of bioprinted acellular scaffolds have been reported, such as muscular tissues, liver tissues, cartilage, bone, skin, etc.² The specific material that composes the scaffold and any potential biological agents must be selected to recreate the nature of the engineered tissue. In this section, we focus on constructs with high mechanical strength, typically for engineered bone. The materials and AM processes for acellular scaffolds based on soft engineered tissues (e.g., skin, liver) are similar to the cellladen ones and are, therefore, described in the section “Manufacturing Soft Materials for Cell Encapsulation” later in this article.

Materials

Four categories of materials are highlighted based on their chemical nature. The first category is polymers,¹¹ such as collagen (Product No. C5483, C7624