Elsevier

Acta Biomaterialia

Volume 56, 1 July 2017, Pages 3-13
Acta Biomaterialia

Review article
3D printing for the design and fabrication of polymer-based gradient scaffolds

https://doi.org/10.1016/j.actbio.2017.03.030Get rights and content

Abstract

To accurately mimic the native tissue environment, tissue engineered scaffolds often need to have a highly controlled and varied display of three-dimensional (3D) architecture and geometrical cues. Additive manufacturing in tissue engineering has made possible the development of complex scaffolds that mimic the native tissue architectures. As such, architectural details that were previously unattainable or irreproducible can now be incorporated in an ordered and organized approach, further advancing the structural and chemical cues delivered to cells interacting with the scaffold. This control over the environment has given engineers the ability to unlock cellular machinery that is highly dependent upon the intricate heterogeneous environment of native tissue. Recent research into the incorporation of physical and chemical gradients within scaffolds indicates that integrating these features improves the function of a tissue engineered construct. This review covers recent advances on techniques to incorporate gradients into polymer scaffolds through additive manufacturing and evaluate the success of these techniques. As covered here, to best replicate different tissue types, one must be cognizant of the vastly different types of manufacturing techniques available to create these gradient scaffolds. We review the various types of additive manufacturing techniques that can be leveraged to fabricate scaffolds with heterogeneous properties and discuss methods to successfully characterize them.

Statement of significance

Additive manufacturing techniques have given tissue engineers the ability to precisely recapitulate the native architecture present within tissue. In addition, these techniques can be leveraged to create scaffolds with both physical and chemical gradients. This work offers insight into several techniques that can be used to generate graded scaffolds, depending on the desired gradient. Furthermore, it outlines methods to determine if the designed gradient was achieved. This review will help to condense the abundance of information that has been published on the creation and characterization of gradient scaffolds and to provide a single review discussing both methods for manufacturing gradient scaffolds and evaluating the establishment of a gradient.

Introduction

Consideration of a tissue-engineered scaffold’s architecture on the macro-, micro- and nanoscale is crucial for proper nutrient and waste transport, cellular interactions, mechanical stability, and ultimately functional tissue formation [1]. Despite developments in material selection and scaffold design, it can still be difficult to achieve tissue biomimicry in these scaffolds, which significantly affects the resulting tissue regeneration [2]. As observed by Zhang et al., a scaffold carefully designed to mimic the architecture of the extracellular matrix (ECM) of native tissue can be expected to induce and direct cells to develop toward functional tissue [3]. In addition to the importance of material selection, inclusion of proper biochemical or biophysical stimuli could contribute to appropriate development. ECM can have specific spatial arrangement or be haphazardly organized depending on the type of tissue. Specific properties of the scaffold, if made to carefully mimic the native tissue, contribute to functional tissue formation. For example, scaffolds with uniform pore size and porosity similar to that of natural bone can lead to bone tissue formation [4]. Similarly, electrospun scaffolds with areas of poly(lactic-co-glycolic acid) (PLGA) fibers in an aligned or random orientation, designed to mimic collagen type I fibril orientation found in tendons, were created and seeded with fibroblasts [5]. Cells on aligned fibers elongated along the direction of the fibers and produced collagen type 1 in an organized fashion, as seen in native tendon fibroblasts. This approach, biomimicry of a component of the native environment, could be improved as research moves past biomaterial scaffolds with homogenously distributed composition or structural properties [6]. Native tissue often exists as a series of connected and graded transitional zones to build distinct functional regions. The lack of heterogeneity within previous scaffolds does not support the diverse populations of cells and surrounding environment of the native tissue. Thus, it is critical that the engineer incorporates graded properties within scaffolds to properly guide the development of new tissue. The advent of 3D printing and additive manufacturing has enabled the design of tissue scaffolds with precise designs incorporating graded properties that support the heterogeneous population of cells and matrix components that will eventually populate them.

3D printing, also referred to as additive manufacturing, rapid prototyping, or solid-freeform technology, emerged in a few fields in the mid 1980 s, and today has found industrial applications in the automotive, aerospace, construction, and even the cosmetic industry [7]. This technology has swept through the commercial world, transforming established practices and disrupting manufacturing techniques much like an industrial revolution, and has contributed to numerous biomedical applications within the research setting [8]. Specifically in the field of tissue engineering, 3D printing has been applied to virtually all tissues in the body and has even been utilized to fabricate whole organs [9].

Before the development of additive manufacturing techniques, tissue engineers used traditional scaffold fabrication techniques such as particulate leaching, electrospinning, phase separation, and gas foaming to achieve architectural variety in fabricated scaffolds [10]. While each of these methods have been extensively studied and optimized, they have inherent limitations. For example, these conventional methods are incapable of precisely controlling pore geometry, interconnectivity, and pore size, and especially of creating regions of variance within a single scaffold [11]. In addition, several of these techniques are contingent upon using organic solvents with inherent biocompatibility [12]. Additive manufacturing techniques allow the tissue engineers to circumvent these limitations through precise control over the design of the scaffold. This results in greater reproducibility, higher level details, and even patient-specific constructs [13].

This review focuses on additive manufacturing techniques that are being used to fabricate polymer-based, gradient scaffolds for tissue engineering. While these technologies are already being used to fabricate homogenous and patient-specific scaffolds, the true utility of these technologies is the level of architectural engineering they bring to the field of tissue engineering.

Section snippets

3D printing techniques

There is a vast assortment of 3D printing techniques developed and applied to the field of tissue engineering, with different approaches to reproducing a computer generated model. Within the constraints of the manufacturing method, the engineer can design the external and internal architecture that is to be built utilizing computer-aided design (CAD) software, informed through mathematical equations or values derived from clinical data [14]. CAD-based methods are the most widely used in the

Incorporation of gradients

As described, the human body is a complex multiphase system built from a dynamic interaction between 1) cells, 2) extracellular matrix, and 3) the resulting tissue architecture. In certain tissues, this system creates a microstructure that is highly graded in organization, chemical gradients, and mechanical properties [35]. These gradients contribute to critical processes such as embryogenesis, chemotaxis of polymorphonuclear leukocytes, and cell migration [36], [37], [38]. Additionally,

Evaluation of the achieved gradients

As described, gradient scaffolds are designed to meet a huge set of applications in tissue engineering. Looking collectively at the set described here, there are several analytical tools that are commonly used to investigate whether a gradient was achieved. Depending on the form of gradient, either biochemical or physical characteristic, different tools can provide information.

Challenges and perspectives of 3D printing for gradient scaffolds

The advent of additive manufacturing techniques within the field of tissue engineering has aided in the development of sophisticated biomaterials that better recapitulate the native tissue architecture. The complex heterogeneous microarchitecture of tissue plays a significant role in the differentiation and recruitment of diverse populations of cells. Thus, it is unsurprising that for successful tissue integration one would need to optimize the scaffold architecture to mimic this organized and

Acknowledgements

We acknowledge support by the National Institutes of Health (R01 AR068073, and the Armed Forces Institute of Regenerative Medicine (W81XWH-14-2-0004) for work in tissue engineering (A.G.M.). J.P.F. acknowledges support from the Maryland Stem Cell Research Fund (Grant # 4300811), and L.G.B. is supported by the National Heart, Lung, And Blood Institute of the National Institutes of Health (Award Number F31 HL132541). We also acknowledge support from the Sheikh Zayed Institute for Pediatric

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    Part of the Gradients in Biomaterials Special Issue, edited by Professors Brendan Harley and Helen Lu.

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