Extrusion Based Rapid Prototyping Technique An Advanced Platform

Extrusion Based Rapid Prototyping Technique: An Advanced Platform forTissue Engineering Scaffold Fabrication

M. Enamul Hoque, Y. Leng Chuan, Ian PashbyDepartment of Mechanical, Materials and Manufacturing Engineering, University of Nottingham Malaysia Campus,

Jalan Broga, 43500 Semenyih, Selangor Darul Ehsan, Malaysia

Received 14 January 2011; revised 27 May 2011; accepted 23 June 2011

Published online 9 August 2011 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/bip.21701

This article was originally published online as an accepted

preprint. The ‘‘Published Online’’ date corresponds to the

preprint version. You can request a copy of the preprint by

emailing the Biopolymers editorial office at biopolymers@wiley.

com

INTRODUCTION

Tissue Engineering

Tissue Engineering (TE) is an interdisciplinary field

that brings together the principles of life sciences,

medicine, and engineering.1–6 TE is concerned with

the technology that develops artificial tissue/organ

for the improvement or replacement of lost or dam-

aged human tissue/organ.7 This technology makes physiolog-

ical replacement of impaired tissues or organs to patients,

which restores, maintains, or improves the functions of dam-

aged tissues.1 The implantation of engineered biological sub-

stitute should be functional at the time of implantation or

integrate and form functional tissues at a later stage.6

For scaffold-based therapy, TE treatment approach have

been investigated for the reconstruction of large bone defects:

cells are taken from the patient or a donor, cultured in vitro

and seeded in a scaffold. The cell-construct is then implanted

in the defect with the aim to stimulate new bone formation,

ReviewExtrusion Based Rapid Prototyping Technique: An Advanced Platformfor Tissue Engineering Scaffold Fabrication

Correspondence to: M. Enamul Hoque; e-mail: Enamul.Hoque@nottingham.

edu.my

ABSTRACT:

Advances in scaffold design and fabrication technology

have brought the tissue engineering field stepping into a

new era. Conventional techniques used to develop

scaffolds inherit limitations, such as lack of control over

the pore morphology and architecture as well as

reproducibility. Rapid prototyping (RP) technology, a

layer-by-layer additive approach offers a unique

opportunity to build complex 3D architectures

overcoming those limitations that could ultimately be

tailored to cater for patient-specific applications. Using

RP methods, researchers have been able to customize

scaffolds to mimic the biomechanical properties (in terms

of structural integrity, strength, and microenvironment)

of the organ or tissue to be repaired/replaced quite closely.

This article provides intensive description on various

extrusion based scaffold fabrication techniques and

review their potential utility for TE applications. The

extrusion-based technique extrudes the molten polymer

as a thin filament through a nozzle onto a platform layer-

by-layer and thus building 3D scaffold. The technique

allows full control over pore architecture and dimension

in the x- and y- planes. However, the pore height in z-

direction is predetermined by the extruding nozzle

diameter rather than the technique itself. This review

attempts to assess the current state and future prospects of

this technology. # 2011 Wiley Periodicals, Inc.

Biopolymers 97: 83–93, 2012.

Keywords: polymer; rapid prototyping; layer-by-layer;

scaffolds; tissue engineering

VVC 2011 Wiley Periodicals, Inc.

Biopolymers Volume 97 / Number 2 83

thereby repairing the defect. This approach holds promises not

only for bone tissue but also for other organs and tissues.8

The latest TE technology combines cells, scaffolds, and

signaling substances to both physically and biologically

regenerate tissue.8,9 Further research, development, and inte-

gration of tools allow precise and reproducible fabrication of

scaffolds, and also quantitative characterization of the bio-

logical integration of the tissue engineered constructs.10

Results are in substantial progress in the development of

increasingly complex tissue engineered constructs.

TE SCAFFOLDScaffold is a porous structure that assumes the role of a syn-

thetic extracellular matrix (ECM), which permits cell adhe-

sion, proliferation, and differentiation for surgical implanta-

tions.9,11 Fabricated scaffold should mimic the biomechani-

cal properties of the organ or tissue to be replaced.

Therefore, the scaffold should essentially be three-dimen-

sional (3D) to facilitate cell attachment, growth, and neotis-

sue formation.12 Besides, the scaffold being is a suitable car-

rier structure needs to be combined with the cell to hasten

the healing process of damaged tissues.13

Due to all these requirements, there is always a great chal-

lenge associated with the modeling, design, and fabrication of

TE scaffolds to meet various biological and biophysical condi-

tions, for example, designing load bearing scaffolds for bone

and cartilage tissue applications.12,14 Bone and cartilage tissue

scaffolds usually require complex architecture in terms of pore

shape, size, and interconnectivity to provide the needed struc-

tural integrity, strength, transport, and an ideal microenviron-

ment for the growth of cells and tissues in growth.15

So far, limited or even no systematic study has been con-

ducted to assess whether the exact match of mechanical prop-

erties is indeed crucial for optimal tissue regeneration.16 For

instance, since mechanical properties are directly related to the

porosity of porous structures, a stiffer and less porous scaffold

will provide a better integration with the surrounding natural

tissue, or a more deformable and porous one will allow cells to

attach and proliferate in a more efficient way.17 Different TE

applications pose different challenges in the development of

suitable scaffold, for example skin, bone, and cartilage tissues.

Conventionally fabricated scaffolds may not be ideal for skin

TE, because they do not provide the optimal environment for

cell adherence, proliferation, and multiplication.18 Nevertheless,

bone tissue has the capability of self healing/reconstruction

upon injury to some extent. However, when the defect is crit-

ically large it usually remains unhealed, and requires an ideal fil-

ler, such as cadaver bone, coral, hydroxyapatite, or similar min-

eral compound, which consists of tailored structural and me-

chanical properties.19 Likewise, cartilage has poor cell density

and lacks in vascularization therefore, it becomes difficult to de-

velop suitable scaffolds meeting the requirements for cartilage

TE. Scaffolds should also provide mechanical support besides

introducing stimuli to the cells, allowing them to behave as if

they are in their natural in vivo native environment.

SCAFFOLD MATERIALSThere is a broad list of materials that have been currently

used in the fabrication of TE scaffolds. The selection of mate-

rial used in the manufacturing of a TE scaffold depends on

numerous factors including intended application and, scaf-

fold processing technique to be employed.20,21 Materials for

TE scaffolds must be naturally biocompatible, biodegradable,

and essentially be processable into required 3D porous struc-

tures. Among all type of materials, polymers offer great

potential in design and processing of TE scaffolds.22,23 For

example, polymer can be chemically modified to match a

wide range of properties for biomedical applications, such as

mechanical properties, diffusivity, density, hydrophilicity,

etc. Besides, there can be optimal control over specific cellu-

lar interactions with polymeric material, because cells feel

comfort in interacting with proteins that are attached to

some material surfaces, particularly natural polymers.24

Natural polymers offer high degree of biocompatibility

whereby the cells act naturally and thus supports the process

of proliferation and differentiation most.25 Alginate and colla-

gen are among the most popular natural polymers so far used

in scaffold design to construct artificial tissue.26 Other than

natural polymers, poly(ethylene glycol) (PEG), poly(ethylene

oxide) (PEO), polycaprolactone (PCL) and poly-L-lactide acid

(PLLA) are the widely used synthetic polymers in biomedical

field. PEG and PEO are used in the production of hydrogel

scaffolds due to their hydrophilic nature, and their controllable

and reproducible chemistry, which allows modification of par-

ticular properties, such as molecular weight, structure, degra-

dation rate, cross-linking density, mechanical strength, and

stiffness. However, PEG’s major drawback in regard to cell

seeding is that it does not present specific receptors for cell

attachment.26 PCL is another common biocompatible and

biodegradable aliphatic polyester with low melting point and

good solubility in most solvents. This material has been used

in various biomedical devices, such as urethral catheters, drug

delivery systems, resorbable sutures, and has been proposed as

a material for engineering of bone and cartilage.27

The general approaches applied in the TE include cell-

seeded polymeric scaffolds, cell-seeded gels, and cell self-as-

sembly into a cellular matrices.28,29 Among these, cell-seeded

polymeric scaffold approach is the most commonly used

84 Hoque, Chuan, and Pashby

Biopolymers

method for artificial tissue generation, and many consider

this technique as the classic TE approach. It involves 3D po-

rous structure (i.e., scaffold) into or onto which cells are

placed. Then the adhered cells organize themselves into a 3D

assembly having similar characteristics as natural ECM

arrangements and interactions.28,30

However, not all polymeric materials are applicable/suita-

ble for all scaffold fabrication methods. For example, mold-

ing methods are inappropriate for developing hydrogel scaf-

folds. Because, the biomaterial cannot be removed without

damaging both internal and external architecture, which will

most likely cause scaffold breakage.31 Porous hydrogel scaf-

folds are difficult to develop especially, when integration of

tight interconnecting pores is required. There are limited

reports presenting any rapid prototyping (RP) technology

producing scaffolds with consistent pore definition in the

range of 200–400 lm, while retaining a high accuracy of out-

side architecture.7,26

Hydrogel with low viscosity tends to be difficult to use in

constructing scaffolds because of its long gelation time that

results in the collapse of scaffolds due to the mechanical

instability.32 However, recent advances in RP came out with

a new technology called bioplotting, which aims to circum-

vent these inherent problems. This technology allows plot-

ting of various biomaterials into a media bath containing

material of similar flowability, which supports the structure

as it is dispensed, and to initiate cross-linking of the hydro-

gel. By controlling material’s concentration, viscosity, and the

temperature of both the plotting material and the plotting

media, the speed of the hydrogel gelation and cross-linking

in the media bath can be enhanced.7,26,33

Combination of more than one biomaterial provides bet-

ter characteristics than any single type of biomaterial. For

example, the composite consisted of PLLA and b-tricalciumphosphate (TCP) has better combination of properties as a

scaffold material. The biodegradation rate of PLLA is too low

to match the tissue regeneration process after implantation.34

Besides, the acidic degradation products of PLLA, such as

lactic acid tends to cause aseptic inflammation in tissue.35

Unlikely, TCP has a higher biodegradation rate due to its

hydrophilic surface, and its degradation products are also of-

ten alkaline. However, TCP has poor mechanical properties

(e.g., brittle in nature). According to the histological analysis

of the in vivo experiment of PLLA/TCP composite scaffold

manufactured by low temperature deposition manufacturing

(LDM) process (see Figure 1), the scaffold was degraded in

24 weeks after implantation with no trace of aseptic inflam-

mation found. As a scaffold material for bone TE, PLLA/TCP

composite could be a better choice compared to the PLLA or

TCP alone.36

EXTRACELLULAR MATRIXDuring the process of tissue regeneration and repair, cells play

the most important roles due to their extensive proliferation,

cell-to-cell signaling, biomolecular production, formation of

ECM.5,6 Various studies demonstrated that the ECM promotes

key cell signaling pathways, and enables cell proliferation, dif-

ferentiation, and proper cell-to-cell and cell-matrix interac-

tions. The ECM is a vital component of cellular microenviron-

ment, supplying cell and tissue with the appropriate 3D archi-

tecture for normal growth and development. Reproduction of

these condition and structures in vitro would create an oppor-

tunity for rapid and accurate studies of molecular and biologi-

cal events that happen in vivo. Moreover, ECM and a tuned

3D microenvironment are important for any successful clinical

correspondent research of TE, cell based therapy, or stem cell

biology. In vitro cells require sufficient movement of nutrients

with the elimination of metabolites. For this process to be suc-

cessful, high diffusion rates of culture media are required to

enclose the 3D scaffold. High diffusion rates are induced by

smaller distances in the range of 150–400 lm. That is why the

biodegradable scaffolds must incorporate an internal porous

architecture for successful and maximum cell growth. The cells

cultured on a 3D matrix behave almost the same as they

behave in vivo. Three dimensional cell cultures on a biode-

gradable cell scaffold are the basis of TE where the specific cell

can grow and regenerate into a structure similar to tissues or

organs in the living body.37 The proliferation and multiplica-

tion of the transplanted cells are greatly influenced by the

composition and biocompatibility of the biomaterial, and

architecture and the surrounding of the scaffolds. Various 3D

matrices are currently employed for the proliferation and mul-

tiplication of various progenitor cells including adult mesen-

chymal stem cells. However, most of these matrices do not

provide the inherent and unique biological environment where

cells can proliferate and multiply in the same fashion as in

vivo systems.5

Certain criteria were considered necessary for an ideal ma-

trix for cell transplantation. Engineers design biomaterials

with a cellular structure to replace or regenerate tissues in the

body. Natural tubular structures often have a honeycomb-

like or foam-like core supporting a denser outer cylindrical

shell, increasing the resistance of the shell to kinking or local

buckling failure. For example, titanium foam is being consid-

ered as substitute material for trabecular bone.38 Many mate-

rials have a cellular structure, with either a two-dimensional

(2D) array of prismatic cells as in a honeycomb or a 3D array

of polyhedral cells as in foam. Engineering honeycombs and

foams can now be made from nearly all types of materials,

such as polymers, metals, ceramics, glasses, and composites

with pore sizes ranging from nanometers to millimeters.38,39

Extrusion Based Rapid Prototyping Technique 85

Biopolymers

Preferably, honeycomb foam not only provides open cell

porous network, it also has remarkably high specific strength

and stiffness, resulting in a structure with extremely favorable

properties. Honeycomb cellular structure gives rise to a

unique combination of properties, which is exploited in en-

gineering design. Their low weights make them attractive for

structural sandwich panels, their abilities to undergo large

deformations at relatively low stresses make them ideal for

absorbing the energy of impacts, their low thermal conduc-

tivities make them excellent insulators, and their high specific

surface areas make them attractive as substrates for catalysts

in chemical reactions. Honeycombs with their prismatic cells

are referred to as 2D cellular solids, while foams with their

polyhedral cells are 3D cellular solids. The relative density is

the density of the cellular solid divided by that of the solid it

is made from, and is equivalent to the volume fraction of

solid. Foams may be either open (with solid only at the edges

of the polyhedral) or closed (with solid membranes over the

faces of the polyhedral). The properties of the foam depend

on those of the solid making up the cellular material; in

materials such as wood, the cell wall is itself a multilayered

composite material.38,39

It has been reported that the scaffold structures designed

and fabricated with the extrusion based RP techniques were

highly similar to the honeycomb of the bee with its regular

array of identical pores when viewed in the Z-direction of the

fabrication process. Honeycomb-structured scaffold is a suit-

able scaffold for 3D cell cultures, which has enormous poten-

tial in the field of various TE applications. The biodegradable

honeycomb collagen sheet can be cut into suitable thickness

and various sizes depending on the application.5 The

mechanics of honeycombs lends great understanding of how

the 3D scaffolds behave under loading forces.38,40 The main

difference lies in the morphology of the pores; the bee’s hon-

eycomb comprises hexagonal pores surrounded by solid

faces/walls that nest together to fill a plane, whereas the 3D

scaffold structure is built from intercrossing filaments

stacked in horizontal planes and comprises pores surrounded

by solid edges/struts. Even though the pores of the bee’s hon-

eycomb are usually hexagonal in section they can also be in

other polygonal shapes in manmade honeycombs, for exam-

ple triangles and squares.41

SCAFFOLD FABRICATION TECHNIQUES

Limitations of Conventional Techniques

To date, conventional techniques have shown great promise in

scaffold fabrication, and a wide range of techniques and scaf-

fold characteristics have been reported.42,43 Conventional TE

scaffold production techniques include fiber bonding, solvent

casting/particulate leaching, membrane lamination, melt mold-

ing, and gas foaming.44 Generally, the main disadvantage of

these conventional techniques is that they are not able to de-

velop a homogeneous structure. They are also associated with

poor reproducibility, irregularity of pore shape, and insufficient

pore interconnectivity.45 Therefore, these techniques remain

FIGURE 1 Low temperature deposition manufacturing system.36


Biopolymers

impractical to manufacture useful scaffolds as required because

of the following number of limitations20:

Manual Intervention. All conventional techniques involve

multistage manual processes that are labor-intensive and

time consuming. The heavy reliance on user skills and expe-

riences often results in nonuniformity and poor repeatability

of the scaffold architectures and properties.

Reproducibility of Processing Procedures. Conventional

techniques are unable to precisely control the pore size, pore

geometry, and spatial distribution of pores, which results in

inconsistent macrostructure and microstructure of the scaf-

folds. For example, scaffolds produced by solvent casting

and/or particulate leaching cannot guarantee interconnectiv-

ity of pores, because this depends on whether the adjacent

salt particles are in contact.

Use of Toxic Solvents. Most conventional techniques require

extensive use of toxic organic solvents to dissolve the raw

stocks (granules, pellets, or powders) and convert into the

final scaffold. Thereupon, it becomes difficult to remove the

toxic solvents completely from the fabricated scaffolds espe-

cially, in thicker constructs. The residual toxic solvents cause

adverse effects on adherent cells, incorporated biological

active agents, or nearby tissues.46

Use of Porogens. Some techniques (e.g., particulate leaching,

hydrocarbon templating, etc.) use salts or waxes as porogens

to create porosities in the scaffolds. The use of porogens lim-

its the scaffold thickness to �2 mm47 because of the prob-

lems in complete removal of porogens. In addition, it

becomes difficult to prevent the agglomeration of porogen

particles and thus to achieve uniform porogen dispersion.

This phenomenon results in uneven pore size and densities,

and morphologies of the scaffolds, which give rise to anisot-

ropy in scaffold properties.43

Shape Limitation. Some of the techniques use moulds or

containers to manufacture scaffolds as thin membranes or

3D constructs. These techniques are confined to create cer-

tain simple shapes and cannot produce scaffolds with com-

plex and/or desired structural architectures.

Limited Cell Growth. Conventional techniques produce

scaffolds mostly in the form of foams. Cells are then seeded

and expected to grow into scaffold. However, this approach

has resulted in the in vitro growth of tissues with cross-sec-

tions of\500 lm from the external surface.48,49 This is prob-

ably due to the diffusion constrait of the foam, which causes

scarcity in nutrients and oxygen supply, and sufficient re-

moval of waste products.

RAPID PROTOTYPING TECHNOLOGYThe inherent limitations of the conventional methods have led

to use the RP techniques to customize design and fabricate 3D

porous scaffolds with fully interconnected pore networks. RP

technology was first launched in the market in late 1980s with

the introduction of the stereolithography (STL) system by 3D

Systems,50 which is now used almost in all engineering areas.

Presently, RP technique is a powerful tool to fabricate TE scaf-

folds, and this technique is becoming increasingly popular in

TE field. RP technique develops scaffold directly from the

scanned image and the computer model of the defect site to

supply a structurally and mechanically perfect fit scaffolds.51 It

is the process of creating 3D objects through repetitive deposi-

tion of material layers, using computer controlled equipment,

based on the cross-sectional data obtained from slicing a com-

puter aided design (CAD) model of the object.52

As opposed to conventional techniques that involve subtrac-

tion or removal of materials from stock, all RP techniques are

material attachment processes, which employ the same fabrica-

tion mechanism of layer-by-layer additive manufacturing to

produce 3D physical parts from wood, plastics, metals, and

ceramics.53 RP fabrication begins with the development of a

3D volumetric computer model of the desired part that can be

derived from output data generated by surface digitizers or

medical imaging systems, for example, computed tomography

or magnetic resonance imaging. The digital model is then

mathematically sliced into thin layers having a constant thick-

ness that is user-defined. Using RP fabrication, layers of mate-

rial representing the cross-sectional profiles of the desired part

as obtained from the computer-generated slices are formed by

processing solid sheet, liquid, or powder material feedstocks.

The material layers are automatically and precisely stacked and

fused on top of one another to create the desired physical

part.21,54 Furthermore, advances in in vivo imaging, such as

positron emission tomography, make it possible to provide a

confined monitoring of the development and incorporation of

the engineered tissues.55,56

RP techniques having full control over porosity, pore size,

pore shape, and permeability, are able to produce scaffolds

with fully interconnected pore structure. Besides, RP techni-

ques also allow the investigation of the effect of scaffold geom-

etry on cell behavior for further optimization of the scaffold

design.57 This method is particularly useful for TE, since it

allows a very good reproducibility and the production of

almost any kind of structure within the limitations of each

technique used. It is possible to design a structure that mimics

the natural tissue to be replaced.58 It is here where RP offers


Biopolymers

possibilities to compromise such different requirements into

one scaffold, because it adds freedom of varying structural pa-

rameters to the nonvariable bulk mechanical properties of the

material used.16,35 Currently, a number of RP techniques have

been exploited for TE scaffold fabrication that include STL,

selective laser sintering, fused deposition modeling (FDM),

electron beam melting and 3D printing. These RP systems

cover a range of processing technologies such as laser, print, as-

sembly, extrusion, etc.59–61 However, this article focuses on the

systems based on extrusion technology.

Systems Based on Extrusion Technology

Among the available RP techniques, extrusion based techni-

ques are the most advanced scaffold fabrication technique

due to their ability of using different biomaterials, their pos-

sibility of manufacturing scaffolds in a cell-friendly environ-

ment, and their feasibility of controlled drop-on demand

high precision deposition.62 In the material science literature,

another term for extrusion-based systems is ‘‘direct-write’’

technique.51 Lewis and coworkers63 define direct writing

techniques as those that rely on the formulation of colloidal

inks for a given deposition scheme. The techniques employed

in direct writing can perform extrusion of strands/filaments

and/or plotting of dots in 3D.64 The basic principle of extru-

sion-based fabrication technologies involves the extrusion of

a melt to build a 3D scaffold through a jet or nozzle in a lay-

ered fashion guided by the computer model (see Figure 2).16

A computer model (volume model) is sliced into a number

of layers by CAD software and each layer of the original

model is produced in succession (see Figure 3).65 These tech-

niques include multiphase jet solidification (MJS), 3D plot-

ting, precise extrusion manufacturing (PEM), FDM, etc. A

variety of biomaterials can be used to fabricate scaffolds

based on the type of machine used.

Multiphase Jet Solidification (MJS). MJS process was devel-

oped by a Germany’s leading organization of applied research

‘‘The Fraunhofer-Gessellschaft.’’ The MJS process is able to

produce metallic or ceramic parts. It uses low-melting point

alloys or a powder-binder mixture, which is squeezed out

through a computer-controlled nozzle to build the part

layer-by-layer. The main components of the apparatus used

for the MJS process comprise of a personal computer, a com-

puter-controlled positioning system, a heated chamber with

a jet and a hauling system. The material is supplied as pow-

der, pellet, or bar. The extrusion temperature of the molten

FIGURE 2 Overview of rapid prototyping (RP) technology that uses layer-by-layer build-up process.16


Biopolymers

material can reach up to 2008C. Extrusion orifices vary from

0.5 to 2.0 mm.

The MJS process is usually used to produce high density

metallic and ceramic parts. A feedstock consists of powder-

binder-mixture is heated in a process chamber above the

melting point of the binder, and thus only the binder is

liquefied during the process. A piston squeezes out the low

viscous mixture through an x-y-z controlled jet. The feed

rate of the piston controls the material flow. The material is

deposited layer-by-layer by moving the jet and solidifies

when touches the base platform or the previous layer due to

temperature decrease. Following the aim to build up metallic

and ceramic parts, most investigations were performed using

powder-binder-mixtures of stainless steel. However, for med-

ical applications the constraints and aims of using MJS are

quite different. In this case, the modeling material used is a

polymeric material instead of powder-binder-mixture and

supplied as powders, pellets, or bars.

Koch et al.66 studied the use of MJS process to build 3D

hollow scaffolds made of poly(D, L-lactide) for bone and car-

tilage TE. The scaffold pore size was found to be in the range

of 300–400 lm and the structure supported ingrowth of

human bone tissues. However, there was no report on the

detailed scaffold morphology using microscope analyses or

on any mechanical study of the scaffold properties. Calvert

and Crockett 67 developed an in-house extrusion-based RP

technique to manufacture scaffolds. They built scaffolds with

typical layer heights of 0.2–1.0 mm and a resolution of �0.5

mm. Xiong et al.36 built an RP technique called PEM to fab-

ricate 3D scaffolds. They manufactured and tested porous

scaffolds with PLLA and TCP for bone TE (see Figure 4).

Three-Dimensional Plotting. This system was developed by

the researchers at the University of Freiburg68 and was

termed as ‘‘bioplotter.’’ This technique involves a moving ex-

truder head (x-y-z control) and uses compressed air to force

out a liquid or paste-like plotting medium. The process gen-

erates an object by building micro strands or dots in a layered

fashion. Unlike 3D bioplotter, 3D printing (3DP) build scaf-

folds layer-by-layer by bonding starch particles together fol-

lowed by infiltration and partial cross linking of starch with

lysine ethyl ester diisocyanate. The 3D bioplotting allows 3D

dispensing and reactive processing of oligoether urethanes

derived from isophorone diisocyanate, oligoethylene oxide,

and glycerol.

Landers et al.69 compared 3DP and 3D bioplotting in

manufacturing biodegradable polyurethane scaffolds using

aliphatic polyurethanes based on lysine ethyl ester diisocya-

nate and isophorone diisocyanate. Depending on the type of

dispenser head, a variety of materials can be used to build

scaffolds. Landers and Mulhaupt Rolf68 have made use of a

wide variety of polymer hot melts as well as pastes, solutions

and dispersions of polymers and reactive oligomers. The bio-

plotter built in house at the Freiburg Materials Research Cen-

ter, Albert Ludwigs University, Freiburg, Germany was

employed to fabricate scaffolds for TE applications, which

investigated the versatility of this technique.70,71

Geng et al.51 present a technique based on 3DP, referred

to as dual dispensing for the design and fabrication of scaf-

folds (see Figure 5). The dual dispensing method excrete the

high sensitivity to material concentration compared with the

method of dispensing plotting materials into a fluid medium,

as the precipitation occurs when the dispensing material and

the coagulant medium merge on the base or on the previous

layer. There is then no precipitated lump forming at the noz-

zle and no movement of the fluid medium to affect the shape

of the precipitated strands of the scaffold. The chitosan scaf-

folds built by this dual dispensing method exhibits excellent

FIGURE 3 CAD images of generated scaffold assemblies (top) and their fabricated counterparts (bottom).65


Biopolymers

uniformity, sufficient strength, good reproducibility, and cal-

ibration. The qualities of the surface, the edges of scaffolds,

and uniformity of the top layer are still well maintained.

Moreover, greater flexibility can be achieved with different

dispensers to suit the nature of the fluid to be dispensed.

Ang et al.69,72 fabricated chitosan scaffolds layer-by-layer

with a pre-programmed lay-down pattern (see Figure 6).

Scaffolds fabricated show good attachment between

layers allowed the chitosan matrix to form fully intercon-

nected channel architecture. The results of this preliminary

study using the RP robotic dispensing (RPBOD) system

demonstrated its potential in fabricating 3D scaffolds with

regular and reproducible macropore architecture. Besides,

results of in vitro cell culture studies revealed the scaffolds’

biocompatibility.

Xiong et al.36 developed another wise RP technique, called

low temperature deposition manufacturing (LDM) system

that processes slurry material into frozen scaffold under low

temperature (e.g., 08C). The research team fabricated the

PLLA/TCP composite scaffolds for bone TE. First, the mate-

rial slurry was fed into the material hopper with a soft pipe

connected to a screw pump nozzle. Then the computer con-

trolled the nozzle to move in the X-Y plane to extrude the

material slurry out and deposited it onto the platform in the

area defined by the digital models. The layer of deposited

materials was frozen on the platform that built the scaffold.

In comparison to other depostion methods, the LDM process

can better preserve bioactivities of scaffold materials because

of its nonheating liquefying processing of materials.

Fused Deposition Modeling. FDM is another prominent

extrusion-based RP process. The FDM technique was first

invented by Crump SS of Stratasys Inc73, to develop 3D

objects, and further improved by Comb et al.74 FDM system

is capable of using multiple build materials in a build/sup-

port relationship. The FDM technique requires preformed

filaments with specific size and material properties to build

scaffold. The filament is heated in a liquifier to a semi-liquid

state and pump through a nozzle directly onto the build plat-

form following a programmed model. The incoming fila-

ment acts as a piston to extrude the semi-molten material

along a thin orifice and deposit it through a nozzle tip onto

the platform below. FDM has extended its applications to

medical modeling due to its low cost process and the use of

cheap materials. The feasibility of manufacturing bioresorb-

able and ceramic implants make FDM a suitable process for

producing implants at low cost. To date, FDM technique has

FIGURE 4 Precision extrusion deposition (PED) system that pro-

vides better mixing of polymers and thus avoids possible air bubble

formation.36

FIGURE 5 Dual dispensing system (left) and top views of chitosan scaffolds fabricated by this system (right).51


Biopolymers

shown promising success in the fabrication of 3D scaffolds in

terms of pore reproducibility and interconnectivity.

An interdisciplinary group75 has studied and patented the

parameters to process PCL and several composites (PCL/HA,

PCL/TCP ets.) by FDM technique. For more than 3 years,

this first generation of scaffolds (PCL) has been studied with,

and without cells in a clinical setting. Employing FDM tech-

nique, the second generation of scaffolds for bone TE was

fabricated from polymer and CaP composites. These scaf-

folds, as certained, favorable mechanical and biochemical

properties (degradation and resorption kinetics). The

strength is conferred via ceramic phase whereas; toughness

and plasticity are attained via polymer phase. In addition,

these scaffolds offer improved cell seeding, and enhanced

incorporation and immobilization of growth factors. How-

ever, the commercially available FDM system imposes some

constraints on TE scaffold fabrication. The nature and oper-

ating limits of the FDM system reduce the choice of scaffold-

ing materials to thermoplastics and to preform filament.

The major contribution of the further developed FDM sys-

tems is that the techniques apply the same modeling principle as

FDM but employ different driving forces like, the external gas

pressure or the pressure created by turning screw to extrude the

polymer melt. Therefore, the scaffolding material can be directly

melted and extruded without filament preparation, which

greatly widens the material processing windows. However, dif-

ferent FDM systems have inherent advantages and limitations.

For example, the system that extrudes the polymer melt by

external gas pressure is easy to operate and maintain cleanliness.

However, there is a possibility that the air bubbles may remain

entrapped in the polymer melt (if any) because of lack of appro-

priate mixing, which may cause imperfect filament deposition.

In the case, maintaining a bit higher super heat in the polymer

melts and holding for a while may facilitate easy escape of the

air bubbles. However, the system that extrudes the polymer melt

using the pressure created by turning screw is tedious to main-

tain the cleanliness, and without appropriate cleaning of the sys-

tem, the scaffold might be contaminated. However, the advant-

age is that the polymer melt is mixed thoroughly and thus avoid

the possibility of air bubbles entrapment.

Woodfield et al.,76 developed 3D fiber deposition (3DF)-

bioplotter and tested for TE scaffold fabrication, considering

a system for the extrusion of highly viscous polymer. They

investigated the poly(ethylene glycol)-terephthalate-poly(bu-

tylene terephthalate) (PEGT/PBT) block-copolymers and

polyethyleneoxide-terephtalate/polybutylene-terephtelate

(PEOT/PBT) to process into 3D scaffolds that can modulate

their viscoelastic properties to accomplish mechanical

requirements for tailored tissue engineered applications. The

3DF-bioplotter device was able to fabricate structures with

high reproducibility and flexibility, and it offered a wide vari-

ety of solutions in terms of different architectural and geo-

metrical configurations. The 3DP method generally involves

low cost with minimal specialized equipment. However, reso-

lution is the primary limiting factor, which is determined by

the size of dispensing tip.

Wang et al.77 developed a variation of FDM process called

precision extruding deposition (PED) system to fabricate

FIGURE 6 Rapid prototyping robotic dispensing (RPBOD) system and SEM images showing the top view of the chitosan-40% HA scaffold.69


Biopolymers

interconnected 3D scaffolds at Drexel University. The major

difference between PED and conventional FDM is that in the

PED system the scaffolding material can be directly extruded

without filament preparation. PED system fused pellet-

formed PCL by two electrical band heaters connected with

two respective thermo-couples. Then the PCL melt was

extruded by the pressure created by a tuning precision screw

to build the scaffolds layer-by-layer. The scaffold morphol-

ogy, internal microarchitecture, mechanical properties, and

biocompatibility of the as-fabricated scaffolds were eval-

uated. The test results reported the structural integrity, con-

trolled pore size (250 lm), pore interconnectivity, a favorable

mechanical property and basic biocompatibility of the PED-

fabricated PCL scaffolds.

Shor78 proposed a new system called mini-extruder

mounted on a high-precision positioning system deliveries the

PCL or PCL-HA in a fused form through the deposition noz-

zle. Similar to mini-extruder, Hoque et al.79 developed a desk-

top robot based rapid prototyping (DRBRP) system, which is

able to fabricate 3D scaffolds with virtually any thermoplastic

synthetic polymers of any form (e.g., pellet, lump, powder,

etc.). The mini-extruder and DRBRP systems can be used with

bulk material in any form (e.g., granule, pellet, powder etc),

which can be avoided in most of the material preparation

steps in a filament-based system. This configuration opens up

the opportunity for the use of a wider range of materials, mak-

ing the system more versatile and realizable alternative manu-

facturing process for composite scaffold materials. The as fab-

ricated scaffolds with DRBRP system were tested for their bio-

compatibility using rabbit smooth muscle cells that showed

excellent cell adhesion and tissue formation.79

CONCLUSIONSScaffolds built by polymeric deposition and/or extrusion

based RP techniques, always showed good attachment with

regular and reproducible scaffold architecture. The dispens-

ing process has certain advantages, such as flexibility in sup-

port for the scaffold as it forms. However, the main disad-

vantage of this method is the high sensitivity of materials

concentration/viscosity on the nozzle tip. Furthermore, due

to frequent filament distortion during the extrusion of mate-

rial causes failure of the process, therefore limiting choices of

material used. Consequently, this problem prevents an auto-

matic and continuous process reducing the main advantage

of a filament based scaffolding system.

The existing CAD-based scaffolds are based on cubic latti-

ces with straight edges and sharp turns or those derived from

Boolean intersections of geometric primitives, such as

spheres and cylinders. Neither of these partitions provides a

biomorphic environment suitable for cell attachment, migra-

tion and proliferation. The aggregates of cells, foam/ECM,

typically have cells separated by curved partitions. The bio-

morphic geometry that best mimics this structural configura-

tion would be surfaces that are continuous through space

and divided into two (pore and nonpore) not necessarily

equal by a nonintersecting two-sided surface.

The biocompatibilities of various polymers have not been

conclusively proved till now. In spite of today’s availability of

numerous of biodegradable materials, usually no single ma-

terial is able to provide the scaffold with all the required

properties for TE applications. Hence, the major challenge in

developing a scaffold lies primarily in the choice of a suitable

single or blend of biomaterials with the correct combination

of properties. The field is still widely open to design the most

appropriate polymer scaffold with all the vital considerations

and properties for effective in vivo applications.

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Extrusion Based Rapid Prototyping Technique An Advanced Platform

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