The 3D Powder Printing

Project for Tissue Simulation:

A Review and Analysis

Abstract: The PWDR printer is an open-source powder-based three-

dimensional (3D) printer. The goal of the project was to use this

printer to research potential materials to use in a 3D printing process

to create flexible objects. Flexible and realistic models could then be

used to simulate soft human tissue such as skin. A review and analysis

of the work done by the original design team and by the author is

carried out. Both aspects of the project, the printing technology and

the materials science, are discussed. Future work and additions to the

project are discussed and explored. A basic procedure to operate the

PWDR printer is also enclosed.

Aaron Najfach

August 2014

Miami University

Dedication

Thank you to Dr. Jessica Sparks for the opportunity to work on this

project. Both your academic and financial support was much appreciated.

Table of Contents

Introduction…………………………………………………………………………………………………………………………………...1

Printing Technology………………………………………………………………………………………………………………………..1

Desired Attributes…………………………………………………………………………………………………….……….1

State of the PWDR printer……………………………………………………………………………………….………..2

Future Considerations……………………………………………………………………………………………….………3

Materials Science…………………………………………………...………………………………………………………………………3

Prior State of the Materials Science…………….…………………………………………………………………….3

Current State of the Materials Science………….…………………………………………………………………..4

Material Selection and Initial Samples….……………………………………………………………..4

Starch Gelatinization and Retrogradation…….……………………………………………………..5

Pregelatinization…………………………………………….……………………………………………………5

Acid Modification…………………………………………………………………………………….………….6

Future of Materials Science……………………………………………………………………………………….……..7

Starch with Silicon Infusion………………………………………………………………………….……..7

Cross-Linking of Starch………………………………………………………………………………….…….7

Starch with Fatty Acids……………………………………………………………………………………….8

Starch with Salts…………………………………………………………………………………………………8

Starch with Cellulose Derivatives………………………………………………………………………..8

Use of Gelatin as Main Substance……………………………………………………………………….9

Current Operating Procedure for PWDR 3D Printer……………………………………………………………………..10

Converting the Model and Loading to SD Card……………………………………………..………………….10

Uploading Arduino Firmware……………………………………………………………………………..…………….10

Starting a Print Job…………………………………………………………………………………………………………….10

Finishing and Clean-Up…………………………………………..............………………………………………………11

Acid-Hydrolyzed Gel Test Results…………………………………………………………………………………………..……….12

References, Articles, and Academic Papers……………………………………………………………………………..……..13

Miscellaneous Resources………………………………………………………………………………………………….……………..15

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Introduction

3D printing is fast growing technology with application in a variety of fields [3]. One area of interest is applications

in biomedical technology. A goal within the field is the rapid reproduction of body parts. The idea is that lost or

severely damaged body parts can be replaced with 3D printing using biologically compatible materials [1]. These

materials and model body parts can simulate the properties of the natural tissue. Often, a 3D scaffold is produced,

at which point cells may be introduced [2].

The goals of this project have a similar goal in mind. The desired outcome is that 3D printing can be used to

replicate soft tissue. What makes this goal different is that virtually all 3D printing will produce only a rigid

structure. A softer, flexible structure is more complicated. Furthermore, anything printed that is in fact soft or

flexible is likely not produced using a powder method of 3d printing. It is the goal of this project to explore 3D

powder printing technology and to explore materials that could potentially produce flexible models which simulate

soft human tissue, notably skin tissue.

A 3D powder printing process works in a similar manner as any other type of 3D printing process. A layer of

powder is deposited. A print head then deposits some form of a binder which causes the powder material to bind

together. Another layer of powder is deposited on top of the first layer, more binder is deposited, and so on. As

each layer is deposited, the structure is built higher and higher producing a three dimensional product [4].

Currently, powder printing has little to offer in terms of flexible products. Furthermore, powder printing is

currently a fairly expensive process. It is the goal of this project to explore new materials with a low-cost powder

machine. The project can be viewed as having two aspects: Printing Technology and Materials Science. The current

progress and future of each aspect is discussed below.

Printing Technology

Desired Attributes

Since the overarching goal of the project is to produce a flexible model with a powder-based 3D printer, the printer

itself needs to have certain qualities. Over the course of research, a wide variety of materials may need to be

tested. The effects of these materials could potentially cause harm to the machine. To deal with this risk, the ideal

printer needs to be low cost and adaptable. Many 3D powder printers on the market are very specialized and can

cost thousands of dollars (or even tens of thousands). Replacing damaged parts could be costly and the machine

design may limit the modifications possible. A particular printer may only be specified for a small range of

materials. Therefore, it would not be wise to test new powders and binders on such a machine.

It is these requirements that makes the PWDR printer an attractive choice. PWDR is an open source design from a

student team from the University of Twente in the Netherlands made without any highly specialized parts or

materials [II]. The open-source design allows for any possible modifications to both the hardware and software,

allowing the maximum level of adaptability. The low cost of materials means assembly of the printer is much more

affordable and can be easily replaced. For instance, the printer head uses a standard inkjet cartridge to deposit a

binder. An inkjet cartridge is a common item found in office supply stores. This allows for a variety of binders to be

tested in the printer with much more room for error. If a potential binder damages the cartridge, it is easy to

replace for a relatively low cost.

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The PWDR 3D printer. Photo: https://pwdr.github.io/ [II]

State of the PWDR printer

Throughout the 2013-2014 academic year, the original PWDR team of Corey Bush, Kelly Oswald, Maria Fedyszyn,

and Katie Jonas purchased parts and assembled the printer [I]. This process included assembling the frame,

installing motors, belts, and shafts, and wiring the components. Unfortunately, at the end of the academic year,

the PWDR printer was not fully operational [I]. Problems included binder not being depositing properly, the printer

head not moving over the same spot as needed, and the roller used to deposit new powder layers not moving

properly. The printer head was not depositing binder in the proper shape. Often only a random splatter would be

deposited. The printer head would attempt to deposit binder outside of the build bin workspace. The roller which

is meant to deposit new layers of powder onto the build bin would move in a ‘backwards’ fashion, moving in a way

as if the build bin and powder storage bin were switched i.e powder bin on the left toward the electronics and

build bin on right.

Over the following summer, some improvement was made. With a change in the wiring for the motor controlling

the y-axis motion, the motor orientation was switched, allowing the roller to move in the proper direction – left to

right, powder bin the build bin – when depositing powder. Rewiring the printer head allowed for a more consistent

binder patter to be deposited. Instead of a seemingly random splatter, the printer head consistently deposited two

short dashes. Although the reason is not completely clear, the printer head also started to deposit the ink over the

same area consistently rather than trying to print of the build area. By ignoring the ‘Initialize position’ button in the

PWDR Interface used to control the printer and jogging the print head to the center of the build space, one can

take advantage of this fact. The print head will return to whatever position it started at after the roller moves to

deposit a new layer of powder. (It should be noted that no powder has been tested with the printer up to this

point. References to depositing powder refer simply to the motion of the roller. It’s implying what the printer

would do if powder was present.)

One possible reason the PWDR printer is not yet fully operational is the fact that there are too many resources

posted on the PWDR website and the associated Github pages. Since the printer is an open-source design, several

other builder have contributed changes, corrections, and improvements to the printer. Although there is an

original schematic and original code available, there are also several circuit diagrams and several versions of the

Arduino code throughout the site [II]. It is not necessarily clear which diagram and which version of the code is

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best. This creates difficulties in troubleshooting as it is not clear if the current problems with the printer are due to

the code, the wiring scheme, or perhaps a change in the operating procedure.

A procedure for operating can be found on page 10.

Future Considerations

To improve the PWDR printer and to get the printer fully operational, consulting with an Electrical Engineering

major, Computer Science major, or someone with a high knowledge of computing will likely be necessary. Ideally,

it would be such a person’s expertise that would aid in identifying where problems lie and how to correct those

problems. It would also be potentially beneficial to continue contact with one of the original designer of the

printer, Alex Budding.

Another thing to consider is use of an entirely different printer. While there are few 3D printers that meet the

needed criteria, there is one that could soon be available. A group out of San Diego, under the name Creo

Machines, has been developing their Sandbox 3D printer [III, IV]. At this point, it appears this printer is very similar

to the PWDR printer. It is claimed that the Sandbox is low-cost, uses an inkjet cartridge, is highly adaptable, and

will be open source. There is also the claim of additional functions, such as Wi-Fi adaptability. The hope is that this

printer will be available in an already assembled form and not as a build-it-yourself concept. This would avoid the

same potential problems that PWDR has had.

Regular checks for Sandbox updates should be conducted. There are websites for Creo Machines and the Sandbox

printer which have a link for a newsletter sign up. Both sites have a News/Blog page where updates may be

posted. The only problems so far with updates is that the last update has not been for several months, almost a

year. It is unknown if the group is still actively working on the Sandbox, if there will be an upcoming release date,

etc.

Once a basic process is established, additional functionality of the PWDR printer and the 3D printing processes may

be explored. In order to enhance the realism of any simulated tissue, the addition of pigment to the material

would be a likely next step. The ability to control the density of a printed material would also be beneficial.

Furthermore, the ability to take MRI scan or CT scan data and convert the data into a printable format would allow

for the personalization of simulated tissue models produced with the 3D printer. Many of these process additions

would likely require the expertise of computer science students, software engineering students, or electrical

engineering students. If not, examining published works a given topic, such as the work out of the University of

Sheffield, would be beneficial [5]. It is also possible that tools and programs currently exist and are available for

purchase would be able to provide these desired functions.

Materials Science

Prior State of the Materials Science

When members of the original PWDR team were not working on the printer, members were exploring materials to

use with the printer as well as any pre-processing or post-processing steps [I]. The main set of materials the team

explored was based on another paper that involved 3D printing and biocompatible materials [2]. A powder mixture

of gelatin, starch, and dextrose (rather than dextran like the referenced paper) was used to create flat, cylindrical

structure by hand. [I]. A layer of powder was deposited, then water was applied. This process was repeated for

several layers. The cylindrical samples were then subject to soaking solutions as based on the same paper. The

soaking solutions used 2% concentrations of Poly(d,l-lactide), polycaprolactone, and Poly(l-lactide).

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The problem with these materials was that they did not produce results relevant to the goal of flexible materials.

The resulting structures were structures that were primarily rigid structures [2]. It is unclear why the PWDR group

chose to pursue research of these materials and their use. These materials could have potential benefits instead in

demonstrating powder 3D printing as a whole or in the examination of the structural integrity of various shapes

and models printed. Since the team chose to explore those materials, it become necessary to explore alternative

materials.

Current State of the Materials Science

Material Selection and Initial Samples

Since the time of the work of the original PWDR group, there has been significant advancement and direction with

the materials aspect of the project. Through the summer of 2014, much research and work has been conducted

with starch. Starch from maize (corn) was chosen as a primary material for two main reasons. The first reason was

the fact that there was a significant amount of maize starch left over from the original PWDR group. The second

reason was that a common item that exhibited the desire material properties used starch as a primary ingredient –

Play-Doh While Play-Doh may be considered a children’s toy, it is still representative of a desired outcome. It’s

flexible and retains its shape well. As an added advantage, since starch is a biocompatible material [6, 9], any

formulations used in the printer would likely be safe to incorporate into biological systems if the situation required

it.

The testing of powder samples was done in a similar manner as the procedure carried out by the original PWDR

group. A thin layer of powder would be deposited by hand into either an aluminum weight boat or aluminum

cupcake liner. This simulated the printer depositing powder. The layer may then have been patted down for an

even layer. Next, enough water would be introduced to sufficiently moisten the powder layer. Initially, the water

was deposited with a syringe. Later, in order to better simulate the effects of the printer head, a common ironing

spray bottle was used. Finally, a new layer of dry powder would be deposited onto the moist layer. This process

would be repeated for 2-3 layers.

So far, the most promising powder material to be used with the printer is a combination of starch powder and

gelatin powder with water used as the binder. When a mix of 40-60% cornstarch with gelatin was subject to water,

it was found that samples had several desired properties. Qualitatively, there was a moderate degree of flexibility,

cohesiveness, and compressive elasticity. Tensile strength was relatively poor however; but despite this, the

samples showed significant promise in simulating a soft, biological tissue.

One other aspect of this sample that is significant is that the binder used was cold water. Technically, starch and

gelatin need to be subject to hot water to enact their gelling properties [6, 8, 9, 10, 11] (see later section on Starch

gelatinization). While the samples could potentially be stronger with the use of hot water, use of cold water still

had a positive effect. The printer head currently does not have the ability to heat binder material. While a heated

binder could be inserted into the printer cartridge, it would be difficult to maintain the binder at a constant

temperature over the course of an entire print cycle.

One theory as to why this combination works well is that the properties of gelatin and starch work well together.

Gelatin by itself, when subject to cold water, would still form a spongy, somewhat cohesive material. Starch, when

subject to cold water, will form a rigid (although very brittle) structure. The starch acts in a similar manner as

plaster. When combined with gelatin, the starch gave the sample additional rigidity and cohesiveness while the

gelatin gave flexibility. In combination, these two substances worked well to give the properties observed.

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Starch Gelatinization and Retrogradation

While gelatin was a major component of the sample described above, further work was done with a focus on

starch. Starch also will form a gel with through processes called gelatinization and retrogradation [11, 12, 15].

Starch is a polysaccharide made up of chains of glucose molecules [9, 11, 12, 15]. There are two types of chains:

amylose and amylopectin. Amylose is consists of straight chains of molecules while amylopectin consists of

branched molecules [9, 10, 13]. These chains are found within a starch granule. When starch is subject to heat and

water, swelling of the granule occurs. When a high enough temperature, the gelatinization temperature, is

reached, the granule will irreversibly burst and will release the amylose and amylopectin contained within [13].

When this occurs the starch suspension becomes more viscous and forms a paste. This process is called

gelatinization. Starch from different sources will have different gelatinization temperatures [9, 11, 12, 15]. Factors

that may affect gelatinization temperature include, but are not limited to, granule size, pH of solution, and the

presence of salts, sugars, fats, and proteins [9, 11, 12]

As the paste cools down, retrogradation occurs. The amylose and amylopectin that was released from the granule

will begin to recrystallize. It is this recrystallization that forms the starch gel. The strength and consistency of the

gel depends on the relative amounts of amylose and amylopectin [11, 13]. A higher percentage of amylose will

tend to form a strong, firm gel while higher percentage amylopectin will tend to form a softer gel or even a viscous

paste [6, 11]. Retrogradation can also be broken down into two parts. The first part occurs over a short time

period. The amylose chains rapid crystalize first. Later, long term changes in crystallization are attributed to

amylopectin.

Starches for different sources will have different amounts and ratios of amylose and amylopectin. Cornstarch in

particular has a relatively high percentage of amylose at about 21% compared to other native, unmodified starches

[7]. High amylose (HA) starches are also available. HA starches have a much higher percentage of amylose than

other starches at 80% or more [6].

This process of a retrogradation can be affected by different processes and the presence of different substances. It

was in the continued work with starch and the retrogradation process where such processes and substances were

explored.

Pregelatinization

A particularly relevant process that starch can be subject to is pregelatinization. Effectively, starch may be

gelatinized and then dried back into a powder. The resulting powder will then form a gel in cold water [42, 43].

Current applications for pregelatinized starch include use in pies, fillings, and other foods where gelling is desired

without further cooking [42, 43, 44]. It is of interest to the project since cold water is a possible choice for binder

within the print head.

In industrial settings, an extruder, spray dryer, or drum dryer are common mechanical/thermal apparatus used to

produce a pregelatinized starch powder [41, 45, 46]. Common methods of pregelatinization tend to use

mechanical/thermal methods; however, chemical methods are also available. One common chemical method of

gelatinization is suspending starch powder in an alcohol-alkaline solution [38, 39, 40].

The difficultly in using pregelatinized starches is our ability to pregelatinize starch in lab. It is of interest to have this

ability because it would then be possible to treat and modify starch with other processes and substances prior to

pregelatinization. Such processes may require the use of water which would cause gelation of the starch is already

pregelatinized. Alternatively, if an already pregelatinized starch is modified further, it may be necessary to return

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the starch to its powdered form. Currently in lab, the mechanical/thermal apparatus mentioned above are not

present. Simply drying a starch paste in a conventional oven will not suffice, as a starch plastic will form. This

plastic, even when ground to a fine powder, will not produce a gel in cold water. Chemical methods may become

costly in the long term and must be studied and refined over a period of time before becoming an effective and

efficient means of producing pregelatinized starch, modified or unmodified.

Since pregelatinized starch is currently unable to be produced in lab, a rather large quantity was obtained from an

outside source. Grain Processing Corporation, a producer of corn-based products, supplied samples for 3 types of

pregelatinized starches at 5 kg each. One type, Instant Pure-Cote B792, is meant as a food coating and produces a

runny, non-vicious solution. It is not beneficial to the project’s goals at this time. The other two types, B656 and

B658, appear virtually identical and can be treated as such.

Use of these modified starches can be further explored. Currently, the gels they produce do have the capability to

form structures that are somewhat self-supporting. Nevertheless, there are some differences. When the

pregelatinized starches are prepared in a solution rather than through the method by-hand described earlier, its

takes a higher concentration to yield similar results as gels form from unmodified starches. Gels formed from the

modified starch are very sticky and difficult to handle. Ways to reduce the stickiness of these gels would be highly

beneficial as this is virtually the only real barrier in using the pregelatinized starch to make 3D structures. Drying

times, coatings, soaking solutions, or finding ways of increasing the strength of the pregelatinized gels could very

possibly aid in removing this barrier.

Acid modification

When the amylose and amylopectin chains are introduced to an acid with heat, the chains will undergo acid

hydrolysis. These polysaccharide chains will break apart and form dextrin or simple sugars. Over an extended

period of time and/or in a highly concentrated acid, acid hydrolysis will significantly break down the amylose and

amylopectin chains [21]. It is the amylose chains which contribute to gel strength; therefore, by significantly

subjecting starches to acid hydrolysis, gel strength will be reduced.

However, it has been suggested that if acid hydrolysis is carried out correctly, gel strength could actually increase.

(Hoover; pharma tablet paper) During hydrolysis, non-crystalline regions among the amylopectin chains will be

hydrolyzed first [17, 19, 24]. By hydrolyzing these regions and removing their effect on the retrogradation process,

the amylose chains and more crystalline amylopectin chains will contribute to gel formation. As retrogradation is a

crystallization process, the less non-crystalline chains, the better the overall crystallization; and therefore, a

stronger gel may be formed. The ideal time of hydrolysis and acid concentration still needs to be identified. Some

sources perform acid hydrolysis for several hours at a given concentration with positive results, while another

source carried out hydrolysis for several days (although in a different context/goal) [17-26].

In order to test the validity of acid hydrolysis, mechanical testing on an Instron 3340 Series with Bluehill software.

Before testing, unmodified cornstarch was subject to acid hydrolysis for 1 hour at various concentrations. To

produce each batch, 30 g of starch powder was added to 75 mL of hydrochloric acid at 50° C. After 1 hour, the

suspension was neutralized with NaOH solution then vacuum filtered. Drying then took place in a vacuum oven.

Batches were made using .01 M, .05 M, .10 M, .25 M, .5 M, and 1.0 M HCl.

The acid-modified starches (as well as unmodified starch) were gelatinized. Approximately 7.5 g of each sample

were suspended in 75 mL of deionized water. The starch was added to the water at 40-50° C. The suspension was

then held at 80-90° C for 10 min with gentle stirring, uncovered. At the end of the time period, the pastes were

poured into aluminum weigh boats and then compared. Qualitatively, it was determined that the acid-treated

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starch gels produced in .01 M HCl and .05 HCl maintained their shape well and showed similar properties as

untreated cornstarch. It would be the gels from those starches, along with unmodified starch gel, that would be

examined with the Instron machine.

Gels that were 35 mm in diameter and approximately 15 mm in height (varied slightly) were made with

unmodified cornstarch, .01 M HCl-treated starch, and .05 M HCl-treated starch. 6 gels were made from each starch

course. 3 gels were tested that were made within several hours of being poured; while the other 3 gels were left

over night to allow for an extended period of retrogradation. Each gel was compressed at a constant rate over 10

seconds until 10% strain (1.5 mm for a gel height of 15 mm). The compression was held for an additional 90

seconds.

The results of the mechanical testing can be found on page 12. From an initial analysis of the data, the “Fresh” gels

give no obvious results as to whether acid modification under the given conditions increased maize starch gel

strength. The “Overnight” gels give a clearer result. The data suggests that starch gels produced with .01 M HCl-

treated starch produces a stronger gel than the other starch sources. To reach a more convincing conclusion,

further statistical analysis may be performed. This may include analysis with a Student’s t-test, analysis of variance

(ANOVA), or hypothesis testing. Also, as stated before, the acid hydrolysis reaction can be further examined by

testing various time periods over which the reaction is carried out.

Future Materials Science Testing

When continuing the materials science work associated with this project, starch will likely a substance that will be

continually researched. Another substance that has shown promise and will be examined more closely is gelatin.

Starch with Silicon Infusion

As of now, there is only one published example where the end result of a 3D powder printing process has yielded a

flexible material. Out of the University of Sheffield, a research team has been exploring the use of 3D printing to

create prosthetic nose [5]. While the focus of the team’s published research is the color of the prosthetic for the

purpose of matching natural skin pigmentation, the materials used to produce the prosthetic structures are of

great relevance. Starch was used to create a 3D structure of a patient’s soon-to-be nose, then the structure was

soaked in silicon fluid. While no explicit data on the materials texture or mechanical properties was given, it is

reasonable to hypothesize that the final prosthetics had some degree of flexibility, compressive strength, and

tensile strength. It would worth experimenting with different silicon fluids or various soaking methods.

Crosslinking of Starch

A very important modification of starch that should be investigated is cross-linking. With cross-linking, any starch

gels or other starch structures could potentially be strengthened. The amylose and amylopectin chains would be

linked to each other via a cross-linking agent in addition to being crystallized with retrogradation. There are several

materials that can be used to cross-link starch such as Phosphorous Oxychloride (POCl3) and Epichlorohydrin (EPI)

[48]. One of the most common cross-linking agents for starch is Sodium Trimetaphosphate (STMP) [47, 48, 50, 51].

Starch is often cross-linked with STMP for food applications, medical applications, and other technologies.

Research with starch cross-linking would be beneficial for the project as it could enhance the strength of objects

produced with 3D printing technology.

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Research with starch cross-linking would be beneficial for the project as it could enhance the strength of objects

produced with 3D printing technology. The specific task associated with this method of treatment is to find a way

to make it feasible process. One possible course of action to explore is subjecting solid, pregelatinized starch

powder to STMP (or another cross-linking agent) prior to the application of binder (i.e water). It has been found

that starch can be cross-linked in a solid state, rather than in solution, with microwave irradiation [47]. The hope

would be that cross-linking would occur and would somehow be effective without the need for the solid starch to

be a large chuck of material. Ideally, the starch would be a fine powder when used in the PWDR printer.

A more likely course of action would be to enable a cross-linking reaction by soaking a starch gel or other starch

structure in STMP solution prior to printing. With this method, the starch would already be in the desired shape.

Cross-linking could then aid in the ability of the starch in maintaining its shape and mechanical properties. This

method is similar to the starch and silicon example. There are also similarities to 3D printing processes that have

already been established, although those processes yield structures that are very rigid rather than flexible.

Starch with Fatty Acids

In one paper, it has been suggested that the addition of fatty acids, specifically myristic and stearic acids, can aid in

increasing the storage modulus of starch gels [32]. The proposed mechanism is that the fatty acid forms a complex

with the amylose and amylopectin changing the gelatinization temperatures which can have an effect on gel

strength. If this method is proven to work, then it would be necessary to explore ways of adapting the method to a

3D powder printing process.

Starch with Salts

It has been shown that certain salts can increase starch gel strength [33-35]. Certain ions can either inhibit or aid

the aggregation of the amylose chains. Aggregation is essentially the grouping together of molecules. With ions

known as salting-in ions, aggregation is inhibited by increasing the interactions between the amylose and solvent

(water), while reducing amylose-amylose interactions. With salting-out ions, the opposite occurs. Aggregation of

the amylose is increased due to increased amylose-amylose interactions. What is known as the Hofmeister series

describes which ions are salting-out and salting-in well. Salting-out ions tend to include, but are not limited to,

sulfate anions and iron cations. Therefore, it has been shown that the addition of salts with such ions to starches

tend to aid in increasing gel strength.

A possible course of action would be to explore if the same effects occur with pregelatinized starch. The salt

compounds could potentially be added to water to form the binder that is deposited by the printer head. An

alternative could be to soak a printed starch gel in a salt solution. Various compounds and concentrations could be

tested.

Starch with Cellulose Derivatives

The addition of insoluble cellulose derivatives to gel matrices have been shown to increase the storage modulus if

the gels [29-31]. The retrogradation process was accelerated in the presence of those substances. In one

experiment [30], it was found that carboxymethylcellulose (CMC), alkaline soluble fibrous cellulose (ASC), and

powdered microcrystalline cellulose (MCC) all enhanced the retrogradation and increased the storage modulus of

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sweet potato starch gel. All three substances are insoluble in water. Methylcellulose, which is soluble in water,

decreased retrogradation and therefore storage modulus. The proposed mechanism was that the insoluble

derivatives act as nuclei for the crystallization process. The amylose chains are given a point within the gel matrix

to begin crystallizing.

The potential benefit from this work would be that a cellulose derivative in powder form could be added to a

pregelatinized starch powder. If the addition of the insoluble cellulose derivatives to unmodified starch enhances

gel properties, it stands to reason that a similar effect may be seen with a modified form of starch.

Use of Gelatin as main substance

While the use of starch to this point has taken center stage, other materials should not be left unexamined. As

mentioned prior, a very promising and very simple powder mixture of starch and gelatin was used. So far, only

potential enhancements to the starch have been explored. It could be highly beneficial to examine ways to

enhance the properties of gelatin as well.

A very promising process could involve the use of gelatin with glutaraldehyde (GTA). In experiments with gelatin

films, GTA was found to act as a cross-linking agent within the gelatin, therefore strengthen the films [56, 57]. It

stands to reason that if GTA can enhance films, it can also enhance the properties of gelatin gels of any shape. A

gelatin printed gelatin structure could be soaked in GTA solution post-printing. The key would be to explore

various concentrations of GTA and varied soaking times. Furthermore, different shapes may require different

soaking times due to changes in their surface area to volume ratio.

One potential concern with GTA is that it is a toxic substance and can be harmful if not handled properly. However,

low GTA concentrations on the order of 1%, the approximate concentration used in referenced experiments, pose

less of a health risk [57]. Also, it has been shown that after a period of time after cross-linking, any uncross-linked

GTA molecules left within the gelatin matrix will be released. Once all of the unbound GTA is released, a particular

gelatin sample will pose minimal risk.

Several other materials that have been used with gelatin include low acyl gellan gum [52-55], enzymes with gelatin

and chitosan blends [59], κ-carrageenan [61], and ferulic and tannin acids [58]. Further analysis of published data

will more definitely conclude whether or not a given material or process could be beneficial to a 3D powder

printing process.

Just like with starch, there are many possibilities to explore when it comes to enhancing gelatin’s mechanical

properties. The challenge, once a given method has proven beneficial, is to adapt that method so it can be

applicable to a potential 3D printing process and yield our desired properties. An additional task would be to

combine various methods of treating starch and gelatin. It will take time to explore and assess all the potential

materials that could be used with the PWDR printer. It will take even more time to adapt the materials into

something that is useful for the printing process.

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Current Operating Procedure for PWDR 3D Printer

The following is the currently used procedure for operating the PWDR printer. Since the printer is not yet fully

operational, either some or all of this procedure will likely change in the future. These steps are from a blend of

the User Manual on the PWDR website and PWDR Github repository [II] and researcher changes.

Converting the Model and Loading to SD card

1. In Processing 1.5.1, Open “Pwdr_GUI_V0_3” pde file. The file can be found under Pwdr-Model-0.1-master >

Processing > Pwdr_GUI_VO_3

2. Under the ‘Serial’ Tab, line 9, make sure the line of code reads: arduinoSerial = new Serial(this, "COM4", 9600);

Note: ‘COM4’ will change depending on computer. Check the port name by plugging in the Arduino with USB and

Open Device Manager (If using Windows).

3. Run the program

4. Click ‘Load Model’ and select the desired STL file in the pop-up browser window.

5. Click the Settings button in the bottom left corner to adjust margins, number of layers, etc.

Note: It has not yet been conclusively determined what effect adjusting the margins has. It is also supposedly

necessary to ensure all margin values are divisible by 12.

6. Click the first symbol in the top left corner to return to the original window.

7. Click ‘Convert’

8. Close Processing 1.5.1

9. Open the Processing folder within the PWDR Master folder and Open Folder “Pwdr_GUI_V0_3”.

10. Copy “config” to the folder ‘PWDR’.

11. Copy the entire folder ‘PWDR’ to SD card.

12. Close all windows and insert SD card to the Arduino.

Uploading Arduino Firmware

1. In Arduino software, Open the desired version of the firmware code (Pwdrfirmware2_0_mk1). Go to Pwdr-

Model-0.1-master > Arduino > PwdrFirmware2_0_mk1

2. Under ‘Tools’, Highlight ‘Board’ and select the proper Arduino hardware model (Arduino Mega 2560 or Mega

ADK).

3. Under ‘Tools’, Highlight ‘Serial Port’ (if available) and select proper Serial Port. (Check in Window’s Device

Manager if needed).

4. Connect the Arduino hardware via USB cable to Computer.

5. Upload the code

6. Under ‘Tools’, select ‘Serial Monitor’ to check that the code was properly uploaded. If successful, one of two

possible messages will appear. You will see either ‘SD initialization failed’ or ‘Ready to Print’. The former means

that the SD card is not properly being read by the Arduino

7. Close Arduino software. You can leave the USB cable connected.

Starting a Print Job

1. Lubricate motor shafts and put powder into powder bin (closer to the electronics).

Page | 11

2. Reopen the Processing program ‘Pwdr_GUI_V0_3’ and run the program.

3. Click the USB symbol near the top left corner. Connect the proper serial port i.e COM4, COM3…

4. Click the third symbol in the top left corner to bring up the print control interface.

5. Connect power to the PWDR printer.

6. Jog the y-axis motor so the roller bar is either aligned or just behind (relative to the build bin) the center divide

between the build bin and powder bin.

7. Jog the x-axis to desired location. Centering the printer head is recommended.

8. In the printer interface, Click ‘reset position’.

9. Click ‘New layer’ to deposit a new layer of powder. Do this for several layers.

10. Click ‘Print file’.

Finishing and Clean-Up

1. After printing is complete, leave the printed object in the build bin to allow setting. Time will depend on

materials used.

2. Disconnect the power to the PWDR printer

3. By hand, move the print head along the x and y axes to the far corner away from the electronics and opposite

the y-axis motor.

4. Recover any unused powder.

Page | 12

Acid-Hydrolyzed Gel Test Results

Fresh Starch Gels

Overnight Starch Gels

Sample Set Peak Values (N)

Highest Peak Value

(N)

Average Peak (N)

Avg. Peak w/ Lowest Value Dropped (N)

Pop. Standard

Deviation, σ (N)

Pop. Standard

Dev., Lowest Value

Dropped

Unmodified 0.11597 0.24019 0.54578

.54578 .3006

.3930

.1806 .15280

.01 M 0.25823 0.3613 0.80147

.80147 .4737

.5814

.2356 .2201

.05 M 0.29085 0.16609 0.38264

.38264 .2799

.3367

.0887 .0459

.10 M 0.23695 0.29667 0.29878

.29878 .2775

.2977

.0287 .0011

Sample Set Peak Values (N)

Highest Peak Value

(N)

Average Peak (N)

Avg. Peak w/ Lowest

Value Dropped (N)

Pop. Standard

Deviation, σ (N)

Pop. Standard

Dev., Lowest Value

Dropped

Unmodified 0.07356 0.37843 0.19472

.37843 .21557 .28658 .1253 .09186

.01 M 0.25862 0.11299 0.33678

.33678 .23613 .2977 .0927 .03908

.05 M 0.0824 0.16 0.4623

.4623 .2349 .3112 .1638 .15115

.10 M 0.31678 0.26114 0.18568

.31678 .2545 .2890 .0537 .02782

Page | 13

References, Articles, and Academic Papers

3D Printing

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rapid prototyping for tissue engineering." Biomaterials: 6020-6041.

Web.

[2] Lam, C.X.F, X.M Mo, S.H Teoh, and D.W Hutmacher. "Scaffold development

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Engineering: C: 49-56. Web.

[3] Pfister, Andreas, Rudiger Landers, Andres Laib, Ute Hubner, Rainer

Schmelzeisen, and Rolf Mulhaupt. "Biofunctional rapid prototyping for

tissue-engineering applications: 3D bioplotting versus 3D printing."

Journal of Polymer Science Part A: Polymer Chemistry: 624-638. Web.

[4] Suwanprateeb, Jintamai. "Improvement in mechanical properties of three-

dimensional printing parts made from natural polymers reinforced by

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approach." Polymer International: 57-62. Web.

[5] Xaio, Kaida, et al. "Color reproduction for advanced manufacture of soft tissue

prostheses." Journal of Dentistry: e15-e23. Web.

Starches – General

[6] Ashogbon, Adeleke O., and Emmanuel T. Akintayo. "Recent trend in the

physical and chemical modification of starches from different

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[7] Bates, F. Leslie, Dexter French, and R.E. Rundle. "Amylose and Amylopectin

Content of Starches Determined by their Iodine Complex Formation."

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[8] Chiu, Chung-wai, and Daniel Solarek. "Modification of Starches." Starch:

Chemistry and Technology. : , 2009. Web,

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[10] Fredriksson, H., J. Silverio, R. Andersson, A.-C. Elaisson, and P. Aman. "The

influence of amylose and amylopectin characteristics on gelatinization

and retrogradation properties of different starches." Carbohydrate

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[11] "Learning, Food Resource”, Oregon State University, Corvallis, OR N.p., 23

May 2012. Web. 1 Aug. 2014.

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[14] Rai, T, and Sangeetha Mishra. "Morphology and functional properties of corn,

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g-starch-functionality.aspx>. Web.

[16] Willett, Julious L., and Sanghoon Kim. "Isolation of Amylose from Starch

Solutions by Phase Separation."Starch - Stärke: 29-36. Web.

Starch Gelatinization – Acid modification

[17] Bello-Pérez, Luis A. et al. "Effect of acid treatment on the physicochemical

and structural characteristics of starches from different botanical

sources." Starch - Stärke: 115-125. Web.

[18] Ferrara, Peter J. Acid-Modified Starches and Flours and Method of Making the Same. Keep Chemical Company, assignee. Patent 3,479,220. 18 Nov. 1968. Print.

[19] Hoover, R.. "Acid-Treated Starches." Food Reviews International: 369-392. Web.

[20] Nishinari et al. "Effects of adding acids before and after gelatinization on the viscoelasticity of cornstarch pastes." Food Hydrocolloids: 909-914. Web.

[21] Poutanen, Kaisa et al. "Molecular Weight Characterization and Gelling Properties of Acid-Modified Maize Starches." Starch - Stärke: 64-69. Print.

[22] Rao, M.A, and E.K. Chamberlain. "Rheological properties of acid converted waxy maize starches in water and 90% DMSO/10% water." Carbohydrate Polymers: 251-260. Web.

[23] Varavinit, Saiyavit, and Napaporn Atichokudomchai. "Characterization and utilization of acid-modified cross-linked Tapioca starch in pharmaceutical tablets." Carbohydrate Polymers: 263-270. Web.

[24] Wang, Linfeng et al. "Structures and rheological properties of corn starch as affected by acid hydrolysis." Carbohydrate Polymers: 327-333. Web.

[25] Wang, Ya-Jane, and Linfeng Wang. "Structures and Physicochemical Properties of Acid-Thinned Corn, Potato and Rice Starches." Starch - Stärke: 570. Web.

[26] Wuttisela et al. "Amylose/amylopectin Simple Determination In Acid Hydrolyzed Tapioca Starch." Journal of the Chilean Chemical Society: 1565. Web.

Starch Gelatinization – Alkali Treatment

[27] Cameron, R.E, and S.A. Roberts. "The effects of concentration and sodium hydroxide on the rheological properties of potato starch gelatinisation." Carbohydrate Polymers: 133-143. Print.

[28] Fazilah, A. et al. "Pasting and retrogradation properties of alkali-treated sago (Metroxylon sagu) starch." Food Hydrocolloids: 1044-1053. Web.

Starch Gelatinization – Cellulose derivatives and other binders

[29] Donald, A.M., R.E. Cameron, and C.M. Sansom. "The interactions between hydroxypropylcellulose and starch during gelatinization." Food Hydrocolloids: 181-193. Web.

[30] Nishinari, Katsuyoshi, and Kaoru Kohyama. "Cellulose Derivatives Effects on Gelatinization and Retrogradation of Sweet Potato Starch." Journal of Food Science: 128-131. Print.

[31] Okoye, et al. "Comparative study of some mechanical and release properties of paracetamol tablets formulated with cashew tree gun, povidone and gelatin as binders." African Journal of biotechnology 8: 3970-3973. Web.

Page | 14

Starch Gelatinization – Fatty Acids

[32] Saxena, S.K, Narpinder Singh, and Jaspreet Singh. "Effect of fatty acids on the rheological properties of corn and potato starch." Journal of Food Engineering: 9-16. Web.

Starch Gelatinization – Salts and Sugars

[33] Hong, Yan , Ling Zhu, and Zhengbiao Gu. "Effects of sugar, salt, and acid on tapioca starch and tapioca starch-xanthan gum combinations." Starch - Stärke 66: 436-443. Web.

[34] Shi, Lui et al. "Effect of salts on textural, color, and rheological properties of potato starch gels." Starch - Stärke 66: 149-156. Web.

[35] Williams, Peter A., and Fasihuddin B. Amhad. "Effect of Salts on the Gelatinization and Rheological Properties of Sago Starch." Journal of Agricultural and Food Chemistry: 3359-3366. Web.

Starch Gelatinization – Misc.

[36] Lawal, O. "Composition, physicochemical properties and retrogradation characteristics of native, oxidized, acetylated and acid-thinned new cocoyam (Xanthosoma sagittifolium) starch." Food Chemistry: 205-218. Web.

[37] Xie, B.j et al. "Characters of rice starch gel modified by gellan, carrageenan, and glucomannan: A texture profile analysis study." Carbohydrate Polymers: 411-418. Web.

Starch Pregelatinization – Chemical Methods

[38] Bernetti, Raffaele et al. Process For Preparing Pregelatinized Starches. Corn Products Company, assignee. Patent 3,399,081. 27 Aug. 1968. Web.

[39] Chen, J., and J. Jane. “Preparation of Granular Cold-Water-Soluble Starches by

Alcoholic-Alkaline Treatment.” Cereal Chemistry: 618:622. Web.

[40] Jane, Jay-lin, and Paul Seib. Preparation of Granular Cold Water Swelling/Soluble Starches by Alcoholic-Alkali Treatments. Kansas State University Research Foundation, assignee. Patent 5,057,157. 15 Oct 1991. Web.

Starch Pregelatinization – Mechanical/Thermal Methods [41] Colonna, P. et al. ”Extrusion Cooking and Drum Drying of Wheat Starch. I.

Physical and macromolecular Modifications.” Cereal Chemistry: 538-543. Web.

[42] Klingler, Rudolf and Karl-Georg Busch. Pregelatinized Starches and Processes

for their Production. Bayer Cropscience GmbH, assignee. Patent 7,045,003. 16 May 2006. Web.

[43] Kuchinke, Eduard, and Karl Buchta. Process for the Preparation of

Pregelatinized, Cold-Setting Starch. Boehringer Ingelheim, Assignee. Patent 3,332,785. 25 Jul. 1967. Web.

[44] Protzmann, Thomas, and John Wagoner. Gelatinized Starch Products. A.E.

Staley Manufacturing Company, assignee. Patent 3,137,592. 16 Jun. 1964. Web.

[45] Vallous, N.A., et al. “Performance of a double drum dryer for producing

pregelatinized maize starches.” Journal of Food Engineering: 171-183. Web.

Starch Pregelatinization – Other [46] Yan, Hong, and G.U. Zhengbiao. “Morphology of modified starches prepared

by different methods.” Food Research International 43: 767-772. Web. Starch Cross-Linking [47] Gui-Jie, Mao, et al. “Crosslinking of Corn Starch with Sodium

Trimetaphosphate in Solid State by Microwave Irradiation.” Journal of Applied Polymer Science 102: 5854-5860. Web.

[48] Hirsch, Julie B., and Jozef L. Kokini. “Understanding the Mechanism of Cross-

Linking Agents (POCl3, STMP, and EPI) Through Swelling Behavior and Pasting Properties of Cross-Linked Waxy Maize Starches.” Cereal Chemistry 79: 102-107. Web.

[49] Kasemsuwan, T., et al. “Preparation of clear noodles with mixtures of tapioca

and high-amylose starches.” Carbohydrate Polymers 32: 301-312. Web.

[50] Wattanachant, Saowakon, et al. “Effect of crosslinking reagents and

hydroxypropylation levels on dual-modified sago starch properties.” Food Chemistry 80:463-471. Web.

[51] Woo, Kyungsoo, and Paul Seib. “Cross-linking of wheat starch and

hydroxypropylated wheat starch in alkaline slurry with sodium trimetaphosphate.” Carbohydrate Polymers 33: 263-271. Web.

Gelatin with Gellan Gum [52] Lau, M.H., et al. “Texture profile and turbidity of gellan/gelatin mixed gels.”

Food Research International 33: 665-671. Web. [53] Lee, Heayeon, et al. “Optimizing gelling parameters of gellan gum for

fibrocartilage tissue engineering.” Journal of Biomedical materials Research B: Applied Biomaterials 98B: 238-245. Web.

[54] Lee, Kwang Yeon, et al. “Mechanical properties of gellan and gelatin

composite films.” Carbohydrate Polymers 56: 251-254. Web. [55] Shim, Jaewon. Gellan Gum/Gelatin Blends. Merck & Co., Inc., assignee. Patent

4,517,216. 14 May 1985. Web. Gelatin with Glutaraldehyde [56] Bigi, A., et al. “Drawn gelatin films with improved mechanical properties.”

Biomaterials 19: 2335-2340. Web. [57] Bigi, A., et al. “Mechanical and thermal properties of gelatin films at different

degrees of glutaraldehyde crosslinking.” Biomaterials 22: 763-768. Web.

Gelatin – Other [58] Cao, Na, et al. “Mechanical properties of gelatin films cross-linked,

respectively, by ferulic and tannin acid.” Food Hydrocolloids 21:575-584. Web.

[59] Chen, Tianhong, et al. “Enzyme-catalyzed gel formation of gelatin and

chitosan: potential for in situ applications.” Biomaterials 24: 2831-2841. Web.

[60] De Smedt, S.C., et al. “Characterization of Network Structure of Dextran

Glycidyl Methacrylate Hyrdrogels by Studying the Rheological and Swelling Behavior.” Macromolecules 28: 5082-5088. Web.

[61] Haug, Ingvild J., et al. “Physical behavior of fish gelatin-κ-carrageenan

mixtures.” Carbohydrate Polymers 56: 11-19. Web.

Page | 15

Miscellaneous Resources

[I] Bush, Corey et al. “Powder-based 3D Printer fabrication to Create Synthetic Biological Tissue”. CPE 472 –Engineering

Design II. Miami Univeristy.

[II] "Pwdr." Pwdr - Open source powder-based rapid prototyping machine. University of Twente, n.d. Web. 5 Sept. 2013.

<http://pwdr.github.io/>.

[III] Creo Machines – <creo.io>

[IV] Sandbox 3D Printer - <sandbox3dp.com>