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Shape memory polymers (SMPs) are stimuli-responsive materials, which are able to retain an imposed, temporary shape and recover the initial, permanent shape through an external stimulus like heat. In this work, a novel manufacturing method is introduced for thermoresponsive quick response (QR) code carriers, which originally were developed as anticounterfeiting technology. Motivated by the fact that earlier manufacturing processes were sometimes too time-consuming for production, filaments of a polyester urethane (PEU) with and without dye were extruded and processed into QR code carriers using fused filament fabrication (FFF). Once programmed, the distinct shape memory properties enabled a heating-initiated switching from non-decodable to machine-readable QR codes. The results demonstrate that FFF constitutes a promising additive manufacturing technology to create complex, filigree structures with adjustable horizontal and vertical print resolution and, thus, an excellent basis to realize further technically demanding application concepts for shape memory polymers.
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polymers
Article
Additive Manufacturing of Information Carriers
Based on Shape Memory Polyester Urethane
Dilip Chalissery 1, Thorsten Pretsch 1, *, Sarah Staub 2and Heiko Andrä 2
1Fraunhofer Institute for Applied Polymer Research IAP, Geiselbergstr. 69, 14476 Potsdam, Germany;
dilip.chalissery@iap.fraunhofer.de
2
Fraunhofer Institute for Industrial Mathematics ITWM, Fraunhofer-Platz 1, 67663 Kaiserslautern, Germany;
sarah.staub@itwm.fraunhofer.de (S.S.); heiko.andrae@itwm.fraunhofer.de (H.A.)
*Correspondence: thorsten.pretsch@iap.fraunhofer.de; Tel.: +49-(0)-331/568-1414
Received: 5 April 2019; Accepted: 4 June 2019; Published: 5 June 2019


Abstract:
Shape memory polymers (SMPs) are stimuli-responsive materials, which are able to retain
an imposed, temporary shape and recover the initial, permanent shape through an external stimulus
like heat. In this work, a novel manufacturing method is introduced for thermoresponsive quick
response (QR) code carriers, which originally were developed as anticounterfeiting technology.
Motivated by the fact that earlier manufacturing processes were sometimes too time-consuming
for production, filaments of a polyester urethane (PEU) with and without dye were extruded and
processed into QR code carriers using fused filament fabrication (FFF). Once programmed, the
distinct shape memory properties enabled a heating-initiated switching from non-decodable to
machine-readable QR codes. The results demonstrate that FFF constitutes a promising additive
manufacturing technology to create complex, filigree structures with adjustable horizontal and vertical
print resolution and, thus, an excellent basis to realize further technically demanding application
concepts for shape memory polymers.
Keywords:
additive manufacturing; 3D printing; shape memory polymer; fused filament fabrication;
QR code carrier; thermoplastic polyurethane; filigree structures
1. Introduction
Additive manufacturing (AM) alias three-dimensional (3D) printing is increasingly gaining
importance, especially because of the rapid availability and the infinite design variety of print objects.
Within the commercially established AM technologies, fused filament fabrication (FFF), which is a
hot-melt extrusion-based 3D printing process, is widely used [
1
3
]. It requires a virtual 3D model
and appropriate slicing software to convert the model into thin layers and gain the essential printing
instructions. After melting in the extruder nozzle, the polymer strand is deposited layer-by-layer
on the building platform of a 3D printer by moving the nozzle along a pre-calculated path. Once
deposited, the polymer hardens immediately in the desired arrangement of polymer strands, which
set the final shape of an object. Since FFF is an extrusion-based technique, it easily gives access to new
thermoplastic materials provided they can be processed with filaments that meet the requirements
of a 3D printer. To date, many thermoplastic materials have been investigated via FFF, at which
special attention was devoted to polylactic acid (PLA), acrylonitrile-butadiene-styrene copolymers
(ABS), polycarbonates, and polyamides [
3
]. Other indispensable polymer-based AM methods include
stereolithography (SLA), multi-jet fusion (MJF), selective laser sintering (SLS), and big area additive
manufacturing (BAAM).
Basically, the prospect of developing new applications for 3D printing improves as new functional
materials are developed [
4
,
5
]. In general, many shape memory polymers (SMPs) are thermoresponsive
Polymers 2019,11, 1005; doi:10.3390/polym11061005 www.mdpi.com/journal/polymers
Polymers 2019,11, 1005 2 of 19
thermoplasts. They are able to retain an imposed, temporary shape after programming and to
recover the initial, permanent shape upon exposure to an external stimulus like heat [
6
11
]. Today,
thermoplastic polyurethanes belong to the most intensively studied shape memory polymers [
12
25
].
Intriguingly, there are only a few publications in which FFF has been employed as a printing method
for thermoresponsive polyurethane-based SMPs, the majority of them concentrating on polyether
urethanes. Obviously good printing results could be achieved by Hendrikson et al. [
26
]. who
demonstrated that scaolds can be produced via FFF from the polyether urethane DiAPLEX
®
MM
3520 from SMP Technologies Inc. The scaolds were characterized by a fiber spacing of 982
±
11
µ
m,
a fiber diameter of 171
±
5
µ
m, and a layer height of 154
±
2
µ
m. In another work, Raasch et al.
reported on the extrusion of the thermoplastic polyether urethane DiAPLEX
®
MM 4520 from the same
company and used the obtained filaments to manufacture specimens out of them; the 3D objects were
later examined by three-point bend tests to study the influence of annealing upon shape memory
behavior [
27
]. In a work by Yang et al., the same material was extruded and FFF used to print parts
with shape memory properties [
28
]. Villacres et al. fabricated tensile bars of DiAPLEX
®
MM 4520
and proved how to influence the mechanical properties by varying geometrical parameters like print
orientation and infill percentage [
29
]. The apparently only work on extrusion-based AM of polyester
urethane so far has been reported by Monz
ó
n et al. [
30
], who employed a custom-made 3D printer
to produce parts of Desmopan
®
DP 2795A SMP from Covestro Deutschland AG. The setting of the
individual layer height was selected to be 400
µ
m; the stress recovery behavior of programmed parts
was studied and there was a potential seen to be used as mechanical actuators.
Until today, plenty of applications have been suggested for SMPs [
31
40
]. One of these applications
is switchable information carriers [
41
43
]. According to the underlying concept of SMP Tagnologies
TM
,
e.g., a quick response (QR) code, which can be considered as an example for a complex two-dimensional
structure, is contained in the surface of an SMP. After fabrication, the code can be converted from
machine-readable to unreadable by programming. Upon thermally triggering the shape memory eect,
the QR code returns into the machine-readable state. The special material behavior of information
release on demand can be helpful to verify and identify counterfeit products [
41
] or to supervise cold
chains [
44
]. In the past, the preparation of information carriers turned out to be labor-intensive, since
several manufacturing steps had to be passed through. In fact, once an SMP was processed via, e.g.,
injection molding, “guest diusion” had to be applied to achieve a surface-specific coloration and laser
treatment to generate a two-dimensional code in the polymer surface, before the information carrier
could be obtained in its final shape by going through a cutting process [
41
]. Alternatively, an SMP can be
coated and a code engraved into the resulting top layer, followed, e.g., by laser cutting [
45
,
46
]. Since the
procedures are partly very complex, the primary goal of this work includes the introduction of an easier
approach to fabricate QR code carriers. To keep it as simple as possible, the same polyester urethane
(PEU), which was employed as base material in earlier generations of information carriers, was used.
On the way to the production of QR code carriers, the individual steps of filament manufacturing and
processing via FFF were examined, before appropriate programming paths were explored and the
functionality of the QR code carriers was evaluated. Finally, the results of 3D printing were considered
against the background of other technologies used in additive manufacturing.
2. Experimental Section
2.1. Material
The polyester urethane (PEU) Desmopan
®
DP 2795A SMP from Covestro Deutschland AG
(Leverkusen, Germany) was chosen as model compound and used as received in the form of a
granulate. The hard segment of the PEU is composed of 4,4’-methylenediphenly diisocyanate and
a 1,4-butanediol chain extender. The soft segment is based on poly(1,4-butylene adipate) (PBA).
Further information regarding the thermal and mechanical properties of the PEU is given in previous
publications [25,46,47].
Polymers 2019,11, 1005 3 of 19
2.2. Extrusion
The PEU granulate was dried at 110
C in a Binder vacuum drying chamber VDL 53 from Binder
GmbH (Tuttlingen, Germany) in order to remove water and avoid bubble formation when extruding
filaments at a later stage. The thermal pre-treatment was finalized after 150 min. Subsequently, the
pellets were fed into an extrusion line to produce filaments (Figure 1).
Figure 1.
Technical drawing of an extrusion line as used for the production of PEU filaments: Material
feeding system (A), twin screw extruder (B), conveyor belt (C), water bath (D), and filament winding
machine (E). The extrudate is drawn in red.
The individual units of the extrusion line were put together in such a way that it included a
volumetric material feeding system Color-exact 1000 from Plastic Recycling Machinery (Zhangjiagang
City, China), a Leistritz twin screw extruder MICRO 18 GL from Leistritz AG (Nürnberg, Germany),
characterized by seven heating zones and a screw length of 600 mm, a conveyor belt, a water bath and
a filament winder from Brabender GmbH and Co. KG (Duisburg, Germany). The temperature of the
individual heating zones of the extruder was 180, 185, 190, 195, 200, 190, and 190
C. The screw speed
of the extruder was set to 77 rpm. Initially, the PEU granulate was processed without additives. In
another experiment 0.5 wt % of Irgazin
®
Red DPP BO from Kremer Pigmente GmbH and Co. KG
(Aichstetten, Germany) were added to obtain a red filament. To evaluate the quality of the filaments,
the evolution of filament diameter was manually detected at regular intervals using a Vernier caliper
from Fowler High Precision (Auburndale, FL, USA).
2.3. Virtual Design
The bar code generator goQR.me [
48
] was used to create a QR code (Reed-Solomon error correction,
error correction level H) with the encoded information “Fraunhofer IAP” (Figure 2) [49].
Figure 2.
Technical drawing of a QR code, which was used as structural motif for the production of
information carriers. All data are provided in mm.
The code was saved in the .jpeg image format and used as starting point to build a virtual
information carrier by means of the vector-oriented drawing program AutoCAD from Autodesk, Inc.
(San Rafael, CA, USA) (Figure 3) [50].
Polymers 2019,11, 1005 4 of 19
Figure 3.
Technical drawing of a virtual QR code carrier including a substrate layer (gray color) and a
structural QR code elevation (black color): Top view (
a
), isometric view (
b
), front view (
c
), and left
view (d). All data are provided in mm.
The edge length of the QR code was set to 25 mm (Figures 2and 3a). For the substrate layer a
dimensioning of 30 mm
×
40 mm was selected. As can be seen in Figure 3b–d, the QR code carrier was
built up by two structural units including a substrate layer with a height of 180
µ
m and a structural
QR code elevation with a height of 190
µ
m. The design of the QR code carrier as provided by Figure 3
was used for most of the experiments, which are described in this contribution. The only exception
was an approach in which the target height of the substrate was reduced to 15
µ
m. For convenience, a
terminology for the dierent types of QR code carriers and the associated print settings is introduced
in Section 2.4. After finalizing the design, the 3D models were imported into the slicer program Cura
3.3.1 from Ultimaker B. V. (Watermolen, The Netherlands) [
51
]. As a result, numerically controlled
codes, also denoted as G-codes, were generated, containing the instructions for the 3D printer in the
form of printing paths, information with reference to the amount of extruded material and the spatially
resolved printing parameters. Finally, the codes were transferred to the 3D printer.
2.4. Fused Filament Fabrication (3D Printing)
Fused filament fabrication was used to produce QR code carriers with diering technical
specifications. The experiments were carried out with the commercially available 3D printer Ultimaker
3 from Ultimaker B. V. (Geldermalsen, The Netherlands). The manufacturer provides an XYZ resolution
for Ultimaker 3 of 12.5, 12.5, and 2.5
µ
m [
52
], defining the smallest movement that the 3D printer can
make with regard to the XY plane and in the Z direction. To calibrate the print bed, the Ultimaker build
plate manual leveling calibration method was carried out before the beginning of each experiment.
Therefore, a calibration card characterized by a thickness of about 170
µ
m was used. The process
included a rough leveling of the build plate followed by a fine leveling. The fine leveling was achieved
with the calibration card, at which the knurled nut was adjusted at the rear center, front left, and front
right of the build plate until slight friction occurred, when sliding the card between built plate and
print head.
Basically, the same design of QR code carriers was used as introduced in Section 2.3. The two
print heads of the FFF-printer were either equipped with two nozzles having dierent diameters of 100
and 400
µ
m, respectively, or the same diameter of 400
µ
m. For simplicity, the following terminology is
introduced, pointing out the most relevant variations when producing QR code carriers:
Type 1: The substrate was printed with non-dyed PEU using a 400
µ
m nozzle and a target layer
thickness of 180 µm. The elevation was built from red PEU with a 100 µm nozzle.
Polymers 2019,11, 1005 5 of 19
Type 2: In analogy to the type 1 QR code carrier, the substrate was printed with non-dyed PEU
using a 400
µ
m nozzle. Again, a target layer thickness of 180
µ
m was selected for the substrate,
but the elevation was built with red PEU employing a 400 µm nozzle.
Type 3: Similar as in the previous cases, the substrate was printed with non-dyed PEU using a
400
µ
m nozzle, but this time a reduced target layer thickness of 15
µ
m was selected. The elevation
was built with red PEU using a 400 µm nozzle.
The most relevant settings for the 3D printing processes are listed in Table 1.
Table 1.
Printing instructions for Ultimaker 3 to produce the three dierent prototypes of QR code
carriers based on PEU.
Specifications Substrate
(Non-Dyed PEU)
Elevation
(Red PEU)
Type of QR code carrier 1, 2 3 1 2, 3
Diameter of the nozzle (µm) 400 400 100 400
Temperature of the nozzle (C) 225 225 190 190
Speed of print head (mm·s1)50 50 4 7
Build rate (ml·h1)34.2 34.2 5.4 22.8
Build platform temperature (C) 23 23 23 23
Number of layers 1 1 3 1
Layer height (µm) 180 15 63 190
2.5. Characterization of Thermal Properties
Dynamic mechanical analysis (DMA) was used to investigate the thermomechanical properties of
the PEU. The experiments were carried out on two samples including a cylindrical granulate grain,
having a diameter of 1.8 mm and a length of 4.86 mm, and a sample of a 3D printed substrate of a type
2 QR code carrier having the size of 5.15 mm
×
3.8 mm
×
0.19 mm. The measurements were conducted
with a Q800 DMA from TA Instruments (New Castle, DE, USA) at a frequency of 10 Hz. At first, the
sample was heated to 100
C, before it was cooled to
100
C to finalize the first heating-cooling cycle.
Adjacently, the measurement cycle was repeated once more. For all experiments, heating and cooling
rates of 3
C
·
min
1
were selected and the holding time at the highest and lowest temperature was set
to 10 min. The storage modulus (E
´
), loss factor (tan
δ
) and the glass transition temperature (T
g
) were
determined for the second heating.
The phase transition behavior of the PEU was also studied by dierential scanning calorimetry
(DSC) using a Q100 DSC from TA Instruments (New Castle, DE, USA). The measurements were
performed on a granulate grain, a piece of the filament and a sample of the 3D printed substrate of a
type 2 QR code carrier. In any case, the sample weight was approximately 5 mg. In the experiments a
sample was first cooled to
90
C, before it was heated to 100
C and cooled back to
90
C, which
finalized the measurement. Cooling and heating was carried out with a rate of 10
C
·
min
1
. The
temperature holding time was 10 min at 90 and 100 C, respectively.
2.6. Characterization of Print Quality
Topography measurements were performed on QR code carriers using a FocusCam LV150 confocal
microscope from Confovis GmbH (Jena, Germany), which was equipped with an objective lens of
5
×
/0.15 N.A. Any time, the sample was illuminated with a ring light. The data recorded by the focus
variation microscope was evaluated with the software MountainsMap
®
imaging topography 7.4 from
Digital Surf (Besançon, France) [
53
]. The development of the surface profile with regard to a scanned
cuboid including its surrounding was exemplarily determined for type 1 and type 2 QR code carriers.
For a detailed analysis, a line was inserted along the mid-perpendicular through the cuboid. The
cuboid was characterized with a step measurement to determine its height and width.
Polymers 2019,11, 1005 6 of 19
Further microscopic investigations were carried out with the microscope Axio Scope.A1 from
Carl Zeiss Microscopy GmbH (Jena, Germany) using the imaging software Zen 2.3 lite also from
Carl Zeiss Microscopy GmbH [
54
]. The experiments were conducted to evaluate the resolution of
the QR code in the XY-plane and to estimate the layer thickness of QR code carriers and thus the
Z-parameter. In the latter case, a cut was made with a scalpel along the mid-perpendicular through the
abovementioned cuboid.
The printing results were also mathematically investigated. For this purpose, QR code carriers
were scanned in a first step as gray value images with a resolution of 600 dpi and then loaded into the
software tool ImageJ developed by Wayne Rasband (Bethesda, MD, USA) [
55
]. With assistance of this
tool the images were cropped to the dimension of the original QR code and scaled to the corresponding
resolution. The brightness and contrast were adjusted such that the influence of reflections and possible
shadows was minimized. Then, the gray value images were binarized by the automatic binarization
function in ImageJ. In a next step, the binarized images of the printing results were inverted. Thus,
those areas where there was only the substrate of the QR code carrier were marked in black while
the printed elevation parts were marked white. Adjacently, the inverse images of the printing results
as well as the original QR code were imported into the software tool Paraview from Kitware, Inc.
(Saratoga County, NY, USA) [
56
] and exported into the vtk format. The software Paraview allows
mathematical operations on the values of images. The binary values of the printing results and the QR
code were added up in each pixel. Due to the applied inversion for the printing results, three dierent
gray values V were obtained in the summation (Equation (1)):
V=
0, erraneously not f illed {blue
255, congruent green
510, irregularl y f illed {red
(1)
Based on these values, the print quality of the dierent prototypes was evaluated as a percentage
p by means of Equation (2):
p=# pixels of certain gray value
# pixels (2)
2.7. Programming and Characterization of Shape Memory Properties
The programming of QR code carriers was carried out with an MTS Criterion universal testing
machine from MTS Systems Corporation (Eden Prairie, MN, USA). The device was operated with a
temperature chamber, which was controlled by a Eurotherm temperature controller unit. Two heating
elements were located at the back of the chamber. Liquid nitrogen from a Dewar vessel was fed into
the chamber under a pressure of 1.3 bar as an essential prerequisite for cooling. At the beginning of
programming, a QR code carrier was clamped with a length of 25 mm, corresponding to the edge
length of the QR code, in the pneumatic grips of the universal testing machine, the chamber was
heated to 60
C and a maximum force F
max
of either 5 or 25 N was applied using a loading rate of
300 mm
·
min
1
. The maximum distance between the outer sides of the QR code was immediately
determined by means of a Vernier caliper from Fowler High Precision. The QR code carrier was then
cooled to –15
C, whereby the clamping distance was kept constant. After 10 min, the sample was
unloaded and the chamber was heated to 23 C.
A ZTNG-100B heating plate from Dr. Neumann Peltier-Technik GmbH (Neuried, Germany) was
used to investigate the thermoresponsiveness of the programmed QR code carriers. Therefore, the
temperature was gradually raised from 23 to 60
C and images of the sample were taken in regular
time intervals during shape recovery. After finalizing the experiment, the congruence of the QR code
pattern with regard to the permanent and the recovered shape was determined and used to evaluate
shape recoverability. In this connection, a similar approach was followed as described in Section 2.6,
but this time gray value images were generated for the QR code carrier in its permanent and recovered
shape. The gray value image of the permanent shape was regarded as the standard with which the
Polymers 2019,11, 1005 7 of 19
recovered shape was compared. Therefore, the binarized gray value image of the recovered shape was
inverted and added to the image containing the information of the permanent shape. The resulting
gray values V were evaluated such that in the case of consistent pixels the areas were considered to be
congruent while, for nonexistent pixels, the areas were regarded as incongruent (Equation (3)):
V=(255, congruent (green)
else,incongruent (red)(3)
By analogy with the above procedure, the percentage was determined again according to
Equation (2), but this time it was the measure of shape recoverability.
A multiple cycle experiment was carried out with the MTS Criterion universal testing machine,
which was equipped with a temperature chamber. For loading, a type 2 QR code carrier was clamped
with a length of 25 mm, corresponding to the edge length of the QR code, in the pneumatic grips of
the universal testing machine, heated to 60
C, deformed with a rate of 300 mm
·
min
1
to a maximum
clamping distance of 55 mm, before unloading was carried out at the same temperature with a rate of
150 mm
·
min
1
. In total, 20 cycles of loading and unloading were conducted. In the 21
st
cycle, the sample
was loaded and the imposed shape was fixed by cooling to
15
C. After unloading, the temperature
was raised to 23
C and the machine readability of the QR code was checked. The programmed
QR code carrier was adjacently heated to 60
C where, again, the readability of the QR code was
investigated. To characterize the boundary between substrate and elevation, another programming
was accomplished. In this 22
nd
cycle, a cut was made with a scalpel along the mid-perpendicular
through the abovementioned cuboid and investigated by means of light microscopy. The sample was
finally heated to 60
C and a microscopic investigation was carried out with the microscope Axio
Scope.A1, which was equipped with an objective lens of 20
×
and 40
×
magnification. Following other
programming scenarios, a QR code carrier was folded in the middle or rolled up at 60
C, before it
was cooled under load to
15
C. Afterwards, the thermoresponsiveness was again followed on the
heating plate when triggering the shape memory eect. Independent of the programming technique
applied, the machine readability of QR codes was checked with a Samsung Galaxy S8 smartphone from
Samsung Electronics (Seoul, South Korea), which was equipped with the software “Optical Reader”
version 4.4.07 also from Samsung Electronics Co., Ltd [57].
3. Results and Discussion
The melt extrusion of the physically cross-linked PEU block copolymer led to the production of a
whitish filament whose color can be traced back to the presence of crystals from poly(1,4-butylene
adipate) (PBA); the proof will be given below in a DSC measurement. In another experiment, 0.5 wt %
of Irgazin
®
Red DPP BO was added in the course of PEU extrusion so that a red filament could also
be obtained. It is noteworthy that the two filaments had a homogenous diameter of 2.85
±
0.07 mm,
regardless of whether the dye was added or not (Figure 4).
Before starting with the 3D printing experiments, the design of the QR code carriers was developed
(Figure 3). The objects were sliced to obtain the essential printing instructions. In a next step, a dual
extrusion FFF process was established, in which the already obtained filaments were reprocessed to
build up QR code carriers, characterized by a whitish substrate and a red QR code elevation. The most
relevant settings for the 3D printing processes are provided in Table 1.
For the production of a type 1 QR code carrier, a single-layer substrate with a target height of
180
µ
m was printed, using the white filament and a nozzle with a diameter of 400
µ
m. In contrast, the
QR code elevation having a virtual height of 190
µ
m was then built up in three layers by melting the
red filament in the 100
µ
m nozzle and placing the resulting strands on the substrate. The printing
results are portrayed in Figure 5together with their microscopic characterization.
Polymers 2019,11, 1005 8 of 19
Figure 4.
Evolution of filament diameter over time when extruding PEU. The development in measured
values is also representative for an experiment in which Irgazin
®
Red DPP BO was added during
extrusion of PEU.
Figure 5.
Type 1 QR code carrier as investigated by light and confocal microscopy including an
evaluation of print quality: Top view and inset exhibiting a randomly selected cuboid (
a
), surface
topography of the cuboid and its surrounding (
b
), superposition with a virtual QR code having a
transparency of 60% (
c
), result of a mathematic calculation to determine the congruence of the virtual
QR code with the physical print object: consistent print areas (green color), irregularly filled areas (red
color) and unfilled print areas (blue color) (
d
), side view of a cut through the cuboid and the substrate
(e), and the evolution of layer thickness Z with regard to the cuboid and its surrounding (f).
Polymers 2019,11, 1005 9 of 19
The obtained type 1 QR code carrier exhibited a good spatial resolution with respect to the XY
level as exemplified by the presence of finely resolved rectangles (Figure 5a). In order to better assess
the print quality with regard to the smallest structural unit of the QR code pattern, a cuboid of the
finished part with a virtual edge length of 1.21 mm was microscopically examined (Figure 5a,b). Here,
an edge length of approximately 1.25 mm could be determined. This value exceeded the one of our
CAD model by 40
µ
m corresponding to 3.2% of the object dimension (Figure 3a). Basically, a deviation
from the technical specification was anticipated due to slight fluctuations in filament diameter (Figure 4)
and minor dierences in the print bed height resulting from the calibration [
58
]. However, in the XY
plane the print quality of the overall QR code pattern was pretty good as supported by the result of
a superposition experiment, in which the virtual QR code was put with a transparency of 60% over
the printing pattern (Figure 5c). In addition, a congruence measurement was carried out, subtracting
the overhanging regions of the QR code elevation from the black regions of the virtual code. The
result gave that 90.7% of the code areas were congruent, 8.1% were irregularly filled with red PEU
and 1.2% were erroneously not filled (Figure 5d). Next, the resolution in the Z-direction was closely
investigated for the same cuboid and its nearest surrounding. The substrate of the QR code carrier had
a thickness of about 160
µ
m (Figure 5e). The averaged profile height of the elevation was determined
to be approximately 145
µ
m, corresponding to a mean layer height of about 48
µ
m (Figure 5f). The
layer thickness was slightly below the target value, presumably due to deficits in calibration accuracy.
The production of the QR code elevation took 17 min, culminating for the whole QR code carrier in
a production time of 25 min. For a faster production, the 100
µ
m nozzle was replaced by a 400
µ
m
nozzle and the technical parameters were adjusted accordingly (see Table 1). As a result, a type 2 QR
code carrier was obtained and examined microscopically (Figure 6).
This time, the presence of more imperfect rectangles could be witnessed in the QR code pattern
(Figure 6a,b). Once more, the cuboid was studied, which was located at the same position of the
QR code as in the preceding case (Figure 5a), in order to get a first impression about the precision
in the XY printing plane. Here, a drastically increased edge length was determined as documented
by a value of about 1.52 mm (Figure 6a), exceeding the virtual dimensions of this element by 26%
(Figure 3a). The fact that the horizontal print resolution substantially deteriorated in the whole QR
code area was confirmed by another superimposition experiment. As visible to the naked eye, the
printed regions generously overlapped the black areas of the virtual QR code pattern (Figure 6c).
Against this background, another mathematic calculation was carried out. It turned out that 77.4% of
the code areas were congruent, whereas 22.6% of those code areas, in which no printing was desired,
were covered with red PEU (Figure 6d). However, compared to the type 1 QR code carrier, the same
substrate thickness could be verified as expected, but better control over the vertical print resolution
could be achieved as indicated by an average profile height of 175
µ
m (Figure 6e,f). Furthermore, the
production time of the QR code elevation could be drastically reduced to 3 min and 30 s so that the
printing of the entire QR code carrier was finalized after 11 min and 30 s.
Despite the abovementioned dimensional inaccuracies in the 3D printed objects, the QR codes
enabled an error-free decoding with a standard smartphone, independent of which technical equipment
and parameter settings were used for printing. This clearly shows that the surface contrast was
suciently high as ensured by the processing of the dierently colored filaments.
To investigate the influence of reprocessing via extrusion and FFF on the viscoelastic properties of
the PEU, dynamic mechanical analyses were conducted. Therefore, the raw material in the form of a
granulate grain was studied and compared with the thermomechanical behavior of a sample, which was
taken from the 3D printed substrate of a type 2 QR code carrier. The associated temperature-dependent
evolution in storage modulus E’ and in tan δis provided by Figure 7.
In both cases, E’ exhibits a two-step decrease in the DMA measurement as characteristic for
physically cross-linked PEU [
22
,
59
61
]. The investigation of the granulate grain reveals a strong drop
in E’, starting at
51
C and indicating the presence of a glass transition. The tan
δ
peak is located at
about
18
C. Upon further heating, a weaker decline in E’ takes place, which can be associated with
Polymers 2019,11, 1005 10 of 19
the melting of PBA crystals as earlier verified for the same material [
44
]. The 3D printed sample shows
a similarly pronounced drop in E’, starting again at approximately
50
C, and a tan
δ
peak at
20
C,
which is in accordance with the thermal behavior of the granulate grain. In contrast, the decline in
storage modulus associated with PBA melting is slightly extended toward higher temperatures. This
could be related to an orientation eect as supported by reprocessing via FFF, favoring the formation
of PBA crystals with higher temperature stability. In other words, the conditions under which parts of
the PBA phase of PEU crystallized were expected to be more favorable for the 3D printed sample. To
take another look at this, DSC measurements were carried out (Figure 8).
Figure 6.
Type 2 QR code carrier as investigated by light and confocal microscopy including an
evaluation of print quality: Top view and inset exhibiting a randomly selected cuboid (
a
), surface
topography of the cuboid and its surrounding (
b
), superposition with a virtual QR code having a
transparency of 60% (
c
), result of a mathematic calculation to determine the congruence of the virtual
QR code with the physical print object: consistent print areas (green color) and irregularly filled areas
(red color) (
d
), side view of a cut through the cuboid and the substrate (
e
), and the evolution of layer
thickness Z with regard to the cuboid and its surrounding (f).
Polymers 2019,11, 1005 11 of 19
Figure 7.
Thermal and mechanical properties of PEU as determined by DMA: Evolution of storage
modulus E’ (solid line) and tan
δ
(dashed line) at the second heating of a granulate grain (red color)
and the sample of the substrate of a type 2 QR code carrier as manufactured via FFF (blue color).
Figure 8.
DSC thermograms of PEU: Thermal behavior of a granulate grain (red color), a piece of
filament (green color) and a sample from the substrate of a type 2 QR code carrier as obtained via
FFF (blue color). The thermograms are exhibited for the second heating and cooling. The individual
enthalpies of melting are 25.9 J
·
g
1
(granulate grain), 21.1 J
·
g
1
(filament) and 25.1 J
·
g
1
(3D printed
sample), the enthalpies of crystallization are
25.7 J
·
g
1
(granulate grain),
21.0 J
·
g
1
(filament), and
25.0 J·g1(3D printed sample).
The DSC cooling trace of the 3D printed sample shows an exothermic signal at about 7
C
associated with the recrystallization of the PBA phase [
25
]. Compared with the thermal behavior of
the granulate grain, the peak crystallization temperature increased by about 15
C. This observation
can be taken as further hint that strand deposition in course of 3D printing favored a better alignment
of polymer chains, thus facilitating the recrystallization of PBA. In a third DSC measurement, the
filament of PEU was investigated. Here, another exothermic signal associated with PBA crystallization
appeared on cooling, justifying the whitish color of the filament. In this case, the peak crystallization
Polymers 2019,11, 1005 12 of 19
temperature was closer to the one of the granulate grain. In turn, the DSC heating traces of the three
samples show the presence of two phase transitions. The first one is located at around
50
C and,
thus, close to the point at which E’ started to drop in the DMA measurement. It is related to the glass
transition temperature of the PBA phase, while the endothermic signal in between 20 and 50
C with
a maximum at around 40
C can be assigned to the melting of PBA crystallites [
25
]. Here, the same
trend as in the DMA measurement could be verified, but the melting peak temperature of PBA only
increased by 2
C for the 3D printed sample compared with the granulate grain. Beyond that, the
melting behavior of the crystalline PBA phase appeared to be similar for the filament and the granulate
grain. Most importantly, when considering both the DMA and DSC data, no further evidence was
found that two-step processing, including extrusion and FFF, had a significant impact on the thermal
properties of the PEU.
In a next step, the shape memory properties of a type 2 QR code carrier were investigated
(Figure 9).
Figure 9.
Type 2 QR code carrier: Permanent shape after 3D printing (
a
), temporary shape as obtained
after programming (F
max
=5 N) (
b
), and recovered shape after heating to 60
C (
c
). To visualize shape
recoverability, the image of the permanent shape was converted to black-and-white and superimposed
with a transparency of 60% on the image of the recovered shape (
d
). The result of a mathematical
calculation comparing the permanent shape with the recovered shape: congruent areas (green color)
and incongruent areas (red color) (e).
Therefore, the additively manufactured QR code carrier (Figure 9a) was heated to 60
C, at which
the melting of the PBA phase was completed. Subsequently, a tensile force F
max
of 5 N was applied,
whereupon a maximum distance length of 55 mm between the outer sides of the QR code was detected.
The elongated QR code carrier was fixed by cooling below the crystallization temperature of the PBA
phase and unloaded (Figure 9b). Due to changes in the design of the QR code carrier and in particular
because of the much smaller structural thickness of only 160
µ
m, a significantly lower deformation
force was required to achieve a similar QR code distortion in the programmed shape compared to an
earlier generation of QR code carriers, which was characterized by a thickness of 2 mm and required a
tensile force F
max
of 48 N [
41
]. Intriguingly, the bonding was strong enough to withstand a removal of
the QR code elevation from the substrate in the course of deformation. The programmed shape of the
QR code carrier, which was stable at 23
C, was characterized by the largest distance length between
the outer sides of the QR code of 54 mm, speaking for the excellent shape fixity of the polymer. Due to
its drastic distortion the code was no longer machine-readable. Upon triggering the shape memory
eect, the QR code pattern almost completely returned to the original shape (Figure 9c), which was
accompanied with the restoration of machine readability. For a more detailed study, another image
analysis was carried out. Herein, the superimposed QR codes of the original shape and the recovered
shape turned out to be almost identical (Figure 9d). The distinct shape recoverability was evidenced
by another mathematic calculation, unveiling that 87.8% of the code areas of the permanent shape and
the recovered shape were congruent (Figure 9e). Overall, the pronounced shape memory properties,
Polymers 2019,11, 1005 13 of 19
which were detected in the first experimental series, raised the question if type 2 QR code carriers are
able to resist even stronger deformations. To find out the answer, a similar programming experiment
as described above was performed, but this time Fmax was raised to 25 N (Figure 10).
Figure 10.
Type 2 QR code carrier: Permanent shape after 3D printing (
a
), the temporary shape as
obtained after programming (F
max
=25 N) (
b
), and the recovered shape after heating to 60
C (
c
). To
visualize shape recoverability, the image of the permanent shape was converted to black-and-white
and superimposed with a transparency of 60% on the image of the recovered shape (
d
). The result of a
mathematical calculation comparing the permanent shape with the recovered shape: congruent areas
(green color) and incongruent areas (red color) (e).
As a matter of fact, the QR code carrier produced by FFF (Figure 10a) was elongated so that
the outer sides of the QR code had a maximum distance of about 155 mm. After cooling below the
crystallization temperature of the PBA phase and unloading, the temperature was raised to 23
C.
Here the new, even more strongly deformed shape proved to be stable (Figure 10b). The distance
between the outer sides of the elongated QR code measured 153 mm in tensile direction which, again,
revealed the excellent shape fixity of the polymer. It is remarkable that even in this case the QR
code became machine-readable again after triggering the shape memory eect (Figure 10c), which
demonstrates that the concept of information release on demand was still working. Apparently, the
decoding algorithm of the smartphone was able to compensate the residual distortion. The discrepancy
between the QR code pattern of the permanent shape and the recovered shape can be clearly seen
in the corresponding superimposed images (Figure 10d). As quantified in one further mathematical
calculation, 72.7% of the code areas were congruent (Figure 10e). Compared to the previous case, a
weakening of shape recoverability was expected due to the stronger deformation applied. It can be
assumed that this phenomenon of growing residuals with increasing elongation can be traced back to
the flow of amorphous segments in the polymer [62].
To determine the degree of deformation, at which the QR code was no longer machine-readable,
another type 2 QR code carrier was deformed at 60
C with a rate of 0.5 mm
·
min
1
while the machine
readability of the QR code was regularly checked. It turned out that the QR code became unreadable
as soon as a distance length of 30 mm between the outer sides of the QR code was exceeded.
In an attempt to study the reliability of shape memory properties, a type 2 QR carrier was exposed
to a multiple cycle experiment (Figure 11).
The additively manufactured QR code carrier (Figure 11a) was loaded to a clamping distance
of 55 mm and unloaded twenty times, before a loading at 60
C and unloading at
15
C was
accomplished. In this 21
st
cycle, the QR code was non-decodable at 23
C and characterized by a
maximum distance length of 55 mm between its outer sides (Figure 11b). Triggering the shape memory
eect by reheating to 60
C resulted in shape recovery as accompanied with the restoration of the
machine-readable code, characterized by a maximum edge length of 26.8 mm (Figure 11c). This
unequivocally demonstrates the reliability of the concept of switchable information carriers. In the
ensuing 22
nd
cycle, the thermomechanical treatment of the previous cycle was repeated, but neither
Polymers 2019,11, 1005 14 of 19
micro cracks nor delamination could be microscopically detected at the boundary between the substrate
and the elevation both for the programmed shape (Figure 11d) and for the recovered shape (Figure 11e).
This finding indicates good layer coalescence. As expected, the triggering of the shape memory eect
led to an increase in layer thickness. In fact, a recovery from 105 to 155
µ
m for the substrate and from
120 to 165 µm for the elevation could be verified.
Figure 11.
Type 2 QR code carrier: Permanent shape after 3D printing (
a
), initial clamping
distance =25 mm), temporary shape as obtained after 20 loading-unloading cycles (maximum
clamping distance =55 mm) at 60
C, followed by programming (
b
), and the recovered shape after
heating to 60
C in the 21
st
cycle (
c
). Microscopic investigation of a cut through the cuboid and the
substrate as examined in the 22
nd
cycle for the programmed shape (
d
) and the recovered shape (
e
); the
insets show an enlarged view of the boundary between the substrate (below) and the elevation (above).
Next, the deformation scenarios for the programming of QR code carriers were expanded toward
rolling and bending, before the respective thermoresponsivity was studied (Figure 12).
Figure 12.
Thermoresponsiveness of type 2 QR code carriers, which were deformed in a folding
approach (
a
) and a rolling approach (
b
): Programmed shapes (left), sequential shape recovery when
placed on a 60 C hot heating plate (images 2–4) and recovered shapes (images 5–6).
Therefore, two of our type 2 QR code carriers were heated to 60
C, at which the PBA phase of the
PEU was completely amorphous. The first sample was folded in the middle (Figure 12a), the other was
rolled up (Figure 12b). The fixation of the resulting temporary shapes was then achieved on cooling
below the crystallization temperature of the PBA phase. After unloading, the QR code carriers were
placed on a heating plate, which had a temperature of 60
C. In both cases it took about 10 s to finalize
shape recovery, which again was accompanied with the restauration of the machine-readable QR
codes, thus demonstrating that the concept is not restricted to deformation scenarios like elongation or
compression [41,63].
Polymers 2019,11, 1005 15 of 19
Following another design approach, the dimensions of the QR code carrier were altered by
drastically reducing the target substrate thickness from 180 to 15
µ
m. As a result, a type 3 QR code
carrier was obtained (Figure 13).
Figure 13.
Type 3 QR code carrier: Illustration of size and thickness in comparison with a 50 euro cent
coin, which is characterized by a height of 2 mm (
a
) and image of a light microscopic investigation to
estimate the thickness of the QR code carrier (b).
The production time of the type 3 QR code carrier was about 11 min 30 s, corresponding to the
processing time of the type 2 QR code carriers. To illustrate the low thickness, a 50 euro cent coin having
a thickness of 2 mm was placed next to it (Figure 13a). As determined in a microscopic measurement,
the thickness of the PEU substrate varied from about 7 to 10 µm (Figure 13b).
It is also worth mentioning that the weight of all QR code carriers described herein was significantly
lower compared to earlier generations of prototypes, which were obtained by other processing
techniques [
41
,
45
,
46
]. In direct comparison with each other, the introduced type 1 and type 2 QR code
carriers were approximately weighing 340 mg while the weight of the type 3 QR code carrier was
100 mg and, thus, significantly lower, qualifying it for applications, in which the costs for transport
must be kept under control.
For the purpose of comprehensive consideration, the printing results described herein were
compared with those of other printing materials, which were processed by FFF, and additionally with
the results of other 3D printing techniques. For convenience, the same approach was followed as
by Quinlan et al. [
64
], who compared polymer-based processes like fused filament fabrication (FFF),
stereolithography (SLA), big area additive manufacturing (BAAM), multi-jet fusion (MJF) and selective
laser sintering (SLS) with particular emphasis on build rate and layer thickness, the latter of which
can be considered as a measure of Z-direction accuracy. The corresponding results are supplied in
Figure 14.
It can be clearly seen that SLA, BAAM, MJF, and SLS provide higher build rates compared with
FFF. In turn, FFF makes it particularly possible to control the layer thickness, namely, the Z-parameter,
over quite a wide range as apparent for printing materials like ABS and PLA. However, the data points
introduced for the presented QR code carriers do also cover a broad area, which in parts overlaps with
the already existing data for FFF. Due to the printing result of the thin layer as evident for the substrate
of the type 3 QR code carrier, a data point emerges, defining the lowest value for Z. Interestingly,
this reasonably good print resolution could neither be achieved by other groups, working on shape
memory polyurethanes using extrusion-based AM techniques [
26
30
,
65
,
66
] nor by other researchers
who utilized those 3D printing techniques, which were described by Quinlan et al. [
64
]. Admittedly,
two-photon lithography (2PL) is another AM technology, which was not included in our considerations,
but allows obtaining 3D objects, which are characterized by even smaller layer thicknesses of 0.2 to 0.3
µ
m [
67
]. Although being particularly advantageous in resolution, the good print results of 2PL are at
the expense of the build rate. Therefore, a compromise is needed, which seems to be achievable by FFF,
well-balancing the build rate with print resolution and, thus, qualifying it as promising technology to
obtain shape memory polymers in entirely new shapes.
Polymers 2019,11, 1005 16 of 19
Figure 14.
Build rate versus layer thickness for common additive manufacturing processes. The initial
data was extracted from Quinlan et al [
64
]. The red stars represent data points for the QR code elevation
as part of the type 1 QR code carrier (1) and the type 2 QR code carrier (2), while the remaining data
points refer to the substrate of the type 1 and type 2 QR code carrier (3) and the type 3 QR code
carrier (4).
4. Conclusions
Fused filament fabrication is a suitable technique to produce bicolored additively manufactured
QR code carriers in a dual extrusion process as demonstrated for a polyester urethane, which was used
as model compound. The print resolution both in the XY-plane with regard to the QR code pattern
and in Z-direction with reference to the layer height could be controlled by the experimental setup
and the print instructions. This way, filigree, well-resolved structures could be obtained. The objects
were able to resist strong deformations and characterized by distinct shape memory properties. Even
in a multiple cycle experiment no major damage could be witnessed for the print objects. The use of
congruence measurements has proven to be a valuable tool to determine the printing accuracy and
shape recoverability. Although a higher resolution of the QR code pattern was achieved when using a
setup with a 100
µ
m nozzle, with extending the production time, FFF seems to be a practical method
in this scenario as well, which may give access to other technically demanding objects. The main
advantages of the new manufacturing process for QR code carriers are that polymer extrusion can be
easily controlled, a significantly lower amount of base material is needed, facilitating the fabrication
of very thin layers with a thickness below 10
µ
m, and the use of solvents can be avoided. The latter
is of ecological importance. All these aspects emphasize that the novel production process for QR
code carriers is not only attractive for research purposes, but also from an economic point of view,
not least because the material could be qualified for processing with a commercially available 3D
printer. Therefore, FFF could turn out as an enabling technology to realize applications for SMPs in
fields like counterfeit-proof marking of goods at risk of plagiarism and supervision of cold chains.
Future challenges consist in shortening the production time without compromising on resolution and
using the dimension of time to autonomously manipulate 3D printed objects, which is also known as
4D-printing, thus eliminating the need for programming.
Author Contributions:
Conceptualization: T.P. and H.A.; formal analysis: D.C., S.S., and H.A.; funding acquisition:
T.P. and H.A.; investigation: D.C. and S.S.; methodology: D.C. and S.S.; project administration: T.P. and H.A.;
supervision: T.P.; validation: D.C.; visualization: D.C. and S.S.; writing—original draft: D.C., T.P., and S.S.;
writing—review and editing: T.P.
Funding:
This research was funded by Fraunhofer High Performance Center for Functional Integration in Materials,
grant number 630039, and by Fraunhofer Excellence Cluster “Programmable Materials”, grant number 630527.
Polymers 2019,11, 1005 17 of 19
Acknowledgments:
This work was supported as Fraunhofer High Performance Center for Functional Integration
in Materials (project 630039). T.P., S.S., and H.A. also acknowledge support by the Fraunhofer Excellence Cluster
“Programmable Materials” under project 630527. T.P. wishes to thank the European Regional Development
Fund for financing a large part of the laboratory equipment (project 85007031). The authors thank Chris Eberl
(Fraunhofer IWM) for fruitful discussions on two-photon lithography. Tobias Rümmler is kindly acknowledged
for carrying out the DMA measurements and Katrin Hohmann for light microscopic investigations.
Conflicts of Interest: The authors declare no conflict of interest.
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... When exposed to heat, the one-way shape memory effect DOI: 10.1002/mame.202100619 is triggered and the polymer almost completely returns to its permanent shape. [1][2][3][4][5] The advantageous shape-memory behavior of polymers has already been utilized, among others, in connection with the development of biomedical devices, [6][7][8][9][10][11][12][13] the counterfeit-proof marking of goods susceptible to plagiarism, [14][15][16][17][18][19] and for active assembly [20][21][22][23] and disassembly, [22,[24][25][26][27][28] which both requires a rethinking of classical design processes. ...
... [30] A few studies have been reporting on FFF with SMPs, in particular with thermoplastic polyurethanes (TPUs). [19,[31][32][33][34][35] Most importantly, in the past few years, it has become known how to implement internal stresses during AM, which is the so-called "4D printing," [36,37] addressing timeevolving structural functions as unattainable by conventional 3D printing. [38][39][40] The main advantage of such function integration is that the finished objects can be removed directly from the printer without the need for an additional thermomechanical treatment. ...
... For this purpose, the same extrusion line was used as recently reported. [19,49] The obtained filament had a smooth surface and a diameter of 2.85 ± 0.10 mm. The narrow tolerance range ensured that an important processing criterion was fulfilled. ...
Article
Full-text available
Four-dimensional (4D) printing of shape memory polymers enables the production of thermoresponsive objects. In this contribution, a facile printing strategy is followed for an in-house synthesized thermoplastic poly(ether urethane). Processing by means of fused filament fabrication (FFF), in which the difference between nozzle temperature and material-specific glass transition temperature of the polymer is kept as low as possible, allows to obtain highly shrinkable objects whose shape and thermoresponsiveness can be precisely controlled. The effectiveness of the method also applies to the printing material polylactic acid. One possible application lies in highly shrinkable objects for assembly purposes. As proof-of-concept, lightweight hands-free door openers for healthcare applications are functionally simulated and developed. Once printed, such devices shrink when heated to fit on door handles, allowing an easy assembly. At the end-of-use, a heating-initiated disassembling and mechanical recycling are proposed. In perspective, a reuse of the materials in 4D printing can contribute to the emergence of a circular economy for highly functional materials. This article is protected by copyright. All rights reserved
... Shape memory polymers (SMPs) are stimuli-responsive materials that can recover the initial, permanent shape after an imposed, temporary shape during programming (training), by exposure to an external stimulus like heat [1,2]. Among the SMPs, thermoplastic polyurethanes received most of the attention [3], due to their competitive shape memory and mechanical properties compared to other polymers, e.g. ...
... Although there is a broad discussion on the thermomechanical behavior of conventionally produced SMPs [7,8], there is lack of information of SMP materials handled in the era of digital manufacturing. There are only few findings in the literature of fused filament fabrication (FFF) applied as a printing method of shape memory polyurethane [2], unlike to the well documented printing of other thermoplastics in terms of printing strategy and mechanical properties [9,10]. Even in these cases, the focus is on the printability [11À13]. ...
Article
Unique thermal shape recovery and chemical stability make Shape Memory Polymers (SMPs) attractive for critical applications in biomedical, aerospace and energy sectors. While additive manufacturing (AM) of SMPs allows fabrication of functionally-graded structures with tailored, intricate design features, the effect of AM on shape recovery characteristics has not received much attention. To demonstrate that shape recovery characteristics can be significantly enhanced through a variation of AM building strategies and process parameters beyond tuning of material compositions alone, an experimental study was developed. In-situ thermo-micro-mechanical testing was applied to capture the shape memory properties, both during shape programming and post that.
... Once characterized, the PEU was melt-extruded into a filament as essential for further processing via fused filament fabrication (FFF). For this purpose, the same extrusion line was used as reported recently [42]. The obtained filament had a homogenous diameter of 2.85 ± 0.08 mm, so that an important attribute for further processing was fulfilled. ...
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Full-text available
For soft robotics and programmable metamaterials, novel approaches are required enabling the design of highly integrated thermoresponsive actuating systems. In the concept presented here, the necessary functional component was obtained by polymer syntheses. First, poly(1,10-decylene adipate) diol (PDA) with a number average molecular weight M n of 3290 g·mol-1 was synthesized from 1,10-decanediol and adipic acid. Afterward, the PDA was brought to reaction with 4,4'-diphenylmethane diisocyanate and 1,4-butanediol. The resulting polyester urethane (PEU) was processed to the filament, and samples were additively manufactured by fused-filament fabrication. After thermomechanical treatment, the PEU reliably actuated under stress-free conditions by expanding on cooling and shrinking on heating with a maximum thermoreversible strain of 16.1%. Actuation stabilized at 12.2%, as verified in a measurement comprising 100 heating-cooling cycles. By adding an actuator element to a gripper system, a hen's egg could be picked up, safely transported and deposited. Finally, one actuator element each was built into two types of unit cells for programmable materials, thus enabling the design of temperature-dependent behavior. The approaches are expected to open up new opportunities, e.g., in the fields of soft robotics and shape morphing.
... The combination of the materials shape memory effect allows the QR code to be distorted and unscannable when the part is deformed, and scannable once the part has returned to its original shape. Challisery et al. [37] demonstrated this by 3D printing a QR code on a shape memory polymer material. In this work, the applicability of this authentication technique is addressed on the printed Diaplex 9020 SMP. ...
Article
Full-text available
Shape memory polymers (SMPs) are materials capable of changing their structural configuration from a fixed shape to a temporary shape, and vice versa when subjected to a thermal stimulus. The present work has investigated the 3D printing process of a shape memory polymer (SMP)-based polyurethane using a material extrusion technology. Here, SMP pellets were fed into a printing unit, and actuating coupons were manufactured. In contrast to the conventional film-casting manufacturing processes of SMPs, the use of 3D printing allows the production of complex parts for smart electronics and morphing structures. In the present work, the memory performance of the actuating structure was investigated, and their fundamental recovery and mechanical properties were characterized. The preliminary results show that the assembled structures were able to recover their original conformation following a thermal input. The printed parts were also stamped with a QR code on the surface to include an unclonable pattern for addressing counterfeit features. The stamped coupons were subjected to a deformation-recovery shape process, and it was observed that the QR code was recognized after the parts returned to their original shape. The combination of shape memory effect with authentication features allows for a new dimension of counterfeit thwarting. The 3D-printed SMP parts in this work were also combined with shape memory alloys to create a smart actuator to act as a two-way switch to control data collection of a microcontroller.
... In segmented polyurethane (PU), the exact chemical composition has a significant influence on phase morphology while the ratio of hard to soft segments affects phase separation [6][7][8]. Today, physically cross-linked, phase segregated block copolymers like poly(ester urethanes) (PEUs) belong to the most promising SMP families [9][10][11][12][13][14][15][16][17][18]. Here, switching can be accomplished when passing the glass transition (Ttrans = Tg) or the melting transition (Ttrans = Tm) of the polyester soft segment [19,20]. ...
Article
Full-text available
In this work, a novel type of polyester urethane urea (PEUU) foam is introduced. The foam was produced by reactive foaming using a mixture of poly(1,10–decamethylene adipate) diol and poly(1,4–butylene adipate) diol, 4,4′-diphenylmethane diisocyanate, 1,4–butanediol, diethanolamine and water as blowing agent. As determined by differential scanning calorimetry, the melting of the ester-based phases occurred at temperatures in between 25 °C and 61 °C, while the crystallization transition spread from 48 °C to 20 °C. The mechanical properties of the foam were simulated with the hyperplastic models Neo-Hookean and Ogden, whereby the latter showed a better agreement with the experimental data as evidenced by a Pearson correlation coefficient R² above 0.99. Once thermomechanically treated, the foam exhibited a maximum actuation of 13.7% in heating-cooling cycles under a constant external load. In turn, thermal cycling under load-free conditions resulted in an actuation of more than 10%. Good thermal insulation properties were demonstrated by thermal conductivities of 0.039 W·(m·K)−1 in the pristine state and 0.052 W·(m·K)−1 in a state after compression by 50%, respectively. Finally, three demonstrators were developed, which closed an aperture or opened it again simply by changing the temperature. The self-sufficient material behavior is particularly promising in the construction industry, where programmable air slots offer the prospect of a dynamic insulation system for an adaptive building envelope.
... Dans ce cas, c'est le fait que la transition vitreuse se produise sur une large gamme de température qui permet cette mémorisation multiple.ii. Propriétés de mémoire de forme dans les PUs La propriété de mémoire de forme est une caractéristique connue pour les PUs, et a largement été décrite dans la littérature.20,24,[33][34][35][36][37][38][39][40][41][42][25][26][27][28][29][30][31][32] En effet, grâce à leur T g aisément modulable ainsi que leur biocompatibilité, les PUs sont des systèmes particulièrement prometteurs pour des applications biomédicales. ...
Thesis
Cette thèse porte sur l’application du procédé d’extrusion réactive à la synthèse, sans solvant, de polyhydroxyuréthanes (PHUs). D’une part, des PHUs thermoplastiques ont été synthétisés à partir de trois biscarbonates cycliques à 5 chaînons, activés ou non par des fonctions ester ou éther en béta du cycle, et différentes diamines. La conversion totale des fonctions réactives a été atteinte dans la majorité des cas en des temps courts (quelques heures), malgré le caractère très cohésif des substrats biscarbonates employés et notamment ceux comportant un lien amide. D’autre part, via ce même procédé d’extrusion réactive, différents PHUs réticulés qui présentent des propriétés de mémoire de forme ou encore de reprocessabilité ont été synthétisés. En parallèle, une étude via des réactions modèles menées hors extrudeuse a permis de mettre en évidence les conditions expérimentales permettant de fortement limiter la formation d’un produit secondaire de type urée. Dans le cas particulier du dicarbonate de diglycerol (DGDC), un protocole de purification par recristallisation a été mis au point de façon à séparer ses deux formes énantiomères. La polymérisation des deux énantiomères séparés, avec différentes diamines, a révélé que la stéréochimie du monomère biscarbonate joue un rôle déterminant sur les dimensions et les caractéristiques thermomécaniques des PHUs finaux.
... A full shape memory cycle includes two steps, namely programming, which is to fix the temporary shape, and recovery, which is to apply the stimulus to active the SME. From a real engineering application point of view, such as in active disassembly [31,32], deployable structures [33] and anti-counterfeit applications [34][35][36][37][38][39][40], activation of the SME may not be carried out right after programming, but after a period of storage. As such, we need to consider the influence of aging at around room temperature after programming [41], which is a topic that has been less explored so far [42,43], but that is utterly important from an engineering application point of view. ...
Article
Full-text available
In this paper, we experimentally investigate the influence of storage at 40 °C on the shape memory performance and mechanical behavior of a pre-stretched commercial poly(methyl methacrylate) (PMMA). This is to simulate the scenario in many applications. Although this is a very important topic in engineering practice, it has rarely been touched upon so far. The shape memory performance is characterized in terms of the shape fixity ratio (after up to one year of storage) and shape recovery ratio (upon heating to previous programming temperature). Programming in the mode of uniaxial tension is carried out at a temperature within the glass transition range to one of four prescribed programming strains (namely 10%, 20%, 40% and 80%). Also investigated is the residual strain after heating for shape recovery. The characterization of the mechanical behavior of programmed samples after storage for up to three months is via cyclic uniaxial tensile test. It is concluded that from an engineering application point view, for this particular PMMA, programming should be done at higher temperatures (i.e., above its Tg of 110 °C) in order to not only achieve reliable and better shape memory performance, but also minimize the influence of storage on the shape memory performance and mechanical behavior of the programmed material. This finding provides a useful guide for engineering applications of shape memory polymers, in particular based on the multiple-shape memory effect, temperature memory effect, and/or low temperature programming.
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Development of fiber-spinning technologies and materials with proper mechanical properties is highly important for manufacturing of aligned fibrous scaffolds mimicking structure of the muscle tissues. Here, we report touch spinning of a thermoplastic poly(1,4-butylene adipate)-based polyurethane elastomer, obtained via solvent-free polymerization. This polymer possesses a combination of important advantages such as (i) low elastic modulus in the range of a few MPa, (ii) good recovery ratio and (iii) resilience, (iv) processability, (v) non-toxicity, (vi) biocompatibility and (vii) biodegradability that makes it suitable for fabrication of structures mimicking extracellular matrix (ECM) of muscle tissue. Touch spinning allows fast and precise deposition of highly aligned micro and nanofiber without use of high voltage. C2C12 myoblasts readily align along soft polymer fibers and demonstrate high viability as well as proliferation that makes proposed combination of polymer and fabrication method highly suitable for engineering skeletal muscles. This article is protected by copyright. All rights reserved
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3D Printing has become a powerful technology for future advanced manufacturing, however the choice of materials with good performance is still limited. In this research, a type of hybrid photopolymer resin based on silicone epoxy with high UV curing rate and low viscosity was applied in stereolithography (SLA) technology. First, aliphatic silicone epoxy was synthesized through hydrosilylation reaction, and then compounding with acrylates, cycloaliphatic epoxy and photo‐initiators, finally the hybrid photopolymer system for SLA was obtained. The features of UV curing process were studied through photo‐differential scanning calorimetry and real‐time Fourier transform infrared measurements. Scanning electron microscopy test showed interpenetrating polymer network structure was formed in 3D objects. The performances of 3D printed sample such as mechanical properties, thermal mechanical and stability properties were also investigated in detail. Moreover, the 3D printed objects with complicated structures showed high printing accuracy. This new type of hybrid photopolymer system based on aliphatic silicone epoxy was with fabrication ease, good mechanical properties, good thermal stability and high printing resolution and has great potential of applications in industrial design and models making fields.
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Poly(ester urethane)s represent a widely investigated class of thermoplastic polymers that exhibit a thermally triggered dual shape memory effect. This behavior is the result of urethane‐rich hard segments that define the permanent shape, while domains formed by the crystallizable polyester segments act as the memory switch. We show here that blending poly(ester urethane)s with a second polyester having a different melting temperature is a straightforward and possibly general approach to create triple‐shape memory polymers, in which two different temporary shapes can be programmed. To demonstrate this, we blended a poly(ester urethane) containing crystallizable poly(1,4‐butylene adipate) segments with poly(butylene succinate) or poly(hexamethylene dodecanoate). The blends microphase separate and the different polyester segments form separate semicrystalline domains, which serve as switching elements that can be activated at different temperatures. The blends retain attractive mechanical properties and the shape memory characteristics are characterized by high fixities (70%–96%) and recovery rates (82%–94%).
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Extrusion-based additive manufacturing (EBAM) or 3D printing is used to produce customized prototyped parts. The majority of the polymers used with EBAM show moisture sensitivity. However, moisture effects become more pronounced in polymers used for critical applications, such as biomedical stents, sensors, and actuators. The effects of moisture on the manufacturing process and the long-term performance of Shape Memory Polyurethane (SMPU) have not been fully investigated in the literature. This study focuses primarily on block-copolymer SMPUs that have two different hard/soft (h/s) segment ratios. It investigates the effect of moisture on the various properties via studying: (i) the effect of moisture trapping within these polymers and the consequences when manufacturing; (ii) and the effect on end product performance of plasticization by moisture. Results indicate that higher h/s SMPU shows higher microphase separation, which leads to an increase of moisture trapping within the polymer. Understanding moisture trapping is critical for EBAM parts due to an increase in void content and a decrease in printing quality. The results also indicate a stronger plasticizing effect on polymers with lower h/s ratio but with a more forgiving printing behavior compared to the higher h/s ratio.
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Material choice is a fundamental consideration when it comes to designing a solid dosage form. The matrix material will ultimately determine the rate of drug release since the physical properties (solubility, viscosity, and more) of the material control both fluid ingress and disintegration of the dosage form. The bulk properties (powder flow, concentration, and more) of the material should also be considered since these properties will influence the ability of the material to be successfully manufactured. Furthermore, there is a limited number of approved materials for the production of solid dosage forms. The present study details the complications that can arise when adopting pharmaceutical grade polymers for fused-filament fabrication in the production of oral tablets. The paper also presents ways to overcome each issue. Fused-filament fabrication is a hot-melt extrusion-based 3D printing process. The paper describes the problems encountered in fused-filament fabrication with Kollidon® VA64, which is a material that has previously been utilized in direct compression and hot-melt extrusion processes. Formulation and melt-blending strategies were employed to increase the printability of the material. The paper defines for the first time the essential parameter profile required for successful 3D printing and lists several pre-screening tools that should be employed to guide future material formulation for the fused-filament fabrication of solid dosage forms.
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The rapid development of additive manufacturing and advances in shape memory materials have fueled the progress of four-dimensional (4D) printing. With the right external stimulus, the need for human interaction, sensors, and batteries will be eliminated, and by using additive manufacturing, more complex devices and parts can be produced. With the current understanding of shape memory mechanisms and with improved design for additive manufacturing, reversibility in 4D printing has recently been proven to be feasible. Conventional one-way 4D printing requires human interaction in the programming (or shape-setting) phase, but reversible 4D printing, or two-way 4D printing, will fully eliminate the need for human interference, as the programming stage is replaced with another stimulus. This allows reversible 4D printed parts to be fully dependent on external stimuli; parts can also be potentially reused after every recovery, or even used in continuous cycles—an aspect that carries industrial appeal. This paper presents a review on the mechanisms of shape memory materials that have led to 4D printing, current findings regarding 4D printing in alloys and polymers, and their respective limitations. The reversibility of shape memory materials and their feasibility to be fabricated using three-dimensional (3D) printing are summarized and critically analyzed. For reversible 4D printing, the methods of 3D printing, mechanisms used for actuation, and strategies to achieve reversibility are also highlighted. Finally, prospective future research directions in reversible 4D printing are suggested.
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Additive manufacturing (AM) alias 3D printing translates computer-aided design (CAD) virtual 3D models into physical objects. By digital slicing of CAD, 3D scan, or tomography data, AM builds objects layer by layer without the need for molds or machining. AM enables decentralized fabrication of customized objects on demand by exploiting digital information storage and retrieval via the Internet. The ongoing transition from rapid prototyping to rapid manufacturing prompts new challenges for mechanical engineers and materials scientists alike. Because polymers are by far the most utilized class of materials for AM, this Review focuses on polymer processing and the development of polymers and advanced polymer systems specifically for AM. AM techniques covered include vat photopolymerization (stereolithography), powder bed fusion (SLS), material and binder jetting (inkjet and aerosol 3D printing), sheet lamination (LOM), extrusion (FDM, 3D dispensing, 3D fiber deposition, and 3D plotting), and 3D bioprinting. The range of polymers used in AM encompasses thermoplastics, thermosets, elastomers, hydrogels, functional polymers, polymer blends, composites, and biological systems. Aspects of polymer design, additives, and processing parameters as they relate to enhancing build speed and improving accuracy, functionality, surface finish, stability, mechanical properties, and porosity are addressed. Selected applications demonstrate how polymer-based AM is being exploited in lightweight engineering, architecture, food processing, optics, energy technology, dentistry, drug delivery, and personalized medicine. Unparalleled by metals and ceramics, polymer-based AM plays a key role in the emerging AM of advanced multifunctional and multimaterial systems including living biological systems as well as life-like synthetic systems.
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Additive manufacturing, commonly referred to as 3D printing (3DP), has ushered in a new era of advanced manufacturing that is seemingly limited only by imagination. In actuality, the fullest potentials of 3DP can only be realized through innovative breakthroughs in printing technologies and build materials. Whereas equipment for 3DP has experienced considerable development, molecular-scale programming of function, adaptivity, and responsiveness in 3DP is burgeoning. This review aims to summarize the state-of-the-art in stimuli-responsive materials that are being explored in 3DP. First, we discuss stimuli-responsiveness as it is used to enable 3DP. This highlights the diverse ways in which molecular structure and reactivity dictate energy transduction that in turn enables 3D processability. Second, we summarize efforts that have demonstrated the use of 3DP to create materials, devices, and systems that are in their final stage stimuli-responsive. This section encourages the artistic license of advanced manufacturing to be applied toward leveraging, or enhancing, energy transduction to impart device function across multiple length scales.
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Hierarchically structuring materials open the door to a wide range of unexpected and uniquely designed properties. This work presents a novel mechanical metamaterial unit cell with strain‐dependent solid–solid phase changes resultant from hierarchically structured “mechanisms” built into an auxetic unit cell, and further presents a realization of this kind. The interaction of auxetic structure and mechanism allows stable or metastable elastic energy states to be reached as a result of mechanical deformation. The result is a principally elastic analog to a shape memory material with a functional dependency on its negative Poisson's ratio. Prototypes are additively manufactured using direct laser writing, and are subsequently subjected to uniaxial compression with a customized micromechanical test set up. Experimental results depict reversible states initially triggered by deformation; the unit cell is a building block for a programmable material with a nonlinear “if… then…” relationship. Implementing interior mechanisms as a hierarchical level unlocks new directions for mechanical metamaterials research, and the authors see potential impacts or applications in multi‐scale modeling, medicine, micro‐actuation and ‐gripping, programmable matter/materials. The work presented herein depicts an auxetic metamaterial unit cell displaying a time‐dependent deformation behavior analogous to conventional shape memory materials, but achieves this through the structurally hierarchical embedding of a “mechanism”. This proof‐of‐concept paper is a step in the direction of programmable materials, which are materials with reversible, functionalized properties triggered by external stimuli such as a force.
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Four-dimensional (4D) printing has great potential for fabricating patient-specific, stimuli-responsive 3D structures for the medical sector. Porous Shape memory polymers have high volumetric expansion and enhanced biological activity, which make them as ideal candidates for implant materials through minimally invasive surgical procedures. The objective of the present work is to develop a radiopaque, porous, and custom shaped shape memory polyurethane (SMPU) for its application in endovascular embolization. In this paper, the porous SMPU was fabricated by combining extrusion, fused filament fabrication (FFF) and salt leaching. The filament for FFF was produced by extruding the mixture of SMPU, NaCl, and Tungsten at the desired composition. The 3D printed and salt leached porous SMPU was observed to have the porosity in the range of 32.7 - 36% and pore sizes of <250 µm with the interconnected network. The porous Tungsten SMPU showed the improved radiopacity, increased storage modulus, and also, excellent shape holding and shape recovery up to 100%. The feasibility of combining fused filament fabrication and salt leaching technique was established for fabricating the radiopaque porous SMPU having the required characteristics for embolization, which can be explored by the Interventional Radiologist.
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Purpose Material extrusion additive manufacturing, also known as fused deposition modeling, is a manufacturing technique in which objects are built by depositing molten materials layer-by-layer through a nozzle. The use and application of this technique has risen dramatically over the past decade. This paper aims to first, report on the production and characterization of a shape memory polymer material filament that was manufactured to print shape memory polymer objects using material extrusion additive manufacturing. Additionally, it aims to investigate and outline the effects of major printing parameters, such as print orientation and infill percentage, on the elastic and mechanical properties of printed shape memory polymer samples. Design/methodology/approach Infill percentage was tested at three levels, 50, 75 and 100 per cent, while print orientation was tested at four different angles with respect to the longitudinal axis of the specimens at 0°, 30°, 60° and 90°. The properties examined were elastic modulus, ultimate tensile strength and maximum strain. Findings Results showed that print angle and infill percentage do have a significant impact on the manufactured test samples. Originality/value Findings can significantly influence the tailored design and manufacturing of smart structures using shape memory polymer and material extrusion additive manufacturing.