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Surfaces, Interfaces, and Applications

Transformation of 2D Planes into 3D Soft and Flexible Structures with Embedded Electrical Functionality Hyunmin Moon, Namsun Chou, Hee Won Seo, Kyeongyeon Lee, Jinhee Park, and Sohee Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09578 • Publication Date (Web): 21 Aug 2019 Downloaded from pubs.acs.org on August 21, 2019

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ACS Applied Materials & Interfaces

1

Transformation of 2D Planes into 3D Soft and

2

Flexible

3

Functionality

4

AUTHOR NAMES

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Hyunmin Moon,† Namsun Chou,‡ Hee Won Seo,† Kyeongyeon Lee,† Jinhee Park,§ and

6

Sohee Kim*,†

7

AUTHOR ADDRESS

8

†Department

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Technology (DGIST), Daegu, 42988, Republic of Korea

Structures

with

Embedded

Electrical

of Robotics Engineering, Daegu Gyeongbuk Institute of Science and

10

‡Center

for BioMicroSystems, Korea Institute of Science and Technology (KIST), Seoul,

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02792, Republic of Korea

ACS Paragon Plus Environment

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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§Department

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Technology (DGIST), Daegu, 42988, Republic of Korea

3

KEYWORDS

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flexible and soft 3D structure, selective bonding, PDMS, parylene-C, MEMS

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of Emerging Materials Science, Daegu Gyeongbuk Institute of Science and

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ABSTRACT

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Three-dimensional (3D) structures composed of flexible and soft materials have been in

3

demand for implantable biomedical devices. However, the fabrication of 3D structures

4

using MEMS techniques has limitations in terms of the materials and the scale of the

5

structures. Here, a technique to selectively bond polydimethylsiloxane (PDMS) and

6

parylene-C by plasma treatment is reported, with which 2D structures that are fabricated

7

using micro electro mechanical systems (MEMS) techniques are turned into 3D structures

8

by the inflation of selectively non-bonded patterns. The bonding strength and the bonding

9

mechanism were analyzed by mechanical tests and chemical analyses, respectively. We

10

fabricated soft and flexible 3D structures with various patterns and dimensions, even with

11

embedded electrical functions, including light emitting diodes and electrocorticogram

12

electrodes. Based on these results, the flexible, soft, and MEMS-capable 3D structures

13

that are obtained by the developed selective bonding technique are promising for

14

applications in a wide range of biomedical applications.


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TEXT

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1. Introduction

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Three-dimensional (3D) structures are desirable for many applications, such as

4

biomedical electrodes,1-6 biomedical scaffolds,7-10 microelectromechanical systems

5

(MEMS),11-14 batteries,15-19 and further novel structure fabrication.20-22 3D structures with

6

various functions and dimensions, from the nanoscale to the microscale, have satisfied

7

the requirements for research purposes. Despite such efforts towards the generation of

8

3D structures, there are still problems that remain.23,24 The types of materials that are

9

used for MEMS fabrications, such as photolithography, are limited to inorganic silicon.

10

Additionally, because MEMS techniques conventionally aim for elaboration and

11

miniaturization, the feature size of the generated 3D structures is limited to the nanoscale

12

to microscale ranges.

13

Recently, as a solution for the limited scale of 3D structures, a membrane of silicon

14

(as a body) and a polymer of SU-8 (as an epoxy) were used to generate 3D structures.24,25

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The MEMS techniques were processed on a two-dimensional (2D) plane, and


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mesostructures were fabricated by induced compressive buckling. However, although

2

fabricating mesoscale structures was possible, the selection of materials is still limited to

3

inorganic materials, namely, silicon. On the other hand, Whiteside and his colleagues

4

have fabricated a 3D structure for a gaiting robot entirely consisting of soft materials such

5

as polydimethylsiloxane (PDMS) and Ecoflex.26,27 They fabricated the 3D structure using

6

a molding technique and bonding of the top and bottom layers. However, the elaborate

7

alignment between the top and bottom layers during bonding was challenging, and further

8

MEMS processes on the fabricated device were not possible.

9

In particular, in the study of implantable biomedical devices such as neural interfaces,

10

biocompatibility is one of the essential considerations for the selection of materials.

11

Moreover, because of the mechanical properties similar to biological tissues, flexible and

12

soft polymers are given priority over stiff and inorganic materials as the device material.

13

Hence, we developed a novel technique that can transform 2D planar structures into 3D

14

structures by selective bonding between PDMS and parylene-C through plasma

15

treatment on the selected areas. Both materials have the highest biocompatibility of USP


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class VI, high flexibility, and low stiffness. Moreover, this technique enables bonding in

2

one process because plasma is selectively treated on the top surface of the entire

3

structure, unlike other techniques that require bonding of two layers with elaborate

4

alignment. For this reason, additional MEMS processes are possible after the bonding

5

between layers. After extensive mechanical and chemical analyses, we fabricated various

6

3D structures by the inflation of non-bonded areas (hereafter referred to as ‘balloons’)

7

created by the selective bonding technique. Moreover, 3D structures including electrical

8

functions, such as light emitting diodes (LEDs) or electrocorticogram (ECoG) electrodes,

9

were also fabricated by using MEMS processes after the selective bonding step. All

10

electrical connections remained intact after the inflation of balloons, resulting in

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electrically functional, soft and flexible 3D structures in mesoscale. With flexible, soft, and

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biocompatible properties of the used materials, we expect that the flexible 3D electronic

13

devices fabricated by the selective bonding technique can be used for various biomedical

14

applications.


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2. Results and discussion

2

2.1. Soft and flexible 3D structures that integrate electrical functions. Figure 1a shows

3

the principle to fabricate a 3D soft and flexible structure with embedded conductive lines.

4

Owing to the flexibility and stretchability of the used materials, the non-bonded area

5

inflates upon fluid injection, creating a 3D structure. Prior to the fluid injection, conductive

6

lines and patterns can be formed on the 2D plane, just as the conventional MEMS

7

processes. At the bonded area, the PDMS layer adheres covalently onto the parylene-C

8

layer because the hydroxyl and carboxyl groups are terminated at the surface of PDMS

9

and parylene-C, respectively. These groups form a covalent bond between PDMS and

10

parylene-C. More details of the bonding mechanism are presented in Section 2.4. Figure

11

1b shows the method of selective bonding between PDMS and parylene-C. The whole

12

structure consists of a PDMS layer, a parylene-C layer, and a second PDMS layer with

13

thicknesses of 180 μm, 1 μm, and 180 μm, respectively. The first PDMS layer and

14

parylene-C layer adhere firmly during the deposition of parylene-C onto PDMS. Generally,

15

PDMS on the parylene-C layer does not adhere well due to the hydrophobicity of


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parylene-C. However, we developed a technique for selective bonding between PDMS

2

and parylene-C using nitrogen and oxygen plasma treatment. By masking the plasma,

3

non-bonded areas are generated where PDMS and parylene-C can easily detach from

4

each other if enough force is applied. Figure 1c shows the method of fluid injection into

5

the non-bonded areas. A soft and flexible 3D structured device is demonstrated, which

6

was generated by the injection of red-stained liquid into the non-bonded area (Movie S1).

7

More details regarding the methods of selective bonding and fluid injection are explained

8

in Figures S1 and S2, respectively.

9

2.2. Adhesion evaluation depending on plasma conditions. Plasma conditions

10

(nitrogen flow rate, oxygen flow rate, power, and treatment time) enormously affect the

11

bonding strength. We investigated the different plasma conditions as summarized in

12

Table S1. Samples were evaluated by T-peeling tests following ASTM D1876-08 (n=3 for

13

each plasma condition).28 The T-peeling test method is explained in Experimental Section.

14

The samples were prepared such that the region with a length of 15 mm was not treated

15

by plasma and the region with a length of 20 mm was treated by plasma (Figure 2a). We


8

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1

measured the peeling force between the second PDMS layer and the parylene-C layer,

2

as shown in Figure 2b-e. A little force was measured during the first 30 mm of stage

3

movement, in which the non-treated area of the sample was peeled off. The force then

4

started to increase with no detachment of the bonded area at the ‘location of detachment’,

5

where the plasma-treated area began. After that, no increase in force was observed

6

during detachment of the plasma-treated area and then, the force abruptly increased

7

before the layers were entirely detached, which indicates the accumulated force by the

8

stretching of the PDMS layer. In the insets of Figure 2b-e, the forces measured at the

9

moment of detachment are presented. In terms of nitrogen flow rate, there was a tendency

10

for the larger amount of nitrogen gas to generate stronger bonding (Figure 2b). On the

11

other hand, among the oxygen flow rates of 0, 5, 10, 15, 20, 25 and 50 sccm that were

12

used, 10 sccm produced the strongest bonding (Figure 2c). It was assumed that a large

13

amount of oxygen gas produces silica on the surface of the second PDMS layer, which

14

inhibits the penetration of plasma through the PDMS layer. Low oxygen flow rate reduced

15

the reactions at the parylene-C surface by plasma treatment. As the plasma power and

16

treatment time are related to plasma energy, higher plasma energy corresponded to


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stronger bonding (Figure 2d,e). In particular, 200 W of plasma power generated a strong

2

bonding, even underneath the masked area: 6 mm of the masked area from the ‘location

3

of detachment’ was bonded with this strong plasma power. Although high plasma power

4

generates stronger bonding, it can be a problem in creating fine patterns.

5

In addition, we defined five bonding levels according to the observed detachment

6

behaviors after T-peeling tests: over-bonding, critical bonding, partial bonding, increased

7

adhesion, and no bonding. From the T-peeling tests, it was observed that the location of

8

PDMS breakage was different depending on the five bonding levels (Figure 2f-k). We

9

changed the nitrogen flow rate and plasma power, while the oxygen flow rate and

10

treatment time were fixed to be 10 sccm and 30 minutes, respectively. The plasma

11

conditions used to generate each bonding level are presented in Figure S3a. In the over-

12

bonded and critically bonded samples, the bonding between PDMS and parylene-C was

13

generated sharply at the edge of the masked area (Figure 2g,h). But, the over-bonded

14

samples showed the increased force underneath the masked area during T-peeling tests.

15

In the samples with partial bonding, the second PDMS layer was broken in the middle


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region of the plasma treated area (Figure 2i). Lastly, in the samples with the bonding level

2

of increased adhesion or no bonding, the whole PDMS layer was detached from the

3

parylene-C layer because of weak bonding between them (Figure 2j,k). The samples with

4

increased adhesion showed a certain amount of tension in the samples during the T-

5

peeling test, whereas the samples with no bonding level did not. For precise patterning

6

of bonded areas, critical bonding is desired. Figure S3b shows the plasma conditions

7

used to achieve critical bonding.

8

2.3. Quantitative analysis of bonding strength. To measure the quantitative strength

9

of bonding in the orthogonal direction, we conducted pull-off tests following ASTM D4541-

10

09.29 The pull-off test method is explained in Experimental Section. A dolly was pulled

11

with a pressure rate of 0.2 MPa/sec using an adhesion tester (Figure 3a). The maximum

12

bonding strength was measured following the predefined five bonding levels of over-

13

bonding, critical bonding, partial bonding, increased adhesion, and no bonding, which

14

were 4.02±0.21, 3.95±0.21, 2.92±0.24, 2.35±0.11, and 1.19±0.11 MPa, respectively (n

15

≥9 for each bonding level) (Figure 3b). As expected, over-bonded samples showed the


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strongest bonding strength, and slightly low values were obtained in the critically bonded

2

samples. The sample with no bonding level showed the weakest bonding strength. The

3

bonded interface between the second PDMS layer and the parylene-C layer was exposed

4

after pull-off tests as shown in Figure 3c-g. Outside of the dolly, the second PDMS layer

5

was intact, covering the parylene-C layer. In the samples with increased adhesion or no

6

bonding levels, the pull-off test resulted in the transparent surface of exposed parylene-

7

C, because the force was not exerted at the interface of the PDMS and parylene-C during

8

the pull-off test. When the parylene-C layer was stretched by a relatively weak bonding

9

strength, the transparent parylene-C layer was transformed to an opaque surface in the

10

samples with partial bonding and increased adhesion levels. The strong bonding strength

11

in the samples with critical bonding and over-bonding levels generated a surface with

12

torn-out parylene-C. More damages in the parylene-C layer were observed as the

13

bonding strength increased, even the second PDMS layer remained partially in the over-

14

bonded samples. The exposed surfaces were classified into five morphologies according

15

to the five bonding levels: intact PDMS surface, transparent parylene-C surface, opaque

16

parylene-C surface, torn-out parylene-C surface, and remaining PDMS surface.


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2.4. Chemical analysis of the bonded interface. To investigate the mechanism of

2

bond formation between PDMS and parylene-C upon the plasma treatment, five different

3

locations of the exposed surfaces after pull-off tests were analyzed by X-ray

4

photoelectron spectroscopy (XPS). The chemical structures of PDMS and parylene-C are

5

shown in Figure S4. PDMS and parylene-C layers can be recognized by detecting silicon

6

(Si) and chlorine (Cl) peaks, respectively. In the intact PDMS surface and remaining

7

PDMS surface, carbon (C), Si, and oxygen (O) peaks, which compose the PDMS, were

8

detected. In these locations, the mechanism of bond cannot be analyzed because the

9

bonded interface activated by plasma treatment was not exposed. Cl peaks were found

10

at the transparent surface, the opaque surface, and the torn-out parylene-C surface,

11

where the second PDMS layer did not cover the parylene-C layer after the pull-off test

12

(Figure 4a). For more accurate analysis, C and Cl peaks at the parylene-C exposed

13

surfaces are magnified in Figure 4b,c. In the C (1s) spectrum (Figure 4b), the transparent

14

surface that did not make the bond with PDMS contained a peak at 285.1 eV arisen from

15

C-C bond. In the opaque surface with relatively weak bonding and the torn-out parylene-C

16

surface with strong bonding, a new peak at 290.8 eV appeared, indicating the C-O bond


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formation.30,31 In particular, the intensity ratios of the two peaks at 290.8 eV to 285.1 eV

2

were 0, 0.86, and 1.64 at the transparent, opaque, and torn-out parylene-C surfaces,

3

respectively, implying more oxidized parylene-C resulted in stronger bonding. In the Cl

4

(2p) spectrum (Figure 4c), the peaks at 200.4 eV (Cl 2p3/2) and 202.0 eV (Cl 2p1/2)

5

appeared at the transparent surface.32 In addition, the intense peaks at 204.8 eV (Cl 2p3/2)

6

and 207.0 eV (Cl 2p1/2) were observed at the opaque and torn-out parylene-C surfaces,

7

which presumably implies that the migrated Cl atoms from parylene-C to PDMS form the

8

additional bonds with O on the oxidized PDMS side by the plasma treatment.33,34 The

9

intensity ratios of the two peaks at 207.0 eV to 200.4 eV increased according to the

10

bonding strength.

11

The cross-sectional areas of the over-bonded samples were also analyzed by

12

transmission electron microscopy energy dispersive X-ray spectroscopy (TEM-EDS). For

13

TEM-EDS analysis, the samples are required to have a thickness under 200 μm. Hence,

14

we prepared the bonded samples by the cryosectioning method detailed in Figure S5.

15

The surface and line analyses of the cross-sectional samples were conducted as shown


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in Figure S6. Figure 5a shows the cross-sectional analysis of an over-bonded sample.

2

We could certainly distinguish the first PDMS layer (parylene-C deposited PDMS), the

3

parylene-C layer, and the second PDMS layer (plasma-treated PDMS) through a TEM

4

image. The distribution of Si, O, C, and Cl atoms depended on the chemical structure of

5

PDMS and parylene-C. Si and O atoms were dispersed only inside PDMS while C atoms

6

existed in both PDMS and parylene-C. Interestingly, Cl atoms of parylene-C were

7

detected at the upper and lower interfaces between PDMS and parylene-C. As parylene-

8

C was infiltrated into PDMS during deposition,35,36 Cl atoms were expected to be detected

9

at the interface between parylene-C-deposited PDMS and parylene-C. On the other hand,

10

at the interface between plasma-treated PDMS and parylene-C, Cl atoms diffused into

11

PDMS by the nitrogen and oxygen plasma treatment, leading to the formation of an

12

overlapping narrow region with Si and Cl atoms. To see the interface between plasma-

13

treated PDMS and parylene-C more clearly, we conducted line scanning for the cross-

14

section of over-bonded samples (Figure 5b). A distance of 0.5 μm represents the interface

15

between plasma-treated PDMS and parylene-C. In parylene-C, only C was observed

16

while Si, Cl, O and C were detected inside the PDMS. Interestingly, in the region spanning


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from the interface to approximately 0.5 μm into the PDMS, increased intensities of Cl and

2

O were measured. Therefore, we speculate that the migrated Cl reacts with the oxidized

3

PDMS during plasma treatment. Consequently, based on the overall evidences from XPS

4

and TEM-EDS analyses, we propose that the strong covalent bonds were formed

5

between oxidized parylene-C with carboxyl groups37,38 and PDMS with hydroxyl

6

groups39,40 (Figure 5c). Additionally, the C-Cl bonds of parylene-C were partially broken,

7

and this broken Cl newly formed the bonds with O in the oxidized PDMS.

8

2.5. Applications. Using the developed selective bonding technique, we generated

9

soft and flexible 3D mesostructures with various dimensions and fluidic channel patterns.

10

Either air or dyed water was injected into the non-bonded areas for the generation of 3D

11

structures. Figure 6a shows a 3D balloon-shaped structure with a diameter of 20 mm.

12

Figure 6b shows an array of balloons with 5 mm in diameter, where nine balloons were

13

connected through a fluidic channel. Figure 6c shows an array of nine balloons but with

14

three fluidic channels, so that three different fluids can be injected into different balloons.

15

Figure 6d-f show various 3D curved structures with the shapes of normal bending,


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1

trapezoidal bending, and spiral bending, respectively. The force of fluid injection into the

2

balloons created the curved shapes. As shown in the figures, the 3D curved structures

3

were capable of gripping objects with different shapes. The normally bent structure

4

enveloped a tube in a transverse direction and lifted it up. The trapezoidally bent structure

5

could wrap a funnel with inclined surface and pick it up. The spiral structure wrapped a

6

tube in a longitudinal direction and lifted it up.

7

For the integration of electrical functions, we fabricated the 3D structured devices

8

embedding LEDs or ECoG electrodes. Figure 7a,b show three-dimensionally bent

9

structures consisting of five balloons and an inflated balloon, respectively, with

10

enlightened LEDs. All LEDs lit up before and after considerable air injection into the

11

balloons. These lighted LEDs confirmed no disconnection of conductive metal lines after

12

the inflation of balloons. As another application, Figure 8a shows the ECoG electrode

13

array that formed a conformal contact with the cortical surface of a model brain. A stained

14

liquid was injected into the non-bonded area of the ECoG electrode array, clearly

15

demonstrating the conformal contact between the electrodes and the cortical surface, not


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only on the gyrus but also on the sulcus. The soft and flexible 3D structured ECoG

2

electrodes, in conformal contact with the cortical surface, were evaluated in terms of

3

ECoG recording. Seven recording electrodes in the ECoG electrode array were

4

connected to a signal acquisition system through a flexible printed circuit board (FPCB)

5

cable. The emulated action potentials with three different waveforms were injected into

6

the model brain made of agarose. These emulated and recorded signals were band-pass

7

filtered with cuff-off frequency of 1000-8000 Hz. Figure 8b shows the injected and

8

recorded action potentials by the ECoG electrodes. All seven recording electrodes

9

detected the emulated action potentials. The signal to noise ratio (SNR) of the measured

10

signals was 6.62 on average. In addition, three different waveforms could be classified

11

from all seven electrodes because of this high SNR. It is because the developed 3D ECoG

12

electrode array could form conformal contact with the curved cortical surface by filling the

13

space inside the skull stably.

14

3. Conclusion


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1

Using the selective bonding technique, MEMS processed 2D planes could be turned into

2

3D soft and flexible structures with various sizes and patterns of non-bonded areas. The

3

proposed technique generates bonded and unbonded areas between PDMS and

4

parylene-C layers selectively, using plasma treatment on the top surface of the entire

5

structure. After all processes are done on a 2D plane, the selectively bonded 2D structure

6

is then transformed into a 3D structure upon fluid injection into the unbonded area.

7

Previously, to generate soft 3D structures in the mesoscale, the techniques to transfer

8

and stamp patterns on pre-strained substrates or manual bonding of two polymeric layers

9

using glue were used.24-27 Unlike those previous methods, using the proposed technique,

10

the necessary treatment to bond layers is performed only at the top surface of the entire

11

structure consisting of multiple polymeric layers, not at the interface to be bonded. This

12

in turn enables further MEMS processes to fabricate metal patterns in the 2D plane before

13

turning it into the 3D structure, thereby resulting in soft and flexible mesoscale 3D

14

structures with embedded metal patterns in the micrometer scale. However, a few

15

limitations are also identified. As an individual balloon pattern requires a fluid injection

16

inlet composed of small PDMS pieces, the presence of many individual balloons would


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1

enlarge the entire structure. To achieve the critically bonded interface, the thickness of

2

the second PDMS layer is required to be 180 μm or lower. Additionally, to create stable

3

3D structures and not to burst the fluidic channel during fluid injection, the smallest width

4

of the fluidic channel is limited to 500 μm. As the fluidic channel acts as a resistance

5

against fluid flow, a narrow fluidic channel induces a higher pressure than a wide channel.

6

On the other hand, in a state of moderate pressure inside the fluidic channel, the fluid can

7

be injected into the non-bonded areas with various dimensions, that is, from wafer-scale

8

to several hundreds of micrometers.

9

Because PDMS and parylene-C are flexible, soft, and biocompatible, the devices

10

fabricated using this selective bonding technique can be used for implantable biomedical

11

devices. A 3D balloon-shaped or curved device can stably fit to various locations with

12

cavities filled with body fluids. In particular, the fabrication of electrodes or metal patterns

13

using MEMS techniques enables the 3D devices generated by selective bonding to

14

function as various sensors or actuators. Furthermore, if various drugs are injected into

15

the non-bonded areas of the selectively bonded 2D structures, instead of air or water, the


20

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1

device can contain drugs for a certain period of time and deliver them to the implanted

2

site. Consequently, many novel applications of 3D soft and flexible devices that are

3

fabricated using the proposed bonding technique are expected in the field of implantable

4

biomedical devices, ranging from neural interfaces to drug delivery systems.

5

4. Experimental section

6

4.1. Selective bonding between PDMS and parylene-C. PDMS (Sylgard 184, Dow

7

Corning Corp., Midland, MI, USA) and parylene-C (parylene-C dimer, Nuri-Tech Corp.,

8

Incheon, Korea) were used for the generation of soft and flexible 3D structures. The

9

structure was composed of three layers in total: the first PDMS layer, the parylene-C

10

layer, and the second PDMS layer. The PDMS was mixed at a weight ratio of 1:10 (curing

11

agent:monomer) and degassed in a desiccator. The first PDMS layer was spin coated for

12

60 seconds at 500 rpm and cured in a dry oven at 120 °C for 1 hour. A 1 μm parylene-C

13

layer was deposited using a parylene coater (NRPC-500, Nuri-Tech Corp., Incheon,

14

Korea). The parylene-C layer firmly adhered to the first PDMS layer as it was deposited

15

onto the first PDMS layer through chemical vapor deposition.35,36 Then, the second PDMS


21


Page 22 of 52

1

layer was spin-coated at 500 rpm for 60 seconds and cured in a dry oven at 120 °C for 1

2

hour. The bonding area was patterned by masking the second PDMS layer with a PVC

3

film that was patterned using a film cutting plotter (CE5000-40-CRP, Graphtec Corp.,

4

Yokohama, Japan). For the generation of selective bonding between the second PDMS

5

layer and the parylene-C layer, the structure was treated by nitrogen and oxygen mixed

6

plasma using reactive ion etching, RIE (VITA, Femto Science, Gyeonggi-do, Korea). For

7

precise patterning of the bonded area, we needed to achieve critical bonding among five

8

classified bonding levels (see Figure 2f-k). The critical bonding level was obtained using

9

a nitrogen flow rate of 200 sccm, an oxygen flow rate of 10 sccm, power of 150 W, and

10

treatment time of 30 minutes (Figure S3). The detailed processes are explained in Figure

11

S1.

12

4.2. Generation of a 3D structure by fluid injection. After the fabrication of selectively

13

bonded samples in a 2D plane, fluid injection into the non-bonded area turns them into

14

3D structures by inflating the non-bonded area. A PDMS piece with a size of 5 mm by 5

15

mm was covalently bonded to both the first and second PDMS layers by oxygen plasma


22

Page 23 of 52 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

treatment with an oxygen flow rate of 40 sccm and power of 70 W. One side of the PDMS

2

piece with whole sample was excavated using a biopsy punch (Rapid-Core 1.2 mm, WPI,

3

Sarasota, FL, USA). A silicone tube was inserted into the punched hole, and a syringe

4

was connected to the end of the silicone tube. A fluid was injected into the non-bonded

5

area of the structure, which acted as the fluidic channel, using a syringe pump (Fusion

6

100, Chemyx Inc., Stafford, TX, USA). The details of the fluid injection are presented in

7

Figure S2.

8

4.3. T-peeling test for the evaluation of plasma conditions. The experiment utilized a

9

customized stretching setup consisting of an XY-axis stage (CROSS 130-HSM, Owis

10

GmbH, Staufen im Breisgau, Germany), a force meter (ZTA-50N, Imada Inc., Toyohashi,

11

Japan), and a sample holder according to ASTM D1876-08. This setup is described in

12

more detail in Figure S7a. After holding both PDMS layers of the samples, the second

13

PDMS layer was peeled, stretched and detached from the other layer by moving the stage

14

with a direction of 180° and a speed of 500 μm/sec. The induced force from the stretching

15

and detaching was measured by the force meter.


23


Page 24 of 52

1

4.4. Pull-off test for the quantitative evaluation of bonding strength. The pull-off test

2

was conducted using an adhesion tester (PosiTest AT-A, DeFelsko, Ogdensburg, NY,

3

USA) according to ASTM D4541-09. The setup of the pull-off test is detailed in Figure

4

S7b. The first PDMS layer was spin-coated onto the silicon wafer treated by oxygen

5

plasma. When the first PDMS layer was cured, this PDMS layer and the silicon wafer

6

were bonded covalently each other. A dolly with 20 mm diameter was attached onto the

7

second PDMS layer using an epoxy and connected to the adhesion tester. The bonded

8

samples were cut along the edge of the dolly, to keep the area to be pulled constant. The

9

dolly was pulled with a pressure rate of 0.2 MPa/sec. The maximum bonding strength

10

was measured when the second PDMS layer detached from the parylene-C layer.

11

4.5. Cryosectioning of the bonded region for TEM-EDS analysis. The cross-sectional

12

area of the bonded samples was chemically analyzed by TEM-EDS. For sampling of the

13

cross-sectional area, the bonded samples were embedded in an epoxy mixture of Epon-

14

812, DDSA (2-dodecenylsuccinic anhydride), NMA (nadic methyl anhydride) and DPM-

15

30 (2,4,6-tris(dimethylaminomethyl)phenol) with a volume ratio of 45, 36.25, 18.75, and


24

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1

1.25%, respectively. The embedded samples were cured in an oven at 60 °C for 48 hours.

2

The cross-sectional surface of the bonded area was exposed by cutting the embedded

3

sample using an ultramicrotome (EM UC7, Leica Microsystems, Wetzlar, Germany) at -

4

180 °C. The sliced sample, with a thickness of 100 μm, was examined by TEM-EDS

5

analysis. Further details of cryosectioning are shown in Figure S6.

6

4.6. Fabrication of conductive lines and patterns. To integrate electrical functions,

7

selectively bonded samples in a 2D plane were prepared. The parylene-C layer with a

8

thickness of 500 nm was deposited onto the top surface of the whole structure. This layer

9

was used to prevent cracks in the conductive metal layer deposited afterwards.41 Then,

10

the conductive metal layer consisting of Ti/Au was sputtered in thicknesses of 25/300 nm

11

and patterned by wet etching with the help of a positive photoresist (AZ9260, Merck KGaA,

12

Darmstadt, Germany). The insulation layer of parylene-C was deposited in a thickness of

13

4.3 μm and patterned by RIE to create the openings. LEDs (SML-P1, ROHM Co., Kyoto,

14

Japan) were integrated using eutectic gallium-indium (EGaIn, Sigma-Aldrich., St.Louis,

15

MO, USA). The detailed method to integrate electrical functions is found in Figure S8.


25


Page 26 of 52

1


26

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1


FIGURE.

2

3

Figure 1. Fabrication of a soft and flexible 3D structure. (a) Photograph and schematic

4

image of a 3D structure with embedded conductive lines and pads. Fluid injection into the

5

non-bonded area generates the 3D structure. The bonded area is formed between PDMS

6

and parylene-C by the covalent bond between hydroxyl and carboxyl groups. (b)


27


Page 28 of 52

1

Methodology of selective bonding. Using a nitrogen and oxygen plasma treatment, the

2

second PDMS layer and the parylene-C layer are selectively bonded. (c) Generation of a

3

3D structure by the inflation of the non-bonded area upon fluid injection. The scale bars

4

are 5 mm.


28

Page 29 of 52 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

2

Figure 2. Mechanical analysis of bonding strength according to the plasma conditions by

3

T-peeling test. (a) Schematic of the T-peeling test setup with the used sample dimensions.

4

The force was measured during the peeling between layers, according to varying plasma

5

conditions: (b) nitrogen flow rate, (c) oxygen flow rate, (d) power, and (e) treatment time

6

(n=3 for each plasma condition). To investigate the effect of the nitrogen and oxygen flow

7

rates, the power of 50 W and the treatment time of 30 min were fixed. To investigate the


29


Page 30 of 52

1

effect of power and time, the oxygen and nitrogen flow rates of 50 sccm were fixed. (f)

2

The bonding levels were classified into the different locations of PDMS breakage after T-

3

peeling tests. The images following five bonding levels show (g) over-bonding, (h) critical

4

bonding, (i) partial bonding, (j) increased adhesion, and (k) no bonding. The scale bar is

5

10 mm.


30

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1

2

Figure 3. Method and results of pull-off test. (a) Schematic of the pull-off test setup. After

3

the dolly was pulled off, measuring a maximum strength, the interface between the

4

second PDMS layer and the parylene-C layer was exposed. (b) Measured bonding

5

strengths for each predefined bonding level (n ≥ 9 for each bonding level). (c-g)

6

Photographic images of the exposed surfaces after pull-off tests. There are five different

7

morphologies, which are indicated by arrows, according to bonding levels: (c) over-


31


Page 32 of 52

1

bonding, (d) critical bonding, (e) partial bonding, (f) increased adhesion, and (g) no

2

bonding.


32

Page 33 of 52 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

2

Figure 4. XPS results of the exposed surfaces of the bonded interface after pull-off test.

3

Si, O, and C from the PDMS and Cl from parylene-C were detected at the locations of (a)

4

transparent parylene-C surface, opaque parylene-C surface, and torn-out parylene-C

5

surface. Magnified C and Cl peaks detected at three exposed parylene-C surfaces after


33


Page 34 of 52

1

pull-off tests: (b) C 1s spectra including C-C and C-O bonds and (c) Cl 2p spectra

2

containing C-Cl and O-Cl bonds. Stronger bonding exhibits larger shifts of peak intensity

3

from C-C bond to C-O bond and from C-Cl bond to O-Cl bond.


34

Page 35 of 52 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

2

Figure 5. Chemical analysis of the bonded interface between PDMS and parylene-C by

3

TEM-EDS. (a) TEM-EDS results of the cross-sectional area of over-bonded samples,

4

showing the atomic distribution in the cross-section containing the interface (scale bar: 1

5

μm). The line analysis was performed for the 2 μm distance from the center of parylene-

6

C. (b) Intensity levels of different atoms along with the line crossing the interface between

7

plasma-treated PDMS and parylene-C. (c) The proposed mechanism of selective bonding

8

between PDMS and parylene-C.


35


Page 36 of 52

1

2

Figure 6. Generation of various 3D structures with air and dyed water injection. (a)

3

Circular balloon with a 20 mm diameter. Arrays containing nine balloons with (b) one

4

fluidic channel and (c) three fluidic channels. (d) Bent structure enveloping a tube and

5

lifting it up. (e) Trapezoidally bent structure wrapping a funnel and picking it up. (f) Spiral

6

structure wrapping a tube and holding it up. The scale bars are 10 mm.

7


36

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1

2

Figure 7. 3D structured soft and flexible devices embedding electronic components.

3

Demonstration of intact conductive lines by lighting LEDs after the fluid injection in cases

4

of (a) a bent structure consisting of five balloons and (b) a single balloon structure. The

5

scale bars are 5 mm.


37


Page 38 of 52

1 2 3

Figure 8. 3D structured soft and flexible ECoG electrode array. (a) ECoG sensing device

4

fitted on the curved cortical surface, by filling the space between the skull and the brain.

5

For the evaluation of recording performance, emulated action potentials (A.P.) were

6

injected into an agarose brain model (scale bars: 10 mm). (b) Emulated and recorded

7

action potentials by an electrode of the ECoG device. Emulated and recorded action

8

potentials by all seven electrodes in a magnified time scale. Three different waveforms of

9

action potentials were clearly distinguishable by all seven recording electrodes.


38

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1

ASSOCIATED CONTENT

2

Supporting Information

3

The Supporting Information is available free of charge on the ACS Publications website

4

at DOI:

5

Figure S1: Processes of selective bonding between PDMS and parylene-C; Figure S2:

6

Fluid injection to transform 2D structures into 3D structures; Figure S3: Plasma conditions

7

resulting in five different bonding levels and optimization for critical bonding; Figure S4:

8

Chemical structures of PDMS and parylene-C; Figure S5: Cryosectioning of samples

9

containing the bonded interface; Figure S6: Sample preparation and method for TEM-

10

EDS; Figure S7: Experimental setups for the measurement of bonding strength; Figure

11

S8: Fabrication of conductive lines and pads; Table S1: Plasma conditions for the T-

12

peeling test.

13

AUTHOR INFORMATION

14

Corresponding Author


39


Page 40 of 52

1

*Email: [email protected] (S.K.)

2

Author Contributions

3

H.M. and N.C. contributed equally to this work. H.M., N.C., and S.K. conceived the idea

4

and designed the experiments. H.M., N.C., and K.L. fabricated the 3D soft and flexible

5

structures. H.M., and N.C. performed T-peeling tests. H.M. and H.W.S. performed pull-

6

off tests. H.M. and N.C. performed XPS and TEM-EDS. J.P. analyzed XPS and TEM-

7

EDS results. H.M. carried out the fabrication and experiment regarding 3D soft and

8

flexible device with embedded electrical functionality. H.M., N.C., and S.K. wrote the

9

manuscript. All authors have given approval to the final version of the manuscript.

10

Notes

11

The authors declare no competing financial interest.

12

ACKNOWLEDGMENT

13

This work was supported by grants from the Basic Science Research Program (NRF-

14

2017R1A2B2004598) of the National Research Foundation of Korea (NRF) and the


40

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1

DGIST R&D Program (19-RT-01) funded by the Korean government (Ministry of Science

2

and ICT, MSIT). The authors would like to thank H.J., J.P., and H.L. (Center for Core

3

Research Facilities in DGIST) for discussions regarding the parylene-C coater, EDS, and

4

Bio-TEM, respectively.

5

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Transformation of 2D Planes into 3D Soft and ... - ACS Publications

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