**3. Fabrication of ultra-thin films (nanosheets)**

#### **3.1. Preparation and characterization of nanosheets composed of PIPC and SPUPC**

Self-standing nanosheets are easily fabricated by a "sacrificial layer method" as depicted in Fig. 1a [26]. The sacrificial layer can be dissolved with appropriate solvents which do not dissolve nanosheets themselves. In parallel, solvents used for dissolving polymers of nano‐ sheets must not dissolve the sacrificial layer. To this end, we selected poly(vinyl alcohol) (PVA) as a water-soluble sacrificial layer to obtain self-standing nanosheets composed of PIPC-1 and SPUPC-2, because these polymers were soluble in chloroform which did not dissolve PVA. A fabrication procedure of PIPC-1 nanosheet is described as follows. First, an aqueous solution of 10 mg/mL PVA was dropped onto a silicon oxide (SiO2) substrate, which has an extremely flat surface. The substrate was spin-coated at 4,000 rpm for 20 s and then dried. Next, a chloroform solution of 10 mg/mL PIPC-1 was spin-coated on the PVA-coated substrate under the same conditions. When the substrate was immersed into distilled water, the PIPC-1 nanosheet was detached from the substrate due to dissolution of only the PVA layer with water. The obtained PIPC-1 nanosheet was transparent, amazingly flexible, and maintained the size and shape of the SiO2 substrate (Fig. 1b, left). In fact, the thickness was 42 ± 2 nm and the roughness was nanometer scale. Furthermore, the thickness was easily controlled by adjusting the concentration of PIPC-1 just before spin-coating as shown in Fig. 1c. As described in section 2.2, the solubility of PIPC series is dependent on the content of the PC unit. For instance, PIPC-4 with high PC content was insoluble in chloroform but soluble in the aprotic polar solvents as DMSO. However, the PVA sacrificial layer is dissolved with DMSO. To this end, we can select other component of the water-soluble sacrificial layer, *e.g.* sodium alginate (Na-Alg), which is insoluble in DMSO. According to this technique, we could prepare the selfstanding nanosheets composed of SPUPC series [45]. In the case of a chloroform solution of 10 mg/mL SPUPC-2, the thickness of the nanosheet was 66 ± 4 nm. Intriguingly, these nanosheets composed of SPUPC series, which were elastic polymers, tended to shrink after detaching from the substrate as seen in Fig. 1b, right. This tendency suggested that the nanosheet extended on the substrate by the centrifugal force during the spin-coating, and resulted in shrinking due to its elasticity when they were released from the substrate. This tendency was not observed in the nanosheets composed of non-elastic polymers such as PIPC series. In these years, we have prepared the self-standing nanosheets composed of versatile polymers such as polystyr‐ ene and poly(methyl methacrylate), etc., and typical biodegradable polymers such as poly(lac‐ tic acid), their copolymers and polycaprolactone, etc. [24, 25].

Synthesis of New Biocompatible Polymers and Fabrication of Nanosheets http://dx.doi.org/10.5772/59633 11

The flexible and self-standing films could be prepared from these segmented polyurethanes by a solvent casting method using DMSO as a solvent. Then, the elastic mechanical properties were observed for these segmented polyurethane films, where the Young's modulus increased with increasing PC content. Furthermore, the introduction of such a polar phospholipid group was effective in improving the resistance to protein and platelet adhesions on the polymer

film, which was the result of surface properties derived from the PC moiety [21].

**3.1. Preparation and characterization of nanosheets composed of PIPC and SPUPC**

Self-standing nanosheets are easily fabricated by a "sacrificial layer method" as depicted in Fig. 1a [26]. The sacrificial layer can be dissolved with appropriate solvents which do not dissolve nanosheets themselves. In parallel, solvents used for dissolving polymers of nano‐ sheets must not dissolve the sacrificial layer. To this end, we selected poly(vinyl alcohol) (PVA) as a water-soluble sacrificial layer to obtain self-standing nanosheets composed of PIPC-1 and SPUPC-2, because these polymers were soluble in chloroform which did not dissolve PVA. A fabrication procedure of PIPC-1 nanosheet is described as follows. First, an aqueous solution of 10 mg/mL PVA was dropped onto a silicon oxide (SiO2) substrate, which has an extremely flat surface. The substrate was spin-coated at 4,000 rpm for 20 s and then dried. Next, a chloroform solution of 10 mg/mL PIPC-1 was spin-coated on the PVA-coated substrate under the same conditions. When the substrate was immersed into distilled water, the PIPC-1 nanosheet was detached from the substrate due to dissolution of only the PVA layer with water. The obtained PIPC-1 nanosheet was transparent, amazingly flexible, and maintained the size and shape of the SiO2 substrate (Fig. 1b, left). In fact, the thickness was 42 ± 2 nm and the roughness was nanometer scale. Furthermore, the thickness was easily controlled by adjusting the concentration of PIPC-1 just before spin-coating as shown in Fig. 1c. As described in section 2.2, the solubility of PIPC series is dependent on the content of the PC unit. For instance, PIPC-4 with high PC content was insoluble in chloroform but soluble in the aprotic polar solvents as DMSO. However, the PVA sacrificial layer is dissolved with DMSO. To this end, we can select other component of the water-soluble sacrificial layer, *e.g.* sodium alginate (Na-Alg), which is insoluble in DMSO. According to this technique, we could prepare the selfstanding nanosheets composed of SPUPC series [45]. In the case of a chloroform solution of 10 mg/mL SPUPC-2, the thickness of the nanosheet was 66 ± 4 nm. Intriguingly, these nanosheets composed of SPUPC series, which were elastic polymers, tended to shrink after detaching from the substrate as seen in Fig. 1b, right. This tendency suggested that the nanosheet extended on the substrate by the centrifugal force during the spin-coating, and resulted in shrinking due to its elasticity when they were released from the substrate. This tendency was not observed in the nanosheets composed of non-elastic polymers such as PIPC series. In these years, we have prepared the self-standing nanosheets composed of versatile polymers such as polystyr‐ ene and poly(methyl methacrylate), etc., and typical biodegradable polymers such as poly(lac‐

**3. Fabrication of ultra-thin films (nanosheets)**

10 Advances in Bioengineering

tic acid), their copolymers and polycaprolactone, etc. [24, 25].

procedure of the nanosheets by spin-coating. (b) Macroscopic images of PIPC-1 (left) and SPUPC-2 (right) nanosheets suspended in water. (c) Relationship between thickness of the PIPC-1 nanosheets and concentration of PIPC-1 solution before spin coating. **Figure 1.** Fabrication of self-standing nanosheets composed of PIPC-1 and SPUPC-2. (a) Fabrication procedure of the nano‐ sheets by spin-coating. (b) Macroscopic images of PIPC-1 (left) and SPUPC-2 (right) nanosheets suspended in water. (c) Relationship between thickness of the PIPC-1 nanosheets and concentration of PIPC-1 solution before spin coating.

Fig. 1 Fabrication of self-standing nanosheets composed of PIPC-1 and SPUPC-2. (a) Fabrication

We analyzed the mechanical properties of the nanosheets by using a bulging test developed for nanosheets [46]. In fact, the PIPC-1 nanosheet with a thickness of approximately 40 nm was physically adhered to a steel plate with a hole as shown in Fig. 2a. The plate was fixed to a custom-made chamber and air was supplied with a syringe pump until bursting the nano‐ sheets. During the analysis, pressure applied to the nanosheets and its deflection was moni‐ tored in real time by a differential pressure gauge and a stereomicroscope, respectively. Based on the equations as shown in Fig. 2a, we obtained a strain-stress curve as shown in Fig. 2b. From the slope of the elastic region of the curve, the Young's modulus of the PIPC-1 nanosheet was calculated to be 196 ± 9 MPa. This value was 10-folds lower compared to that of the bulk polyimide films (3-7 GPa), indicating that the PIPC-1 nanosheet was softer than the bulk polyimide film. We have demonstrated that the poly(lactic acid) nanosheets with a thicnkess less than 100 nm represent the same tenency [24]. Mattsson *et al*. have explored the relationship between the glass transition temperature (*T*g) and thickness of the ultra-thin films of polystyr‐ ene using a Brillouin light scattering method. In fact, *T*g of polystyrene films with a thickness of approximately 20 nm was decreased to 37°C compared to that of bulk polystyrene (*T*g: 109°C), explaining that the interactions between polymer chains decreased in the ultra-thin films [47]. This may be one of the reasons why the *T*<sup>g</sup> of the PIPC-1 nanosheet would be lower than that of bulk polyimide.

Next, we analyzed the relationship between adhesive strength of the PIPC-1 nanosheets and their thickness with a scratch tester for thin films [48]. As depicted in Fig. 3a, the nanosheets were physically adhered on the SiO2 substrate, and the surface of the nanosheets were horizontally scratched with a diamond tip under the following conditions; radius of curvature of a diamond tip: 25 μm, scratch length: 100 μm, and scratch rate: 10 μm/s. Critical loads just after detaching the nanosheet from the substrate were monitored. Then, the adhesive strength of the nanosheets was difined as the critical loads divided by the thickness of the nanosheets. critical loads divided by the thickness of the nanosheets.

P: pressure (kPa), a: diameter of hole (m)

the nanosheet from the substrate were monitored. Then, the adhesive strength of the nanosheets was difined as the

0

Fig. 2 Mechanical properties of the PIPC-1 nanosheet analyzed by a bulging test. (a) Schematic image of the bulging test. (b) Representative stress-strain curve of the PIPC-1 nanosheet with a thickness of 42 ± 2 nm. **Figure 2.** Mechanical properties of the PIPC-1 nanosheet analyzed by a bulging test. (a) Schematic image of the bulging test. (b) Represetative stress-strain curve of the PIPC-1 nanosheet with a thickness of 42 ± 2 nm. d: diflection (m), h: thickness (m) 0 20 40 60 80 100 120 Strain (%) Fig. 2 Mechanical properties of the PIPC-1 nanosheet analyzed by a bulging test. (a) Schematic image

Figure 2. Mechanical properties of the PIPC-1 nanosheet analyzed by a bulging test. (a) Schematic image of the bulging test. (b) Represetative stress-strain curve of the PIPC-1 nanosheet with a thickness of 42 ± 2 nm. The critical loads of the PIPC-1 nanosheet with thicknesses of 27 and 42 nm were calculated to be (1.6 ± 0.3) × 10<sup>5</sup> and (1.4 ± 0.4) × 10<sup>5</sup> N/m, respectively as shown in Fig. 3b. However, in the region of the thickness over 100 nm, the critical roads were obviously decreased to (0.8 ± 0.2) × 10<sup>5</sup> N/m (thickness: 155 nm) and (0.4 ± 0.2) × 10<sup>5</sup> N/m (thickness: 421 nm). This would be the reason that the nanosheets could conform to the roughness of the substrate due to its flat surface and amazingly flexibility. Actually, these nanosheets can be adhered to various surfaces such as plastics, glasses, steels, and tissues without the utilization of adhesive agents. Once the nanosheets were dried on these surfaces, it was often hard to detach with even washing with water. Consequently, we have demonstrated that the greatest benefit of the nanothickness is high potential to adhere. This phenomenon has been also observed with the poly(lactic acid) nanosheets with the thicnkesses less than 100 nm [24]. The critical loads of the PIPC-1 nanosheet with thicknesses of 27 and 42 nm were calculated to be (1.6 ± 0.3) × 105 and (1.4 ± 0.4) × 105 N/m, respectively as shown in Fig. 3b. However, in the region of the thickness over 100 nm, the critical roads were obviously decreased to (0.8 ± 0.2) × 105 N/m (thickness: 155 nm) and (0.4 ± 0.2) × 105 N/m (thickness: 421 nm). This would be the reason that the nanosheets could conform to the roughness of the substrate due to its flat surface and amazingly flexibility. Actually, these nanosheets can be adhered to various surfaces such as plastics, glasses, steels, and tissues without the utilization of adhesive agents. Once the nanosheets were dried on these surfaces, it was often hard to detach with even washing with water. Consequently, we have demonstrated that the greatest benefit of the nano-thickness is high potential to adhere. This phenomenon has been also observed with the poly(lactic acid) nanosheets with the thicnkesses less than 100 nm [24]. Figure 2. Mechanical properties of the PIPC-1 nanosheet analyzed by a bulging test. (a) Schematic image of the bulging test. (b) Represetative stress-strain curve of the PIPC-1 nanosheet with a thickness of 42 ± 2 nm. The critical loads of the PIPC-1 nanosheet with thicknesses of 27 and 42 nm were calculated to be (1.6 ± 0.3) × 10<sup>5</sup> and (1.4 ± 0.4) × 10<sup>5</sup> N/m, respectively as shown in Fig. 3b. However, in the region of the thickness over 100 nm, the critical roads were obviously decreased to (0.8 ± 0.2) × 10<sup>5</sup> N/m (thickness: 155 nm) and (0.4 ± 0.2) × 10<sup>5</sup> N/m (thickness: 421 nm). This would be the reason that the nanosheets could conform to the roughness of the substrate due to its flat surface and amazingly flexibility. Actually, these nanosheets can be adhered to various surfaces such as plastics, glasses, steels, and tissues without the utilization of adhesive agents. Once the nanosheets were dried on these surfaces, it was often hard to detach with even washing with water. Consequently, we have demonstrated that the greatest benefit of the nanothickness is high potential to adhere. This phenomenon has been also observed with the poly(lactic acid) nanosheets of the bulging test. (b) Representative stress-strain curve of the PIPC-1 nanosheet with a thickness of 42 ± 2 nm.

for thin films. (b) Correlation of adhesive strength of the PIPC-1 nanosheet with its thickness. for thin films. (b) Correlation of adhesive strength of the PIPC-1 nanosheet with its thickness. **Figure 3.** Adhesive strength of the PIPC-1 nanosheet. (a) Schematic image of scratch tester for thin films. (b) Correla‐ tion of adhesive strength of the PIPC-1 nanosheet with its thickness.

Fig. 4 Adhesive strength of the PIPC-1 nanosheet. (a) Schematic image of scratch tester

Fig. 4 Adhesive strength of the PIPC-1 nanosheet. (a) Schematic image of scratch tester

strength of the PIPC-1 nanosheet with its thickness.

with the thicnkesses less than 100 nm [24].

3.2. Biocompatibility of nanosheet surface

3.2. Biocompatibility of nanosheet surface

strength of the PIPC-1 nanosheet with its thickness.

Figure 3. Adhesive strength of the PIPC-1 nanosheet. (a) Schematic image of scratch tester for thin films. (b) Correlation of adhesive

Figure 3. Adhesive strength of the PIPC-1 nanosheet. (a) Schematic image of scratch tester for thin films. (b) Correlation of adhesive

Platelets are one of blood cells and involved in both normal hemostasis and pathological thrombosis [49]. In development of biocompatible materials with the possibility to contact with blood, what the most critical point is to

Platelets are one of blood cells and involved in both normal hemostasis and pathological thrombosis [49]. In development of biocompatible materials with the possibility to contact with blood, what the most critical point is to

#### **3.2. Biocompatibility of nanosheet surface**

the nanosheet from the substrate were monitored. Then, the adhesive strength of the nanosheets was difined as the

the nanosheet from the substrate were monitored. Then, the adhesive strength of the nanosheets was difined as the

Figure 2. Mechanical properties of the PIPC-1 nanosheet analyzed by a bulging test. (a) Schematic image of the bulging test. (b)

Figure 2. Mechanical properties of the PIPC-1 nanosheet analyzed by a bulging test. (a) Schematic image of the bulging test. (b)

0

10

0

0 20 40 60 80 100 120 Strain (%)

0 20 40 60 80 100 120 Strain (%)

Young's modulus: 196 ± 9 MPa

Young's modulus: 196 ± 9 MPa

10

Stress (MPa)

20

20

30

Stress (MPa)

30

40

40

The critical loads of the PIPC-1 nanosheet with thicknesses of 27 and 42 nm were calculated to be (1.6 ± 0.3) × 10<sup>5</sup> and (1.4 ± 0.4) × 10<sup>5</sup> N/m, respectively as shown in Fig. 3b. However, in the region of the thickness over 100 nm, the critical roads were obviously decreased to (0.8 ± 0.2) × 10<sup>5</sup> N/m (thickness: 155 nm) and (0.4 ± 0.2) × 10<sup>5</sup> N/m (thickness: 421 nm). This would be the reason that the nanosheets could conform to the roughness of the substrate due to its flat surface and amazingly flexibility. Actually, these nanosheets can be adhered to various surfaces such as plastics, glasses, steels, and tissues without the utilization of adhesive agents. Once the nanosheets were dried on these surfaces, it was often hard to detach with even washing with water. Consequently, we have demonstrated that the greatest benefit of the nanothickness is high potential to adhere. This phenomenon has been also observed with the poly(lactic acid) nanosheets

The critical loads of the PIPC-1 nanosheet with thicknesses of 27 and 42 nm were calculated to be (1.6 ± 0.3) × 10<sup>5</sup> and (1.4 ± 0.4) × 10<sup>5</sup> N/m, respectively as shown in Fig. 3b. However, in the region of the thickness over 100 nm, the critical roads were obviously decreased to (0.8 ± 0.2) × 10<sup>5</sup> N/m (thickness: 155 nm) and (0.4 ± 0.2) × 10<sup>5</sup> N/m (thickness: 421 nm). This would be the reason that the nanosheets could conform to the roughness of the substrate due to its flat surface and amazingly flexibility. Actually, these nanosheets can be adhered to various surfaces such as plastics, glasses, steels, and tissues without the utilization of adhesive agents. Once the nanosheets were dried on these surfaces, it was often hard to detach with even washing with water. Consequently, we have demonstrated that the greatest benefit of the nanothickness is high potential to adhere. This phenomenon has been also observed with the poly(lactic acid) nanosheets

Figure 3. Adhesive strength of the PIPC-1 nanosheet. (a) Schematic image of scratch tester for thin films. (b) Correlation of adhesive

Figure 3. Adhesive strength of the PIPC-1 nanosheet. (a) Schematic image of scratch tester for thin films. (b) Correlation of adhesive

0 100 200 300 400 500

0 100 200 300 400 500

Thickness of PIPC-1 nanosheet (nm)

Thickness of PIPC-1 nanosheet (nm)

0.0

0

0.0

0

0.5

1.0

1.5

2.0

0.5

Adhesive strength (x105

1.0

Adhesive strength (x105

Fig. 4 Adhesive strength of the PIPC-1 nanosheet. (a) Schematic image of scratch tester for thin films. (b) Correlation of adhesive strength of the PIPC-1 nanosheet with its

Fig. 4 Adhesive strength of the PIPC-1 nanosheet. (a) Schematic image of scratch tester for thin films. (b) Correlation of adhesive strength of the PIPC-1 nanosheet with its

**Figure 3.** Adhesive strength of the PIPC-1 nanosheet. (a) Schematic image of scratch tester for thin films. (b) Correla‐

 N/m)

1.5

 N/m)

2.0

Platelets are one of blood cells and involved in both normal hemostasis and pathological thrombosis [49]. In development of biocompatible materials with the possibility to contact with blood, what the most critical point is to

Platelets are one of blood cells and involved in both normal hemostasis and pathological thrombosis [49]. In development of biocompatible materials with the possibility to contact with blood, what the most critical point is to

Represetative stress-strain curve of the PIPC-1 nanosheet with a thickness of 42 ± 2 nm.

Represetative stress-strain curve of the PIPC-1 nanosheet with a thickness of 42 ± 2 nm.

test. (b) Represetative stress-strain curve of the PIPC-1 nanosheet with a thickness of 42 ± 2 nm.

Coil

Coil

poly(lactic acid) nanosheets with the thicnkesses less than 100 nm [24].

P

Chamber

<sup>a</sup> <sup>d</sup>

2 x d<sup>2</sup> 3 x a<sup>2</sup>

<sup>a</sup> <sup>d</sup>

P

Chamber

Fig. 2 Mechanical properties of the PIPC-1 nanosheet analyzed by a bulging test. (a) Schematic image of the bulging test. (b) Representative stress-strain curve of the PIPC-1 nanosheet with a thickness of

2 x d<sup>2</sup> 3 x a<sup>2</sup>

**Figure 2.** Mechanical properties of the PIPC-1 nanosheet analyzed by a bulging test. (a) Schematic image of the bulging

The critical loads of the PIPC-1 nanosheet with thicknesses of 27 and 42 nm were calculated to be (1.6 ± 0.3) × 105 and (1.4 ± 0.4) × 105 N/m, respectively as shown in Fig. 3b. However, in the region of the thickness over 100 nm, the critical roads were obviously decreased to (0.8 ± 0.2) × 105 N/m (thickness: 155 nm) and (0.4 ± 0.2) × 105 N/m (thickness: 421 nm). This would be the reason that the nanosheets could conform to the roughness of the substrate due to its flat surface and amazingly flexibility. Actually, these nanosheets can be adhered to various surfaces such as plastics, glasses, steels, and tissues without the utilization of adhesive agents. Once the nanosheets were dried on these surfaces, it was often hard to detach with even washing with water. Consequently, we have demonstrated that the greatest benefit of the nano-thickness is high potential to adhere. This phenomenon has been also observed with the

Fig. 2 Mechanical properties of the PIPC-1 nanosheet analyzed by a bulging test. (a) Schematic image of the bulging test. (b) Representative stress-strain curve of the PIPC-1 nanosheet with a thickness of

with the thicnkesses less than 100 nm [24].

Diamond tip

Diamond tip

Scratch

Scratch

thickness.

thickness.

strength of the PIPC-1 nanosheet with its thickness.

3.2. Biocompatibility of nanosheet surface

3.2. Biocompatibility of nanosheet surface

strength of the PIPC-1 nanosheet with its thickness.

tion of adhesive strength of the PIPC-1 nanosheet with its thickness.

Nanosheet

(a) (b)

(a) (b)

with the thicnkesses less than 100 nm [24].

Substrate

Substrate

Nanosheet

critical loads divided by the thickness of the nanosheets.

Strain: <sup>ε</sup> <sup>=</sup> 4 x <sup>h</sup> <sup>x</sup><sup>d</sup>

Pressure gauge

h

P: pressure (kPa), a: diameter of hole (m) d: diflection (m), h: thickness (m)

Stress (σ) Strain (ε)

Strain: <sup>ε</sup> <sup>=</sup> 4 x <sup>h</sup> <sup>x</sup><sup>d</sup>

P: pressure (kPa), a: diameter of hole (m) d: diflection (m), h: thickness (m)

Stress (σ) Strain (ε)

Nanosheet

critical loads divided by the thickness of the nanosheets.

(a) (b)

Nanosheet

(a) (b)

Pressure gauge

Air

Stress: <sup>σ</sup> <sup>=</sup> <sup>P</sup> <sup>x</sup>a<sup>2</sup>

Elastic modulus: E =

h

Air

Stress: <sup>σ</sup> <sup>=</sup> <sup>P</sup> <sup>x</sup>a<sup>2</sup>

Syringe pump

Elastic modulus: E =

Syringe pump

12 Advances in Bioengineering

42 ± 2 nm.

42 ± 2 nm.

Platelets are one of blood cells and involved in both normal hemostasis and pathological thrombosis [49]. In development of biocompatible materials with the possibility to contact with blood, what the most critical point is to inhibit non-specific interactions between platelets and the surface of the materials. To this end, we evaluated the blood compatibility of the surface of the nanosheets composed of PC polymers. Poly(ethylene terephtalate) (PET) plates were used as model surfaces, to which the nanosheets were adhered. The nanosheet-coated PET plates were immersed into 0.5 mL of platelet-rich plasma (PRP) obtained from healthy volunteers and incubated at physiological temperature for 2 h. Finally, PRP was removed and the substrates were washed out with phosphate buffered saline. The surface of the plates was observed with a scanning electron microscope. As shown in Fig. 4, platelets with filopodial extensions were non-specifically adhered to the bared PET plate and the nanosheet-coated PET plate without PC units (PI and SPU). PI is a polyimide obtained by the polycondensation of BAPB with 6FDA followed by the chemical imidization, and SPU is a segmented polyurethane obtained by the polyaddition of 3,5-bis(2-hydroxyethoxy)benzene and PCD (molar ratio: 70/30) with MDI. In the case of the PET plates coated with PIPC-1 and SPUPC-2 nanoheets, reduction of platelet adhesion was clearly observed as compared with PET plate and PI/SPU coated plates. Therefore, it was confirmed that the surface of PC-polymer nanosheets exhibited the good blood compatibility. In other words, these results indicate that sealing of the nano‐ sheets could act as a surface modifier to convert the surface property of the PET plates.

Fig. 4 SEM images of nanosheet surfaces with or without PC unit after contact with platelet-rich plasma for 2h at 37 **Figure 4.** SEM images of nanosheet surfaces with or without PC unit after contact with plateled-rich plasma for 2h at oC. 37°C

#### **3.3. Fragmentation of the nanosheets to coat irregular and uneven surfaces**

As described above, we have succeeded in the preparation of the self-standing nanosheets, which represent unique properties such as good adhesiveness, amazingly flexibility and high transparency. However, such nanosheets possess centimeter size and are only suitable for adhesion to relatively broad surfaces. They are often difficult to adhere to irregular and uneven surfaces because of centimeter size. In our recent study, we have discovered that the frag‐ mented submillimeter-sized nanosheets composed of poly(lactic acid) were adhered to the various surfaces in a spread out configuration that looks like "patchwork" [25, 26]. Once the nanosheets dried on the surface, they were difficult to detach from the surface by even washing with water. Moreover, we have demonstrated that the irregular and uneven surfaces such as a needles and rubbers etc. are effectively coated with the patchwork-like coating of the fragmented nanosheets by just casting or dipping [25, 26]. In this section, we introduce the fragmented nanosheets composed of PIPC and SPUPC series to coat irregular and uneven surfaces and the evaluation of blood compatibility.

Macroscopic image of fragmented PIPC-1 nanosheets (left tube) suspended in distilled water. Right tube shows only distilled water. (c) SEM images of fragmented nanosheet surfaces after contact with platelet-rich plasma for 2h at 37oC. **Figure 5.** (a) Fabrication of fragmented nanosheets composed od PIPC-1 and SPUPC-2. (b) Macroscopic image of frag‐ mented PIPC-1 nanosheets (left tube)suspended in distilled water. Right tube shows only distilled water. (c) SEM im‐ ages of fragmented nanosheet surfaces after contact with platelet-rich plasma for 2h at 37°C.

Fig. 5 (a) Fabrication of fragmented nanosheets composed of PIPC-1 and SPUPC-2. (b)

We herein focus on the fragmented PIPC-1 nanosheets as follows. First, we fabricated abundant self-standing nanosheets with centimeter size by a simple multi-layering process of watersoluble PVA and PIPC nanosheets combined with a peeling technique, according to our reports [25, 26]. Concretely, a 100 mg/mL solution of PVA as a water-soluble sacrificial layer was first spin-coated on a SiO2 substrate at 4000 rpm for 20 s, followed by a drying process as depicted in Fig. 5a. Next, a chloroform solution of 10 mg/mL PIPC-1 was spin-coated on the PVA-coated substrate under the same conditions. Moreover, the multi-layering of PVA and PIPC-1 was repeated twenty times on the substrate. By dissolution of PVA layers in water, tewenty sheets of PIPC-1 nanosheets were obtained. Next, the obtained PIPC-1 nanosheets were fragmented with a homogenzer. When the PIPC-1 nanosheets (size: 40 × 40 mm, thickness: 42 nm) in distilled water were homogenized at 30,000 rpm for 10 min, they were instantly fragmented. The obtained nanosheets were homogeneously suspended in water and the turbidity of the suspension was quite increased as shown in Fig. 5b. In fact, the surface area of one fragmented nanosheet 10 min after homogenizaion was significantly decreased to 6800 ± 208 μm2 , esti‐ mating that the average size of the nanosheet was approximately 80 μm. Using the same prosedure, we also prepared the fragmented nanosheets composed of SPUPC-2 (surface area: 3900 ± 1300 μm2 , thickness: 66 nm).

In order to evaluate the blood compatibility, the fragmented PIPC-1 or SPUPC-2 nanosheets were adhered to a bared PET plate as a model surface. They consisted of a patchwork-like coating in the same manner as the fragmented PLLA nanosheets [25, 26]. The nanosheet-coated PET plates were immersed into PRP and incubated at 37°C for 2 h. As shown in Fig. 5c, very few platelets were adhered to the PIPC-1 and SPUPC-2 coated PET plate. Moreover, some lines were observed on the plates, that correspond to wrinkles (not cracks) formed during drying of patchwork-like coating. In the case of the bared PET plates, abundant platelets were activated and non-specifically adhered. Therefore, we demonstrated that patchwork-like coating with the fragmented nanosheets with PC units acts as an aqueous surface modifier to provide blood compatibility.
