Toward 3D Printing of Medical Implants: Reduced Lateral Droplet

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Towards 3D printing of medical implants: Reduced lateral droplet spreading of silicone rubber under intense IR curing Jan Stieghorst, Daniel Majaura, Hendrik Wevering, and Theodor Doll ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12728 • Publication Date (Web): 11 Mar 2016 Downloaded from http://pubs.acs.org on March 16, 2016

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ACS Applied Materials & Interfaces

Towards 3D printing of medical implants: Reduced lateral droplet spreading of silicone rubber under intense IR curing Jan Stieghorst*,1,2, Daniel Majaura1,2, Hendrik Wevering1,2 and Theodor Doll1,2 1 2

Cluster of Excellence Hearing4all, Hannover, Germany BioMaterial Engineering, Department of Otorhinolaryngology, Hannover Medical School, Carl-Neuberg-Str. 1, 30625 Hannover

* Corresponding author: e-mail [email protected], Phone: +49 511 533 7231, Fax: +49 511 532 18538

Abstract: The direct fabrication of silicone rubber based individually shaped active neural implants requires high speed curing systems in order to prevent extensive spreading of the viscous silicone rubber materials during vulcanisation. Therefore, an infrared laser based test setup was developed to cure the silicone rubber materials rapidly and to evaluate the resulting spreading in relation to its initial viscosity, the absorbed infrared radiation and the surface tensions of the fabrication bed’s material. Different low adhesion materials (Polyimide, Parylene-C, polytetrafluoroethylene and fluorinated ethylenepropylene) were used as bed materials to reduce the spreading of the silicone rubber materials by means of their well-known weak surface tensions. Further, O2-plasma treatment was performed on the bed materials to reduce the surface tensions. To calculate the absorbed radiation, the emittance of the laser was measured and the absorptances of the materials were investigated with fourier transform infrared spectroscopy in attenuated total reflection mode. A minimum silicone rubber spreading of 3.24% was achieved after 2 s curing time, indicating the potentially usability of the presented high speed curing process for the direct fabrication of thermal curing silicone rubbers. Keywords 3D printing, spreading, silicone rubber printing, customized neural implants, individually tailored implants

1 Introduction Flexible silicone rubber based implants can be used to treat or to diagnose (sensori-) neuronal diseases. For instance, hearing of deaf patients can be restored with cochlear implants 1 and epileptic foci can be localized with electrocortical grid arrays 2. Several standard sizes of these implants are available for different anatomical environments, but do not perfectly fit to the individual anatomy of each patient due to their stiffness and inflexibility. Due to this, conventionally available implants are limited by means of resolution and sensitivity caused by large distances between the electrode contacts and the target nerve cells, e.g. in cochlear implants 3 or electrocortical grid arrays 4. Furthermore, electrode arrays with a patient individual adapted number of electrode contacts and individualized sizes are currently not available. To overcome these limitations we presented a fabrication approach 5 for the direct fabrication (e.g. with a material jetting process) of individually tailored silicone rubber based implants using typical medical grade silicone rubber. As the commonly used ultraviolet-curable silicone rubbers for additive manufacturing are not approved for medical implant fabrication, it is required to use the conventionally established heat curing mechanisms for the implant fabrication. Since these silicone rubbers are thermal curing liquids, which undergo a low-viscous region during the timeconsuming heat curing process, a spreading of the silicone rubbers can be expected. As the spreading would result in

fabrication inaccuracies, we developed a high speed curing system to prevent the silicone rubber from extensive spreading. The system uses the strong infrared (IR) absorption properties, in the long-wave infrared region as observed by several authors e.g. Chen et al. (2006) 6 and Efimenko et al. (2002) 7, for heat transfer. In this paper, we describe the evaluation of the silicone rubber spreading with regard to the heat transfer, the initial viscosity of the material and the bed material of the 3Dprinter. A test setup was developed and the spreading of different silicone rubbers was observed. The IR heat transfer was computed by using transmittance data from a fourier transform infrared (FTIR) spectroscopy in attenuated total reflection (ATR) mode and the irradiance from the IR-emitter. Plasma-modifications of the bed materials were performed to reduce the silicone rubber spreading and the resulting surface tensions were characterized with contact angle measurements. According to data presented earlier 5, a modulated continuous-wave (cw) CO2-Laser was used in the experiments, as its emission wavelength of 9.3 µm fits to the high absorption bands of a typical silicone rubber Sylgard 184 (DowCorning GmbH, Germany). Effects of the laser radiation on the biocompatibility of the silicone rubbers were not taken into account, as Green et al. (2010) 8 found no effects on the cell growth and proliferation of fibroblasts on a PDMS surface after laser ablation. In addition, changes in the final cross-linking density of the sili-

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2 J. Stieghorst et al.: Towards 3D printing of medical implants: Reduced lateral droplet spreading of silicone rubber under intense IR curing

cone rubbers are not expected according to earlier published results5. 2 Test setup For the evaluation of the silicone rubber spreading during high-speed curing, a test setup was developed, as shown in Fig. 1. The system comprises of: • a camera (1) to observe the spreading, • an emission controlled CO2-laser (2), • a plano-convex lens (3) to adjust the beam size, • a mirror (4) to deflect the beam, • a drop of the sample material (5), • a printer bed with a low adhesion layer (6) and • a dispenser for the creation of droplets (not shown here) Using this setup, the spreading of the silicone rubber test material can be observed during infrared curing, as shown in Fig. 2. Therefore, different radiative powers can be applied with the IR-laser by a modulation of the laser, according to section 3.3. Furthermore, the impact of different low adhesion layers on the resulting silicone rubber spread can be investigated.

For the dispensing of the test material a self-built syringe pump was used. Thereby, different drop volumes can be applied on the bed material by changing the speed of the extrusion piston as well as with different nozzle diameters. 3 Materials and Methods 3.1 Materials Room-temperature-vulcanization 2-part (RTV-2) poly-(dimethylsiloxane) (PDMS) silicone rubbers with different initial viscosities and curing times were used as test material. Silpuran 2430 (Wacker Chemie AG, Germany) with approx. 9,000 mPas and a pot life of 60-75 min at 23°C and Sylgard 184 (Dow Corning GmbH, Germany) with approx. 3,500 mPas and a pot life of 90 min at 25°C. The silicone rubbers were mixed with their curing agents according to the manufacture’s data sheet. For the printer bed several low adhesion materials and surfaces (Table 1) were tested: polyimide (PI), poly(chloro-p-xylylen) (Parylene-C), polytetrafluoroethylene (PTFE) and fluorinated ethylene propylene (FEP) were used. The PI and FEP were used as available from stock foils, whereas the PTFE was used as a virginal flat material and the ParyleneC was coated in a thickness of approx. 4µm on a glass substrate by using a Labcoater 1 (Diener Electronics GmbH, Germany). Table 1 Data of the low adhesion materials. Polymer

Figure 1 Test setup for the evaluation of the silicone rubber spreading during infrared curing. The system consists of a camera (1), an infrared laser (2), a lens (3), an adjustable mirror (4), a drop of the sample material (5) and a printer bed with a low adhesion layer (6).

Figure 2 The spreading of a test material drop can be observed and the radius can be calculated during the curing process by a circle fitting. The scale bar indicates 1mm.

Product

poly (4,4'-oxydiphenylene- Kapton HN pyromellitimide) 50 µm poly(chloro-p-xylylen) Parylene-C, Dimer DPX-C polytetrafluoroethylene PTFE flat material virginal fluorinated ethylene propylene

Teflon FEP foil 25µm

Manufacturer DuPont, Luxembourg Specialty Coating Systems Inc., USA TECHNOPLAST V.TRESKOW GMBH, Germany DuPont, Luxembourg

* DuPont refers to the DuPont de Nemours S.A.R.L. 3.2 Test procedure To evaluate the resulting spread of the silicone rubber test materials, a drop of the each test material was applied on the adhesion layer of the test setup and immediately cured with the infrared emitter. In the meantime, the spreading of the drop was observed with the camera by taking a picture every 0.8 s until no further increase of the drop radii was measured. Different average emitter powers were used to investigate the influence of the heat flux on the spreading. Therefore, the average emitter power and the absorptance of the materials were determined according to section 3.3 and 3.4. The used modulation parameters of the laser are presented in Table 2. The duty cycles DC25 and DC40 were used for the PTFE, FEP, PI and Parylene-C materials. Higher irradiences were not used in order to prevent overheating and thermal degradation of the materials.


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3

To dispense the test material a 0.3 mm needle was used with a nozzle diameter of 0.2 mm. The speed of the extrusion piston was 3.25 µm/s and the drop volume was 2.11µl±0.12 µl for the Sylgard 184 and 1.85µl±0.19 µl for the Silpuran 2430 material. Initial drop radii of 1.355 mm ± 0.136 mm for the Sylgard 184 and 0.962 mm±0.085 mm for the Silpuran 2430 were obtained. Table 2 Parameters of the laser modulation. Abbreviation

Duty cycle*

Repetition rate

DC25 DC40

25% 40%

5 kHz 5 kHz

* Duty cycle is the percentage of the maximum ontime per cycle that modulation of the laser will be on. 3.3 Infrared heat transfer The heat transfer was calculated as described in an earlier publication 5 using the refractive index and the spectral transmittance of the materials for the calculation of the absorptance. Therefore, the refractive indices of the materials were measured with an Abbemat MWR (Anton Paar GmbH, Germany) and the first two coefficients of the Sellmeier dispersion model9 were calculated to receive an equation for the refractive index in respect to the wavelength1. To analyze the spectral transmittance of the used silicone rubbers, 100 µm thin films of each material was produced with a mold and the spectral transmittance of these samples were determined with FTIR-spectroscopy (Tensor 27, Bruker Corporation, USA) in attenuated total reflection mode. By using these data, the absorptance was computed with the Bouguer’s law of absorption in Lambert’s formulation 10 with the later calculated irradiance from the emitter to the sample and the actual optical thickness of the sample drop. For this purpose, the irradiance can be determined by considering the optical enlargement of the laser beam as well as the emittance of the laser. The latter is given as

=

=

∙

(1)

where M is the emittance, Pavg is the average emitted Power, Ppeak is the peak power of the laser and ABeam is the area of the laser beam. However, as the optical pulse rise and fall times of the laser were neglected with this equation, resulting in an inherent error by using small duty cycles and repetition rates, the emittance was measured with a laser power meter from Thorlabs Inc. (S314C sensor with PM100USB interface). 1

for Sylgard 184: λ1 = 436.4 nm, n1 = 1.425103, λ2 = 657.2 nm, n2 = 1.412642, B1 = 0.969334 and C1 = 0.01137678 nm2 for Silpuran 2430: λ1 = 436.4 nm, n1 = 1.423481, λ2 = 657.2 nm, n2 = 1.41146, B1 = 0.966897 and C1 = 0.01102277 nm2

For the calculation of the optical thickness of the sample material it was assumed that the drop volume of the sample material builds an ideal circular disk with a constant volume on the printer bed. Thereby, the real-time optical thickness was determined by dividing the volume of the drop by its actual surface area, measured with the camera. Reflections and (back) emissions from the samples were not considered in our calculation. Furthermore, a homogenous beam profile was assumed for the calculation. 3.4 Silicone rubber spreading In order to achieve high fabrication accuracies for the implant fabrication in general it is required to prevent the uncured silicone rubber from extensive spreading, as described in section 1. But since RTV-2 silicone rubbers are thermal curing liquids, the spreading behaviour can be described for a steady (1) and for a time-dependent (2) state. Applying the Arrhenius model for the shear viscosity:

= exp #

(2)

) $ = % ' + )' -/ &' - - ( - 1

(3)

!"

where η(T) is the temperature-depending shear viscosity, η0 is the viscosity of the uncured liquid, E is an activation energy, R is the universal gas constant and T is the absolute temperature, the viscosity of the silicone rubber apparently decreases during heating and thus leads to an increased spreading dynamic. However, additional factors determine the resulting spread of polymeric fluids as described by Härth et al. (2012) 11, who reported the relation of a drop volume and the equilibrium contact radius of polymeric fluids with a partially wetted substrate for long spreading times (steady state) as the following (1): & (&

*&

, . 0, .

where V is the drop volume of the polymeric fluid, γL is the surface tension of the liquid, γSL is the interfacial tension, γS is the surface tension of the substrate, re is the equilibrium contact radius, ρ is the density of the test material and g is the gravitational constant. By using this equation, the spreading of a silicone rubber prepolymer drop can be adjusted by the drop volume and the surface tensions of the interface γSL and the substrate material γS. Applying the relation by Owens et al. (1969) 12 for the interfacial tension,

234 = 23 + 24 − 2 823 24 + 823 24 #

(4)

to Eq. (3) the relation can be described as:

$=9

:&' *:;? +

-∙/∙

&' - - ( - 1 , . 0, .


(5)


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where the superscripts D and P refer to the dispersive and the polar part of the surface tension, according to 2 = 2 + 2. By using Eq. (5), the spreading of a given polymeric fluid drop (V, γL, γLD,γLP and ρ) for a steady state (1) can be minimized by reducing the dispersive as well as the polar part of the substrate surface tension. To modify the surface tension of polymers in general, several surface modification techniques can be used, taking the physical and mechanical properties into account. For instance, physical modification techniques like plasma-surface modification can be used to change the surface tension by the formation of oxygen or nitrogen functionalities via ion implantation 13 . Further, changes can be achieved by mechanical modifications, according to Herminghaus (2000) 14 who described roughness-induced changes of the effective interfacial tension by means of a microscopic contact angle at the interface between the substrate and the liquid. However, the differentiation between physical and mechanical modifications is sometimes unclear because of the complex effects on the substrate material. For example, Lee et al. (2006) 15 and Salapare et al. (2013) 16 reported an increased hydrophobicity of a polytetrafluoroethylene (PTFE) substrate after heavily O2-plasma treatment due to an increased roughness, whereas Zanini et al. (2014) 17 and Salapare et al. (2013) 16 reported an increased hydrophilicity after a short period of O2-plasma treatment. The reason for the hydrophilication was described by Zanini et al. (2014) as a result of an OH-functionalization. Similar experiments were performed with a polyimide film. Francioso et al. (2013) 18 reported an increased hydrophilicity after 30 s of O2-plasma treatment and Barshilia et al. (2012) 19 described the hydrophobization after 30 min of O2/Ar-plasma treatment. Thus, to reduce the spreading of the test materials, O2plasma treatment was applied on the layers with a PICO plasma system (Diener Electronics GmbH, Germany). As treatment time 1 min (plasma cleaning / functionalization) and 30 min (plasma etching) were chosen to evaluate the mentioned effects of functionalization and roughness enlargements. The parameters were 30°C, 356 sccm of O2 and 100 W. Prior to the treatments, all surfaces were cleaned with a 70% ethanol/water solution. Changes in the surface tension were evaluated with static contact-angle measurements by using an EasyDrop FM40 goniometer (Krüss GmbH, Germany) with included drop-shape analysis program. Thereby, the polar and the dispersive part of the solid’s surface tension were estimated using the Eq. (4) and the Young-equation Eq. (6),

234 = 23 − 24 @ABC

(6)

where θ is the contact angle between the liquid and the solid, as: &' D(EFG :∙ '

(7)

Furthermore, the test materials were analysed with contact angle measurements to determine the polar and dispersive part of the liquids surface tension. Therefore, three individual spots of every test material were measured ten times. As Eq. (7) comes with four unknown parameters γLP, D γL ,γSP and γSD the measurements of the solid surface tensions were performed for each surface individually with three well-known test liquids, see Table 3. By using first the diiodomethane, whose polar part γLP is zero, and the measured contact angles, the dispersive part of the surface tensions γSD were calculated with Eq. (7). For the calculation of the surface tensions polar components, Eq. (7) was solved with the surface tension values of the test liquids and the calculated dispersive components of the surface tensions γSD. Table 3 Recommended test liquids and their surface tensions according to DIN 55660-2 at 20°C. test liquid* Water Diiodomethane Ethylene glycol

γLD 21.8 50.8 30.9

γL 72.8 50.8 47.7

γLP 51 0 16.8

* the unit of the surface tension is mN/m Additional contact angle measurements were performed with the silicone rubber test materials and the low adhesion layers to determine the ideal bed material with regard to the surface modifications and the initial surface tensions. Besides the steady state for long spreading times, further optimization should be feasible by considering the time-dependent spreading behaviour (2) as mentioned earlier. According to Härth et al. (2012) 11 the spreading for a partial wetting scenario can be described as:

HI = H J1 − exp − L M-' + PL MQ# :&

NO

:RST 1 (Q U- V

M X

#W (8)

where λ is an experimental shape factor, t is the time and t0 is an experimental delay time. Thus, the relation can be described with an asymptotic solution which, besides the early mentioned relations (surface tension, gravitation and density), depends on the previously neglected liquids viscosity. By using this for heat curable polymeric liquids, like silicone rubbers or epoxy resins, the spreading can be reduced by means of an increased initial viscosity η of the polymeric fluid as well as a rising viscosity η(T,t) with progressing cross-linking of the polymeric liquid. Furthermore, by using Eq. 8 with the experimentally determined equilibrium contact radius re, surface tension of the liquid γL, shape factor λ, gravitation g, density ρ and volume V of the applied droplet, a stagnation contact radius r lower than re might be feasible in respect to the viscosity rise during curing.


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5

For the spreading reduction, increased heating rates might result in an increased curing speed which overcomes the liquefying influence of higher temperatures according to Eq. (2). Several studies have already been conducted with (mostly) epoxy resins to investigate the timetemperature-viscosity behaviour with isothermal measurements. Bullions et al. (2002) 20 showed with a phenylthynyl-terminated poly(etherimide) a decreased initial viscosity and an accelerated viscosity gain at higher curing temperatures, which overcomes the initial viscosity drop very shortly. The latter was also presented by Schmitt et al. (1989) 21 with a B-staged epoxy system and Roller et al. (1975) 22 but with different epoxy systems and constant heating rates. To evaluate the influence of different heating rates on the silicone rubber spreading, different heat fluxes were applied on the sample materials, as described in section 3.2, and the resulting spreads were measured with the test setup.

component of the surface tensions, probably as a result of ion implantation of oxygen functionalities, according to Chu et al. (2002) 13. The changes in the dispersive surface tensions components were found to be marginal in comparison to the polar surface tension component. Thus, considering the low treatment time, no roughening of the surfaces and thereby no reduction of the dispersive surface tension could be expected. Contrary to that, the FEP material showed no significant changes in the calculated polar component of the surface tension due to marginal changes in the measured water contact angles. As the latter was also observed by Kang (2013) 31 for short times of an O2plasma etching process, we expect that no oxygenated species were grafted on the surface.

4 Results and discussion 4.1 Evaluation of the surface tension The low adhesion materials were O2-plasma treated and the surface tension components of the bed materials and the silicone rubbers were investigated with static contact angle measurements according to section 3.4. The results of the surface tensions are presented in Table 4 and the surface tensions components are shown in Fig. 3. Further contact angle measurements were performed at the bed material/silicone rubber-interface in order to evaluate the influences of O2-plasma induced functionalization and surface roughening. Table 4 Comparison between experimental and literature surface tension values of the used materials. material

measured γs in mN/m

literature γs in mN/m

reference

Sylgard 184 Silpuran 2430 PI Parylene-C PTFE FEP

19.18 18.84 39.49 36.69 19.80 17.52

19 / 20 * 38.36 / 46.0 27.64 / 37.23 18 / 18.5 14.2 / 20.0

7

/ 23

* 18

/ 24 / 26 27 28 / 29 30 / 25

Figure 3 Calculated surface tension components γP and γD of the untreated (*), 1 min (#) and 30 min (∆) O2-plasma treated bed materials. The light grey bars indicate the dispersive and the dark grey bars, the polar component of the surface tension. The evaluated materials were polyimide (PI), Poly(chloro-p-xylylen) (Parylene-C), polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP), Sylgard 184 and Silpuran 2430.

* no values were found for the Silpuran 2430 material According to Fig. 3, the silicone rubber materials show low dispersive and marginal polar components of the surface tensions. The surface tension of the Sylgard 184 materials was found in the region of other published values, see Table 4, whereas no value for the Silpuran 2430 material was available. However, the measured surface tension was similar to the value of the Sylgard 184 material. Further, the surface tensions of the untreated low adhesion materials were measured and found to be in the range of already published values according to Table 4. As estimated from the literature, the plasma functionalized PI, Parylene-C and PTFE samples showed an increased polar

As estimated from the literature (see Section 3.4), the plasma etched PI, Parylene-C and PTFE materials showed decreasing dispersive surface tension components, probably because of the expected surface roughening. Besides the dispersive component, the polar component of the PI and Parylene-C materials further increased in comparison to the functionalized samples. Thereby, the surface tensions were found to be similar to other published values, e.g. Hwang et al. (2004) 32 for Parylene-C and Inagaki et al. (1992) 33 for PI. Regarding the PTFE material, a decreasing value of the polar surface tension component was calculated arising as a consequence of a highly increased water contact angle (C Z 127°). Similar contact angles were



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also observed by Vandencasteele et al. (2006) 34 and Salapare et al. (2013) 16, but the surface tensions were not calculated. Further examination of the PTFE surface was performed by the mentioned authors, and no significant changes were found by using FTIR 16 and XPS 34 measurements. Following this, the changes in the polar surface component can be expected as a result of the ongoing etching process. Furthermore, the contact angles at the silicone rubber/printer bed-interface were investigated in order to identify the best layer, according to Eq. 3, for long spreading times, see Fig. 4. As the dispersive surface tension of the silicone rubbers are quite high in comparison to its polar component, only the dispersive components of the bed materials determine the spreading behavior for long spreading times. Since the dispersive surface tensions of all PI, Parylene-C and the 1 min treated PTFE materials were high in comparison to the silicone rubbers dispersive surface tensions, no contact angles could be measured due to complete wetting of the substrate (@ABC ] 1; calculated according to Eq. 6).

Thereby, only the results of the PTFE material fit to the estimated behavior that a reduced dispersive surface tension (Fig. 3) of a solid leads to increased contact angles of a dispersive liquid (Fig. 4). In comparison, the contact angles measured with the FEP showed no clear influences in the context of the measured surface tensions. Although the dispersive surface tensions of the FEP material first decreased with the plasma functionalization and then increased with the plasma etching, no increased contact angles could be detected. However, since the dispersive surface tensions and the contact angles of the FEP materials were found in the range of the PTFE materials, we assume influences of the silicone rubber spreading and curing process on the quantitative results of the contact angles measurements. According to the results, the highest contact angle was measured with the O2-plasma etched PTFE material. Following the Eq. 3 for long spreading times and the Youngequation (Eq. 6) with the dispersive silicone rubbers, the lowest silicone rubber spreading can be estimated with the PTFE etched material. 4.2 Infrared heat transfer and silicone rubber spreading For the correlation of the silicone rubber’s spreading to the surface tensions of the bed materials and the received heat fluxes, first the attenuation coefficients were calculated according to section 3.3. The calculated values were 7,639 cm-1 for the Sylgard 184 and 8,057 cm-1 at 9.3 µm wavelength, using the measured FTIR-data 2 . Considering the optical enlargement of the system, the irradiance was calculated to values of 48.5 mW/mm² for the DC25 and 78.4 mW/mm² for the DC40 configuration. Then, the silicone rubber spreading was evaluated on the low adhesion layers with regard to their O2-plasma treatment and the modulation setting DC25 and DC40. The results are shown in Fig. 5 and 6. By using the measured radial spreading, the calculated irradiances and the drop volume for each material, the average heat fluxes from the emitter to the samples were calculated for all test materials and for the whole test time, as presented in Fig 5 for the Sylgard 184 and in Fig. 6 for the Silpuran 2430.

Figure 4 Static contact angle between the untreated (*), 1 min (#) and 30 min (∆) O2-plasma treated bed materials and the Sylgard 184 (dark grey) resp. Silpuran 2430 (light grey). The bed materials were polyimide (PI), Poly(chlorop-xylylen) (Parylene-C), polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP), Sylgard 184 and Silpuran 2430.

2

for Sylgard 184: Transmittance = 27.87% , penetration depth = 1.67 µm, n = 1.461 for Silpuran 2430 Transmittance = 25.24% , penetration depth = 1.71 µm, n = 1.473


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7

Figure 5 Resulting spread of Sylgard 184 drops (bars) after roomtemperature (blue) and high speed curing with the laser modulation parameter DC25 (red) and DC40 (green). The related average heat fluxes are grouped for each material (data points). The tests were performed on the untreated (*), 1 min (#) and 30 min (∆) O2plasma treated bed materials. The bed materials were polyimide (PI), Poly(chloro-p-xylylen) (ParyleneC), polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP). Each bar represents five measurements. Overall, the high speed curing experiments showed an apparently decreased radial spreading with an increased irradiance and an increased initial viscosity, independently. All high-viscous Silpuran 2430 samples with a DC25 and DC40 modulation parameter showed a lower radial spreading than the Sylgard 184 samples, except for the lowviscous Sylgard 184 samples with a DC40 modulation parameter and a PI* and PI∆ bed material. Regarding the Silpuran 2430 material, marginal changes in the radial spreading were observed within the FEP, PI and PTFE bed materials, whereas with the Parylene-C material higher radial spreads were measured. Although the similar behavior was observed with the Sylgard 184 material, a high amount of fluctuations could be observed within the results, perhaps as a consequence of the time-dependent spreading behavior according to Eq. 8. Thereby, an increased spreading dynamic can be expected with a reduced viscosity using the Sylgard 184 material instead of the Silpuran 2430 material. Contrary to this, the room-temperature cured samples showed a different behavior. The maximum and the mini-

mum spreading value could be observed with the Sylgard 184 material, whereas the changes of the Silpuran 2430 spreading were quite low in comparison. As expected from results of the surface tension measurements, the highest spreading was measured with the highest dispersive bed materials (Parylene-C and PI). However, as this behavior was not found within the Silpuran 2430 samples, inhibiting effects of the higher initial viscosity and the curing speed (see section 3.1) are presumably the reason for this. Comparing the room-temperature cured samples with the high speed cured ones, all high speed cured samples showed a decreased radial spreading besides the Sylgard 184 material with the functionalized FEP# material. In accordance with Eq. 8, the highest reductions of the radial spreads were observed with the fast curing Silpuran 2430 material. Since the different measured radial spreads of the Silpuran 2430 material were found in a similar dimension, effects of the underlying bed material were not assumed. In comparison, the slowly curing Sylgard 184 showed marginal changes in the radial spread, using the bed materials

Figure 6 Resulting spread of Silpuran 2430 drops (bars) after room-temperature (blue) and high speed curing with the laser modulation parameter DC25 (red) and DC40 (green). The related average heat fluxes are grouped for each material (data points). The tests were performed on the untreated (*), 1 min (#) and 30 min (∆) O2plasma treated bed materials. The bed materials were polyimide (PI), Poly(chloro-p-xylylen) (ParyleneC), polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP). Each bar represents five measurements.



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with the lowest dispersive tension (FEP and PTFE), and large changes using the bed materials with the highest dispersive tension (PI and Parylene-C). Thereby, the results indicate the early formation of a surface tension related steady state for the FEP and PTFE materials (1) and a reduction of the radial spread for the PI and Parylene-C material due to the high speed curing (2). Further improvement with the low-viscous Sylgard 184 material appears to be feasible by using a SF6 or C4F8 plasma for the reduction of the dispersive surface tension, as proposed by Bi et al. (2014) 35 for Parylene-C. Concerning the heat transfer, increased average heat fluxes were calculated with increasing irradiances. As the thickness of the test materials decreased during spreading and thus should lead to decreased heat fluxes, the heat fluxes increased as estimated from the theoretical approach due to the overproportionally increased absorption surface area of the test materials during spreading. Since the observed spreading of the DC25 samples was higher than the spreading of the DC40 samples, the differences between the average heat fluxes of the DC25 and the DC40 modulation parameters were consequently lower in comparison to the calculated irradiances of 48.5 mW/mm² for the DC25 and 78.4 mW/mm² for the DC40 parameters. As a consequence of this nonlinear behavior, no direct correlation of the average heat flux to the measured radial spreads could be determined, as estimated from the theoretically (nonlinear) time-dependent spreading behavior according to Eq. 8. According to the results, the lowest measured spread was 3.24% for the Sylgard 184 material on an untreated PI material (PI*) and 4.83% for the Silpuran 2430 material on a 30 min O2-plasma treated PTFE material (PTFE∆). 5 Conclusion In this paper we presented and evaluated an infrared based high speed curing system for the 3D-printing of conventionally available medical grade room-temperature curing silicone rubber. Therefore, two different silicone rubbers were applied on PI, PTFE, FEP and Parylene-C bed materials and cured using different irradiances of the curing system. Further, influences of the bed material’s surface tensions were evaluated after different O2-plasma treatments by measuring the spreading of the silicone rubbers during curing. A clear reduction of the spreading could be observed using the high-viscous silicone rubber (Silpuran 2430) with all bed materials and the low-viscous silicone rubber (Sylgard 184) with the high dispersive PI and Parylene-C bed materials. Marginal benefits of the high speed curing process were observed with the low-viscous silicone rubber (Sylgard 184) and the bed materials with the lowest dispersive surface tension (FEP and PTFE), because of the already achieved, surface tension related, spreading limitation. Overall, reduced spreads of the silicone rubbers were determined by increasing the silicone rubber’s initial viscosity and the applied irradiances, independently.

In comparison to our earlier work 5, a reduced minimum curing time of 2 s instead of 120 s was achieved. The lowest silicone rubber spread was measured to a value of 3.24% with the untreated PI material and the highest irradiance (DC40), indicating the potential usability of the curing system for 3D-printing processes of silicone rubber. Acknowledgement This project is supported by the Deutsche Forschungsgemeinschaft (DFG), Cluster of Excellence ‘Hearing4All’ and the Bundesministerium für Wirtschaft und Energie, Deutschland, KF3474601AK4. Further, the authors would like to thank the Institut für Anorganische Chemie, Leibniz Universität Hannover, for supporting us with their Bruker Tensor 27 spectrometer. References (1) Clark, G. M.; Hallwoeth, R. J.; Zdanius, K. A Cochlear Implant Electrode J. Laryngol. Otol. 1975, 89, 787-792. (2) Yang, T.; Hakimian, S.; Schwartz, T. H. Intraoperative ElectroCorticoGraphy (ECog): Indications, Techniques, and Utility in Epilepsy Surgery Epileptic Disord. 2014, 16, 271-279. (3) Min, K. S.; Jun, S. B.; Lim, Y. S.; Park, S. I.; Kim, S. J. Modiolus-Hugging Intracochlear Electrode Array with Shape Memory Alloy Comput Math Methods Med 2013, 2013, 1-9. (4) Wang, W.; Degenhart, A. D.; Collinger, J. L.; Vinjamuri, R.; Sudre, G. P.; Adelson, P. D.; Holder, D. L.; Leuthardt, E. C.; Moran, D. W.; Boninger, M. L.; Schwartz, A. B.; Crammond, D. J.; Tyler-Kabara, E. C.; Weber, D. J. Human Motor Cortical Activity Recorded with Micro-ECoG Electrodes, during Individual Finger Movements Conf Proc IEEE Eng Med Biol Soc. 2009, 2009, 586589. (5) Stieghorst, J.; Bondarenkova, A.; Burblies, N.; Behrens, P.; Doll, T. 3D Silicone Rubber Interfaces for Individually Tailored Implants Biomed.Microdevices 2015, 17, 1-10. (6) Chen, K.; Wo, A. M.; Chen, Y. Transmission Spectrum of PDMS in 4-7µm Mid-IR Range for Characterization of Protein Structure NSTI-Nanotech 2006, 2, 732-735. (7) Efimenko, K.; Wallace, W. E.; Genzer, J. Surface Modification of Sylgard-184 Poly(Dimethyl Siloxane) Networks by Ultraviolet and Ultraviolet/Ozone Treatment J. Colloid Interface Sci. 2002, 254, 306-315. (8) Green, R. A.; Ordonez, J. S.; Schuettler, M.; Poole-Warren, L. A.; Lovell, N. H.; Suaning, G. J. Cytotoxicity of Implantable Microelectrode Arrays Produced by Laser Micromachining Biomaterials 2010, 31, 886-893. (9) Tan, W. C.; Koughia, K.; Singh, J.; Kasap, S. O. In Optical Properties of Condensed Matter and Applications; Singh, J., Ed.; John Wiley & Sons Ltd: Chichester, West Sussex, England, 2006; Chapter 1, pp 7-9.


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Toward 3D Printing of Medical Implants: Reduced Lateral Droplet

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