CHAPTER 6 FABRICATION
6.1 BIOCOMPATIBLE MATERIALS
6.1.1 SILICONE
Silicones are a series of siloxane backed polymers with a functional group attached. They are elastomers with good heat resistance, and low chemical reactivity. Silicones begin as a highly viscous liquid, which cures into a solid. There are three methods to polymerize silicone: platinum- based, condensation, and peroxide cures. The acetoxy cure system is common in silicone caulking.
Most medical grade silicones are based on a platinum cure, since the process produces no byproducts.
Platinum cure silicones are two component silicones: a part A that contains silicones with vinyl groups and a platinum catalyst, and a part B with similar vinyl grouped silicones and a hydrogen cross-linker [6.1]. The process is an addition chemistry, where the cross-linker and vinyl groups result in an ethylene bridge (double carbon bond). The resultant silicone is biocompatible because the process does not produce byproducts and pure silicone is relatively chemically inert. Silicone has a high elasticity and a low Young’s modulus, which makes silicone a good material for medical applications. The compliance of silicone approaches that of the human body better than harder materials such as steel or glass. For these reason, silicone has made its way into all sorts of medical implants, where mechanical compliance is important. Cochlear implant uses silicone to sheath its platinum electrodes, cables and transmitter coil [6.2]. Silicones also have high gas and water vapor permeability, making them ideal for the semipermeable membrane used in this thesis.
Processing silicone is a well understood process with soft lithography of poly(dimethylsiloxane) (also called PDMS) being a common process in microfluidics. The techniques involve pouring and curing PDMS over a mold (pattern) and peeling it afterwards. The PDMS part can then be bonded to another PDMS part or to a substrate. Bonding to itself is generally done by using different mixtures of part A and part B, where each piece would have an excess of a different part. PDMS can also be bonded to substrates like glass using oxygen plasma to make both the glass and the PDMS hydrophilic. Manufacturing techniques for silicone parts will be discussed further below.
Silicone NuSil MED4-4210 was predominantly used in the devices, as it is considered a medical grade material and was used successfully in animal implants. These facts reduced biocompatibility concerns in the experiments with rabbits.
Manufacturing techniques for silicone parts will be discussed further below.
6.1.2 PARYLENE
Parylene is a name for polymer called poly(p-xylylene). There are several different Parylene polymers (Figure 6.1), all of which contain a phenyl (6 carbon) ring connected to a carbon atom as the monomer. Each Parylene type contains different elements attached to the basic carbon structure.
Figure 6.1: Poly(p-xylylene) chemical structures. Parylene-C is the most commonly deposited polymer with low gas and moisture impermeability. Parylene D has better barrier properties but general has worse film uniformity. Parylene Ht is a fluorinated parylene with good high temperature properties. Parylene A and AM contain an amine group making them slightly hydrophilic and more reactive than other parylenes.
Parylene is applied to substances by chemical vapor deposition, in which the Parylene dimer is decomposed into a monomer in a Pyrolysis tube at high temperature (690ºC for Parylene-C) and then deposited onto a substrate downstream. The polymer is coated at vacuum in specialized CVD machines, such as the ubiquitous Specialty Coating SystemsTM PDS2010, and the Specialty Coating SystemsTM PDS2035. These machines have 4 parts: a vaporizer chamber, where the dimer
sublimates at temperatures between 120ºC and 180ºC, a pyrolysis tube, where the dimer is cleaved into a polymer, a deposition chamber, where the monomer polymerizes at ambient temperature onto the target device, and a cold trap that collects any remaining monomer to protect the downstream mechanical vacuum pump (Figure 6.2).
Figure 6.2: Parylene deposition process; reprinted from PDS 2010 Owner’s Manual.[6.3].
This vacuum process conformally coats the polymer onto surfaces with very few, if any, defects.
The resultant defect-free polymer coat is used as a barrier material for electronics due to its low gas permeability (0.042Ba for O2 for Parylene-C) and low moisture (WTVR 0.08g·mm·m-2·day-1) permeability [6.4], [6.5], [6.6]. The polymer is very chemically inert and biocompatible (USP rating class VI [6.7]). These properties make the material ideal for medical use and for the oxygenator.
6.1.3 USE OF PARYLENE AND SILICONE IN A DEVICE
In the devices, Parylene was used to protect electronics from water ingress and to restrict oxygen permeation. Silicone was used for the semipermeable membrane to allow for transport of oxygen while leaving the salts in solution. Figure 6.3 shows that a 5µm Parylene-C coat on one side of a diffusor greatly reduces the oxygen readings on that side.
Figure 6.3: Oxygen tension on opposite sides of a device coated with parylene on one side.
5µm Parylene-C coated on side 2 of the device. Side 1 is only comprised of silicone. An oxygen probe was placed against the device in anoxic deionized water.
However, Parylene is difficult to bond to given its chemical inertness. The fact that silicone does not bond to Parylene allows to use it as a mold release on micropatterned molds for silicone. However, when Parylene is deposited on silicone, it forms a strong bond (see below). It is reported that Parylene-C penetrates several microns into PDMS, with a peel force of 1.4N for an 8mm wide Parylene-C film [6.8]. This mechanical bond in which Parylene “caulks” silicone lowers silicone’s gas permeability.