Hubbry Logo
ParyleneParyleneMain
Open search
Parylene
Community hub
Parylene
logo
8 pages, 0 posts
0 subscribers
Be the first to start a discussion here.
Be the first to start a discussion here.
Parylene
Parylene
Parylene
View on Wikipedia
from Wikipedia
Repeating unit of parylene
The para-xylylene monomer

Parylene is the common name of a polymer whose backbone consists of para-benzenediyl rings −C
6H
4− connected by 1,2-ethanediyl bridges −CH
2−CH
2−. It can be obtained by polymerization of para-xylylene H
2C=C
6H
4=CH
2.

The name is also used for several polymers with the same backbone, where some hydrogen atoms are replaced by other functional groups. Some of these variants are designated in commerce by letter-number codes such as "parylene C" and "parylene AF-4". Some of these names are registered trademarks in some countries.

Coatings of parylene are often applied to electronic circuits and other equipment as electrical insulation, moisture barriers, or protection against corrosion and chemical attack (conformal coating). They are also used to reduce friction and in medicine to prevent adverse reactions to implanted devices. These coatings are typically applied by chemical vapor deposition in an atmosphere of the monomer para-xylylene.

Parylene is considered a "green" polymer because its polymerization needs no initiator or other chemicals to terminate the chain; and the coatings can be applied at or near room temperature, without any solvent.

History

[edit]

Parylene was discovered in 1947 by Michael Szwarc as one of the thermal decomposition products of para-xylene H
3C−C
6H
4−CH
3 above 1000 °C. Szwarc identified para-xylylene as the precursor by observing that reaction with iodine yielded para-xylylene di-iodide as the only product. The reaction yield was only a few percent.[1][2]

A more efficient route was found in 1965 by William F. Gorham at Union Carbide. He deposited parylene films by the thermal decomposition of [2.2]paracyclophane at temperatures exceeding 550 °C and in vacuum below 1 Torr. This process did not require a solvent and resulted in chemically resistant films free from pinholes. Union Carbide commercialized a parylene coating system in 1965.[1][2]

Union Carbide went on to undertake research into the synthesis of numerous parylene precursors, including parylene AF-4, throughout the 1960s into the early 1970s. Union Carbide purchased NovaTran (a parylene coater) in 1984 and combined it with other electronic chemical coating businesses to form the Specialty Coating Systems division. The division was sold to Cookson Electronics in 1994.[3]

There are parylene coating service companies located around the world, but there is limited commercial availability of parylene. The [2.2]paracyclophane precursors can be purchased for parylene N, C, D, AF-4 and VT-4. Parylene services are provided for N, C, AF-4, VT-4 and E (copolymer of N and E).

Varieties

[edit]

Parylene N

[edit]

Parylene N is the un-substituted polymer obtained by polymerization of the para-xylene intermediate.

Chlorinated parylenes

[edit]
Repeating unit of parylene C

Derivatives of parylene can be obtained by replacing hydrogen atoms on the phenyl ring or the aliphatic bridge by other functional groups. The most common of these variants is parylene C, which has one hydrogen atom in the aryl ring replaced by chlorine. Another common variant is parylene D, with two such substitutions on the ring.

Parylene C is the most used variety, due to its low cost of its precursor and to the balance of its properties as dielectric and moisture barrier properties and ease of deposition. A major disadvantage for many applications is its insolubility in any solvent at room temperature, which prevents removal of the coating when the part has to be re-worked.

Parylene C is also the most commonly used because of its relatively low cost.[4] It can be deposited at room temperature while still possessing a high degree of conformality and uniformity and a moderate deposition rate in a batch process.

Also, the chlorine on the phenyl ring of the parylene C repeat unit is problematic for RoHS compliance, especially for the printed circuit board manufacture. Moreover, some of the dimer precursor is decomposed by breaking of the aryl-chlorine bond during pyrolysis, generating carbonaceous material that contaminates the coating, and hydrogen chloride HCl that may harm vacuum pumps and other equipment. The chlorine atom leaves the phenyl ring in the pyrolysis tube at all temperatures; however, optimizing the pyrolysis temperature will minimize this problem. The free-radical (phenyl radical) generated in this process is not resonance-stabilized and mitigates the deposition of a parylene-like material on the downside of the pyrolysis tube. This material becomes carbonized and generates particles in situ to contaminate clean rooms and create defects on printed-circuit boards that are often called "stringers and nodules". Parylene N and E do not have this problem and therefore are preferred for manufacturing and clean room use.

Fluorinated parylenes

[edit]

Another common halogenated variant is parylene AF-4, with the four hydrogen atoms on the aliphatic chain replaced by fluorine atoms. This variant is also marketed under the trade names of parylene SF (Kisco) and HT parylene (SCS). The −CF
2− unit that comprises the ethylene chain is the same as the repeating unit of PTFE (Teflon), consistent with its superior oxidative and UV stability. Parylene AF-4 has been used to protect outdoor LED displays and lighting from water, salt and pollutants successfully.

Another fluorinated variant is parylene VT-4 (also called parylene F), with fluorine substituted for the four hydrogens on the aryl ring. This variant is marketed by Kisco with the trademark Parylene CF. Because of the aliphatic −CH2− units, it has poor oxidative and UV stability, but still better than N, C, or D.

Alkyl-substituted parylenes

[edit]

The hydrogen atoms can be replaced also by alkyl groups. Substitution may occur on either the phenyl ring or the ethylene bridge, or both.

Specifically, replacement of one hydrogen on the phenyl ring by a methyl group or an ethyl group yields parylene M and E respectively.

  • Parylene M
    Parylene M
  • Parylene E
    Parylene E
  • Parylene AM-2
    Parylene AM-2

These substitutions increase the intermolecular (chain-to-chain) distance, which makes the polymer more soluble and permeable. For example, compared to parylene C, parylene M was shown to have a lower dielectric constant (2.48 vs. 3.2 at 1 kHz). Parylene E had a lower tensile modulus (175 kpsi (1.21 GPa) vs. 460 kpsi (3.17 GPa)), a lower dielectric constant (2.34 vs. 3.05 at 10 kHz), slightly worse moisture barrier properties (4.1 vs. 0.6 g·mil/(atom·100 in2·24 hr) (11 vs. 1.6 kg·m·pmol−1·m−2·s−1)), and equivalent dielectric breakdown 5–6 kV/mil for a 1-mil coating) but better solubility.[5][6] However, the copolymer of parylene N and E has equivalent barrier performance of parylene C.

Replacement of one hydrogen by methyl on each carbon of the ethyl bridge yields parylene AM-2, [−(CH
3)CH−(C
6H
4)−(CH
3)CH−]
n (not to be confused with an amine-substituted variant trademarked by Kisco). The solubility of parylene AM-2 is not as good as parylene E.

Reactive parylenes

[edit]

While parylene coatings are mostly used to protect an object from water and other chemicals, some applications require a coating that can bind to adhesives or other coated parts, or immobilize various molecules such as dyes, catalysts, or enzymes.

These "reactive" parylene coatings can be obtained with chemically active substituents. Two commercially available products are parylene A, featuring one amine substituent −NH
2 in each unit, and parylene AM, with one methylene amine group −CH
2NH
2 per unit. Both are trademarks of Kisco.

Parylene AM is more reactive than the A variant. The amine of the latter, being adjacent to the phenyl ring, is in resonance stabilization and therefore less basic. However, parylene A is much easier to synthesize and hence cheaper.

Another reactive variant is parylene X, which features an ethinyl group −C≡CH attached to the phenyl ring in some of the units. This variant, which contains no elements other than hydrogen and carbon, can be cross-linked by heat or with UV light and can react with copper or silver salts to generate the corresponding metalorganic complexes Cu-acetylide or Ag-acetylide. It can also undergo "click chemistry" and can be used as an adhesive, allowing parylene-to-parylene bonding without any by-products during processing. Unlike most other variants, parylene X is amorphous (non-crystalline).

Colored parylenes

[edit]

It is possible to attach a chromophore directly to the [2.2]paracyclophane base molecule to impart color to parylene.[citation needed]

Parylene-like copolymers

[edit]

Copolymers[7] and nanocomposites (SiO2/parylene C)[8] of parylene have been deposited at near-room temperature previously. With strongly electron withdrawing comonomers, parylene can be used as an initiator to initiate polymerizations, such as with N-phenyl maleimide. Using the parylene C/SiO2 nanocomposites, parylene C could be used as a sacrificial layer to make nanoporous silica thin films with a porosity of >90%.[9]

Properties

[edit]

Transparency and crystallinity

[edit]

Parylene thin films and coatings are transparent; however, they are not amorphous except for the alkylated parylenes, e.g. parylene E. As a result of this semi-crystallinity, they scatter light. Parylene N and C have a low degree of crystallinity; however, parylene VT-4 and AF-4 are highly crystalline ~60% in their as-deposited condition (hexagonal crystal structure) and therefore are generally not suitable as optical materials.

Parylene C will become more crystalline if heated at elevated temperatures until its melting point at 270 °C.

Parylene N has a monoclinic crystal structure in its as-deposited condition and it does not appreciably become more crystalline until it undergoes a crystallographic phase transformation at ~220 °C to hexagonal, at which point it becomes highly crystalline like the fluorinated parylenes. It can reach 80% crystallinity at anneal temperatures up to 400 °C, after which point it degrades.

Mechanical and chemical

[edit]

Parylenes are relatively flexible (0.5 GPa for parylene N),[10] except for cross-linked parylene X (1.0 GPa),[11] and have poor oxidative resistance (~60–100 °C, depending on failure criteria) and UV stability,[12] except for parylene AF-4. However, parylene AF-4 is more expensive due to a three-step synthesis of its precursor with low yield and poor deposition efficiency. Their UV stability is so poor that parylene cannot be exposed to regular sunlight without yellowing.

Nearly all the parylenes are insoluble at room temperature, except for the alkylated parylenes, one of which is parylene E,[6] and the alkylated-ethynyl parylenes.[13] This lack of solubility has made it difficult to re-work printed circuit boards coated with parylene.

Permeability

[edit]

As a moisture diffusion barrier, the efficacy of halogneated parylene coatings scales non-linearly with their density. Halogen atoms such as F, Cl and Br add much density to the coating and therefore allow the coating to be a better diffusion barrier; however, if parylenes are used as a diffusion barrier against water then the apolar chemistries such as parylene E are much more effective. For moisture barriers the three principal material parameters to be optimized are: coating density, coating polarity (olefin chemistry is best) and a glass-transition temperature above room temperature and ideally above the service limit of the printed-circuit board, device or part. In this regard parylene E is a best choice although it has a low density compared to, for example, parylene C.

Industry specifications

[edit]
Properties[14] Parylene N Parylene C Parylene D Parylene HT/AF4
Melting point (°C) 420 290 380 >500
Continuous service temperature (°C) 60 80 100 350
Short-term service temperature (°C) 80 100 120 450
Linear coefficient of thermal expansion at 25 °C (ppm) 69 35 38 36
Thermal conductivity at 25 °C (W/(m·K)) 0.126 0.084 — 0.096
Specific heat at 20 °C (J/(g·K)) 0.837 0.712 — 1.04
Young's modulus (psi) 350,000 400,000 380,000 370,000
Tensile strength (psi)[clarify] 7,000 10,000 11,000 7,500
Yield strength (psi)[clarify] 6,100 8,000 9,000 5,000
Elongation to break (%) 250 200 200 200
Yield elongation (%) 2.5 2.9 3.0 2.0
Density (g/cm3) 1.1–1.12 1.289 1.418 1.32
Water absorption (% after 24 hours) <0.1 <0.1 <0.1 <0.01
Rockwell hardness R85 R80 R80 R122
Static coefficient of friction 0.25 0.29 0.33 0.15
Dynamic coefficient of friction 0.25 0.29 0.31 0.13

Coating process

[edit]

Parylene coatings are generally applied by chemical vapor deposition in an atmosphere of the monomer para-xylylene or a derivative thereof. This method has one very strong benefit, namely it does not generate any byproducts besides the parylene polymer, which would need to be removed from the reaction chamber and could interfere with the polymerization.

Parts to be coated need to be clean in order to ensure good adherence of the film. Since the monomer diffuses, areas that are not to be coated must be hermetically sealed, without gaps, crevices or other openings. The part must be maintained in a relatively narrow window of pressure and temperature.[15]

The process involves three steps: generation of the gaseous monomer, adsorption on the part's surface, and polymerization of adsorbed film.

Polymerization

[edit]

Polymerization of the adsorbed p-xylylene monomer requires a minimum threshold temperature. For parylene N, its threshold temperature is 40 °C.

The p-xylylene intermediate has two quantum mechanical states, the benzoid state (triplet state) and the quinoid state (singlet state). The triplet state is effectively the initiator and the singlet state is effectively the monomer. The triplet state can be de-activated when in contact with transition metals or metal oxides including Cu/CuOx.[16][17] Many of the parylenes exhibit this selectivity based on quantum mechanical deactivation of the triplet state, including parylene X.

Polymerization may proceed by a variety of routes that differ in the transient termination of the growing chains, such as a radical group −CH•
2 or a negative anion group CH−
2:

Possible parylene polymerization routes

Physisorption

[edit]

The monomer polymerizes only after it is physically adsorbed (physisorbed) on the part's surface. This process has inverse Arrhenius kinetics, meaning that it is stronger at lower temperatures than higher temperatures. There is critical threshold temperature above which there is practically no physisorption, and hence no deposition. The closer the deposition temperature is to the threshold temperature the weaker the physisorption. Parylene C has a higher threshold temperature, 90 °C, and therefore has a much higher deposition rate, greater than 1 nm/s, while still yielding fairly uniform coatings.[4] In contrast, the threshold temperature of parylene AF-4 is very close to room temperature (30–35 °C), as a result, its deposition efficiency is poor.[18]

An important property of the monomer is the so-called 'sticking coefficient', that expresses the degree to which it adsorbs on the polymer. A lower coefficient results more uniform deposition thickness and a more conformal coating.

Another relevant property for the deposition process is polarizability, which determines how strongly the monomer interacts with the surface. Deposition of halogenated parylenes strongly correlates with molecular weight of the monomer. The fluorinated variants are an exception: the polarizability of parylene AF-4 is low, resulting in inefficient deposition.

Monomer generation

[edit]

From the cyclic dimer

[edit]
The cyclic para-xylylene dimer [2.2]paracyclophane

The p-xylylene monomer is normally generated during the coating process by evaporating the cyclic dimer [2.2]para-cyclophane at a relatively low temperature, then decomposing the vapor at 450–700 °C and pressure 0.01–1.0 Torr. This method (Gorham process) yields 100% monomer with no by-products or decomposition of the monomer.[19][20][21]

The dimer can be synthesized from p-xylene involving several steps involving bromination, amination and Hofmann elimination.[22]

The same method can be used to deposit substituted parylenes. For example, parylene C can be obtained from the dimeric precursor dichloro[2.2]para-cyclophane, except that the temperature must be carefully controlled since the chlorine-aryl bond breaks at 680 °C.

Gorham process for parylene AF-4
Gorham process for parylene AF-4

The standard Gorham process[5] is shown above for parylene AF-4. The octafluoro[2.2]para-cyclophane precursor dimer can be sublimed below <100 °C and cracked at 700–750 °C, higher than the temperature (680 °C) used to crack the unsubstituted cyclophane since the −CF2−CF2− bond is stronger than the −CH2−CH2− bond. This resonance-stabilized intermediate is transported to a room temperature deposition chamber where polymerization occurs under low pressure (1–100 mTorr) conditions.[18]

From substituted p-xylenes

[edit]

Another route to generation of the monomer is to use a para-xylene precursor with a suitable substituent on each methyl groups, whose elimination generates para-xylylene.

Selection of a leaving group may consider its toxicity (which excludes sulfur and amine-based reactions), how easily it leaves the precursor, and possible interference with the polymerization. The leaving group can either be trapped before the deposition chamber, or it can be highly volatile so that it does not condense in the latter.[23]

For example, the precursor α,α'-dibromo-α,α,α',α'-tetrafluoro-para-xylene (CF
2Br)
2(C
6H
4) yields parylene AF-4 with elimination of bromine.[24]

The advantage to this process is the low cost of synthesis for the precursor. The precursor is also a liquid and can be delivered by standard methods developed in the semiconductor industry, such as with a vaporizer, vaporizer with a bubbler, or a mass-flow controller. Originally the precursor was just thermally cracked,[25] but suitable catalysts lower the pyrolysis temperature, resulting in less char residue and a better coating.[26][27] By either method an atomic bromine free-radical is given off from each methyl end, which can be converted to hydrogen bromide HBr and removed from monomer flow. Special precautions are needed since bromine and HBr are toxic and corrosive towards most metals and metal alloys, and bromine can damage viton O-rings.

α,α'-dimethoxy-p-xylene

A similar synthesis for parylene N uses the precursor α,α'-dimethoxy-p-xylene.[28] The methoxy group H
3CO− is the leaving group; while it condenses in the deposition chamber, it does not interfere with the deposition of the polymer.[23] This precursor is much less expensive than [2.2]para-cyclophane. Moreover, being a liquid just above room temperature, this precursor can delivered reliably using a mass-flow controller; whereas the generation and delivery of the gaseous monomer of the Gorham process are difficult to measure and control.[29]

α,α'-dimethyl-α,α'-dimethoxy-p-xylene

The same chemistry can generate parylene AM-2 can be generated from the precursor α,α'-dimethyl-α,α'-dimethoxy-p-xylene.

α,α'-diphenoxy-α,α,α',α'-tetrafluoro-p-xylene

Another example of this approach is the synthesis of parylene AF-4 from α,α'-diphenoxy-α,α,α',α'-tetrafluoro-para-xylene. In this case, the leaving group is phenoxy CH
5O−, which can be condensed before the deposition chamber.[30]

Characteristics and advantages

[edit]

Parylenes may confer several desirable qualities to the coated parts. Among other properties, they are

  • Hydrophobic, chemically resistant, and mostly impermeable to gases (including water vapor) and inorganic and organic liquids (including strong acids and bases).
  • Good electrical insulator with a low dielectric constant (average in-plane and out-of-plane: 2.67 parylene N and 2.5 parylene AF-4, SF, HT)[31]
  • Stable and accepted in biological tissues, having been approved by the US FDA for various medical applications.
  • Dense and pinhole free, for thickness above 1.4 nm[32]
  • Homogeneous and uniformly thick, even within cavities.
  • Stable to oxidation up to 350 °C (AF-4, SF, HT)
  • Low coefficient of friction (AF-4, HT, SF)

Since the coating process takes place at ambient temperature in a mild vacuum, it can be applied even to temperature-sensitive objects such as dry biological specimens. The low temperature also results in low intrinsic stress in the thin film. Moreover, the only gas in the deposition chamber is the monomer, without any solvents, catalysts, or byproducts that could attack the object.

Parylene AF-4 and VT-4 are both fluorinated and as a result very expensive compared to parylene N and C, which has severely limited their commercial use, except for niche applications.

Applications

[edit]

Parylene C and to a lesser extent AF-4, SF, HT (all the same polymer) are used for coating printed circuit boards (PCBs) and medical devices. There are numerous other applications as parylene is an excellent moisture barrier. It is the most bio-accepted coating for stents, defibrillators, pacemakers and other devices permanently implanted into the body.[33]

Molecular layers

[edit]

The classic molecular layer chemistries are self-assembled monolayers (SAMs). SAMs are long-chain alkyl chains, which interact with surfaces based on sulfur-metal interaction (alkylthiolates)[34] or a sol-gel type reaction with a hydroxylated oxide surface (trichlorosilyl alkyls or trialkoxy alkyls).[35] However, unless the gold or oxide surface is carefully treated and the alkyl chain is long, these SAMs form disordered monolayers, which do not pack well.[36][37] This lack of packing causes issues in, for example, stiction in MEMS devices.[38]

The observation that parylenes could form ordered molecular layers (MLs) came with contact angle measurements, where MLs thicker than 10 Å had an equilibrium contact angle of 80 degrees (same as bulk parylene N) but those thinner had a reduced contact angle.[32] This was also confirmed with electrical measurements (bias-temperature stress measurements) using metal-insulator-semiconductor capacitors (MISCAPs).[39] In short, parylene N and AF-4 (those parylenes with no functional groups) are pin-hole free at ~14 Å. This results because the parylene repeat units possess a phenyl ring and due to the high electronic polarizability of the phenyl ring adjacent repeat units order themselves in the XY-plane. As a result of this interaction parylene MLs are surface independent, except for transition metals, which de-activate the triplet (benzoid) state and therefore the parylenes cannot be initiated. This finding of parylenes as molecular layers is very powerful for industrial applications because of the robustness of the process and that the MLs are deposited at room temperature. In this way parylenes can be used as diffusion barriers and for reducing the polarizability of surface (de-activation of oxide surfaces). Combining the properties of the reactive parylenes with the observation that they can form dense pin-hole-free molecular layers, parylene X has been utilized as a genome sequencing interface layer.

One caveat with the molecular layer parylenes, namely they are deposited as oligomers and not high polymer.[32] As a result, a vacuum anneal is needed to convert the oligomers to high polymer. For parylene N that temperature is 250 °C, whereas it is 300 °C for parylene AF-4.

Typical applications

[edit]

Parylene films have been used in various applications, including[1]

  • Hydrophobic coating (moisture barriers, e.g., for biomedical hoses)
  • Barrier layers (e.g., for filter, diaphragms, valves)
  • Microwave electronics (e.g., protection of PTFE dielectric substrates from oil contamination)
  • Implantable medical devices
  • Sensors in rough environment (e.g., automotive fuel/air sensors)
  • Electronics for space travel and defense
  • Corrosion protection for metallic surfaces
  • Reinforcement of micro-structures
  • Protection of plastic, rubber, etc., from harmful environmental conditions
  • Reduction of friction, e.g., for guiding catheters, acupuncture needles and microelectromechanical systems.

See also

[edit]

Conformal coating

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Parylene

View on Grokipedia
from Grokipedia
Parylene is a family of polymers based on poly(para-xylylene), deposited as ultrathin, pinhole-free conformal coatings through a room-temperature (CVD) process, providing superior barrier protection against moisture, chemicals, solvents, and environmental contaminants. Discovered unintentionally in 1947 by chemist Michael Szwarc during studies of by-products, parylene was later commercialized in 1965 via the Gorham process, which enabled controlled vapor-phase for practical applications. These materials are prized for their inertness, , and versatility, making them essential in fields such as encapsulation, biomedical implants, and components. The primary variants include Parylene N (unsubstituted, crystalline structure with high elongation up to 250%), Parylene C (monochlorinated for enhanced moisture resistance and most common in industry), Parylene D (dichlorinated for improved thermal stability), and Parylene HT (fluorinated for high-temperature endurance up to 450°C). The deposition process involves three stages: sublimation of a solid dimer precursor (e.g., [2.2]paracyclophane) at approximately 150°C, at 680°C to generate reactive monomers, and spontaneous on substrates at 25°C under (0.1 ), yielding uniform films from 0.1 to 75 μm thick without solvents or catalysts. This vapor-phase method ensures complete coverage of complex geometries, such as circuit boards and microelectromechanical systems (), distinguishing parylene from liquid-applied coatings. Key properties of parylene coatings include outstanding electrical insulation, with dielectric strengths of 5,400–7,000 V/mil and volume resistivities of 10¹⁶–10¹⁷ Ω·cm, making them ideal for high-voltage applications. Mechanically, they exhibit tensile strengths of 7,000–11,000 psi and moduli of 350,000–400,000 psi, with low water absorption (<0.1% over 24 hours) and vapor transmission rates as low as 0.08 g·mm/(m²·day) for Parylene C. Chemically inert up to 150°C, parylenes resist acids, bases, and most solvents, while thermally they remain stable to 80–350°C continuously depending on the type, with melting points ranging from 290°C to over 500°C. In biomedical contexts, their low cytotoxicity and FDA approval for implant use enable applications like neural probes, stents, and tissue scaffolds, where biocompatibility and minimal inflammatory response are critical.

History

Discovery and Early Research

Parylene, specifically poly(p-xylylene), was discovered in 1947 by Michael Szwarc while he was working at the University of Manchester. During experiments on the thermal decomposition of p-xylene, Szwarc observed an unexpected polymeric byproduct forming as a thin film in a cold trap. This discovery occurred as part of his PhD research in physical chemistry under Professor Michael Polanyi, where he earned his degree in 1947. The pyrolysis process involved heating p-xylene to temperatures exceeding 1000°C under low pressure, leading to its breakdown into reactive diradical species, primarily p-xylylene (a quinonoid hydrocarbon derived from dehydrogenation of p-methyl benzyl radicals). These intermediates underwent spontaneous radical polymerization upon cooling, resulting in the formation of poly(p-xylylene) films. Szwarc identified the mechanism as involving free-radical addition, with the diradical nature of p-xylylene enabling efficient chain growth without typical initiators. In the late 1940s and early 1950s, Szwarc conducted further laboratory experiments at Manchester to explore the polymer's properties, focusing on its ability to form conformal films through vapor-phase deposition. These studies involved pyrolyzing p-xylene vapors and directing them onto substrates maintained at room temperature, where polymerization occurred spontaneously, producing thin, transparent coatings described as "snakeskin-like." Such experiments highlighted the potential for controlled deposition but remained at the academic research stage until Szwarc's departure from Manchester in 1952. Szwarc's foundational work was documented in key publications, including his 1947 paper in Discussions of the Faraday Society, where he first described the thermal decomposition and polymerization of p-xylylene. He later co-authored a comprehensive 1958 review in the Quarterly Reviews of the Chemical Society with L.A. Errede, detailing the radical mechanisms and structural analogs of p-xylylene polymerization. These works established the scientific basis for understanding parylene's formation and reactivity.

Commercialization and Initial Adoption

In the early 1960s, William F. Gorham at Union Carbide Corporation developed a controlled vapor deposition process, now known as the Gorham process, which enabled the practical application of parylene films through the pyrolysis and polymerization of a stable dimer precursor in a vacuum chamber. This innovation addressed the challenges of earlier uncontrolled deposition methods, allowing for uniform, conformal coatings on complex substrates without solvents or catalysts. Union Carbide commercialized parylene in 1965, launching it under the trade name "Parylene" specifically for use as a conformal coating in electronics, with the initial announcement made on February 17, 1965. A key milestone was the issuance of US Patent 3,342,754 in 1967 to Gorham, which detailed the para-xylylene polymer deposition method and solidified the intellectual foundation for industrial production. Early military specifications, such as MIL-I-46058C, soon incorporated parylene for protective applications in harsh environments, recognizing its reliability for defense electronics. Initial adoption in the 1960s focused on the electronics industry, where parylene coatings provided essential insulation and moisture protection for components like ferrite cores in core memory systems used in early computers. This application leveraged parylene's pinhole-free, dielectric properties to enhance reliability in miniaturized circuits, marking its transition from laboratory curiosity to a standard protective material in aerospace and military hardware.

Chemical Composition and Varieties

Molecular Structure

Parylene constitutes a family of poly(p-xylylene) polymers, characterized by a repeating backbone structure. The unsubstituted variant, known as parylene N, features the general formula −[\ceCH2−C6H4−CH2]n-[\ce{CH2-C6H4-CH2}]_n, where the para-substituted benzene ring is linked by methylene groups. This polymer exhibits a linear, thermoplastic configuration, formed through the vapor-phase polymerization of p-xylylene monomers generated from the thermal cleavage of di-p-xylylene dimers. The benzene rings in the structure contribute rigidity and thermal stability to the chain, while the intervening methylene (−\ceCH2−-\ce{CH2}- ) groups enable flexibility and facilitate the step-growth addition mechanism during deposition. The polymerization process is depicted by the simplified equation: n \ceCH2−C6H4−CH2→[−\ceCH2−C6H4−CH2−]nn \ \ce{CH2-C6H4-CH2} \rightarrow [-\ce{CH2-C6H4-CH2}-]_n Certain parylene variants incorporate substituents, such as chlorine or fluorine atoms, on the benzene ring to modify performance characteristics.

Standard Types

Parylene N, also known as poly(p-xylylene), is the unsubstituted base form of the polymer family, synthesized via chemical vapor deposition from the di-p-xylylene dimer, or (2.2)paracyclophane. This variant exhibits the highest degree of crystallinity among standard types due to its simple hydrocarbon structure, making it suitable for applications requiring basic electrical insulation, such as in high-frequency components. Parylene C is produced from the dichloro-di-p-xylylene dimer, featuring a single chlorine atom substituted on the benzene ring of each repeating unit, which enhances its barrier properties compared to Parylene N. This monochlorinated structure provides improved resistance to moisture and chemicals, positioning it as the most widely adopted type for conformal coatings in electronics, where it protects circuit boards and components from environmental degradation. Parylene D derives from the tetrachloro-di-p-xylylene dimer, with two chlorine atoms per benzene ring in the polymer chain, resulting in a dichlorinated poly(p-xylylene). The additional chlorination imparts greater thermal stability than Parylene C, allowing it to serve as a protective coating in environments with elevated temperatures, though it is less common due to processing challenges and reduced flexibility. Parylene HT is synthesized from the [2.2]paracyclophane dimer with alpha hydrogens replaced by fluorine, incorporating four fluorine atoms per repeating unit on the methylene bridges (as CF2 groups). This fluorinated composition delivers superior resistance to radiation and high temperatures up to 450°C short-term, making it ideal for demanding applications in and where durability under extreme conditions is essential. The standard types differ primarily in their halogen substitutions, which influence crystallinity, stability, and application suitability, as summarized below:
TypeSubstitutionKey Distinguishing FeaturePrimary Use
Parylene NNone (unsubstituted)Highest crystallinityBasic insulation
Parylene COne Cl on ringImproved barrier properties protection
Parylene DTwo Cl on ringEnhanced stabilityHigh-temperature coatings
Parylene HTFour F on methylene bridgesSuperior UV and high-temperature resistance (up to 450°C short-term) and optical devices

Specialized and Modified Types

Alkyl-substituted parylenes are derived from the base poly(p-xylylene) structure by incorporating n-alkyl chains, typically ranging from methyl to longer variants like hexyl or octyl, directly onto the aromatic rings or bridges via of substituted [2.2]paracyclophane dimers. These modifications enhance in organic solvents, which is otherwise limited in standard parylenes, facilitating easier processing and patterning for applications such as biomedical stents where improved adhesion to substrates like metals is critical. For instance, poly(n-octyl-p-xylylene) exhibits a temperature around 50–60°C and maintains thermal stability up to 350°C, outperforming unsubstituted variants in flexibility without compromising barrier properties. Reactive parylenes incorporate functional groups, such as vinyl or moieties, into the backbone to enable post-deposition cross-linking or covalent bonding with substrates and biomolecules. Vinyl-substituted variants, synthesized from appropriately modified paracyclophane precursors, demonstrate increased reactivity during low-pressure , where the substituents influence polymerization kinetics and allow for subsequent reactions like for surface functionalization. This approach is particularly useful in bioMEMS, where alkyne groups on parylene surfaces promote strong attachments to proteins or cells, enhancing and stability in physiological environments. Colored parylenes are achieved through copolymerization with chromophore-bearing monomers or pigments during vapor deposition, imparting specific hues while retaining the conformal nature of the base . Recent advancements involve doping with organic pigments like derivatives, enabling precise control of chromatism for identification or optical , with coatings maintaining transparency in non-pigmented regions and adhesion strengths exceeding 10 MPa on substrates like . These variants address needs in minimally invasive implants, where color coding aids surgical precision without altering core protective qualities. Halogen-free options, exemplified by ParyFree developed post-2010, substitute chlorine or fluorine with silicon-based or other non-halogenated groups to comply with environmental regulations like RoHS, while preserving vapor deposition compatibility. This variant exhibits no detectable halogens per BS EN 14582:2007 testing and provides improved mechanical toughness, with Young's modulus around 2.5 GPa, alongside enhanced electrical insulation (dielectric strength >200 kV/mm). It supports IPX7/IPX8 ratings for water ingress protection, addressing corrosion concerns in harsh electronics without the ecological drawbacks of traditional halogenated parylenes. Parylene-like copolymers involve hybridizing the base structure with other monomers, such as blending Parylene C with F or incorporating linear alkyl chains, to tailor properties like thermal stability and elasticity. For example, copolymers of Parylene C and F achieve decomposition temperatures above 450°C and energies to substrates increased by 30% compared to homopolymers, via controlled dimer ratios in deposition. Silicone-parylene hybrids, often layered or interpenetrating networks, combine the of silicones with parylene's conformality, yielding flexible coatings for implants with elongation at break up to 20% higher than pure parylene. These modifications enable customized solutions for dynamic environments, prioritizing and durability over exhaustive listings of variants.

Physical and Chemical Properties

Mechanical Properties

Parylene films are characterized by their high tensile strength relative to other coatings, enabling robust performance in demanding applications. For instance, Parylene N exhibits a tensile strength of approximately 45 MPa, while Parylene C reaches 70 MPa, reflecting differences in molecular structure and crystallinity. These values, measured per ASTM D882 standards, underscore parylene's ability to withstand mechanical stress without fracturing. The elongation at break for parylene types N and C typically ranges from 200% to 250%, indicating exceptional that allows the films to conform to intricate substrates and complex shapes without cracking. This flexibility is complemented by a of 2.4 GPa for parylene N and 2.8 GPa for parylene C, with the higher crystallinity of type N contributing to its relatively rigid behavior compared to the more chlorinated type C. Yield strength further supports durability, at 42 MPa for N and 55 MPa for C, ensuring the material deforms elastically under load before permanent deformation occurs. Parylene demonstrates strong fatigue resistance, particularly under cyclic bending and thermal cycling conditions, making it ideal for flexible electronics. In thin-film parylene-metal-parylene devices with 24 μm total thickness, structures endured over 100,000 bending cycles at radii as small as 138 μm with minimal impedance degradation, highlighting preserved mechanical integrity. Film thickness significantly influences overall performance; typical deposition ranges from 0.1 to 50 μm, where thinner layers (e.g., <1 μm) enhance flexibility and tight conformance to microstructures, while thicker films (e.g., 25-50 μm) bolster resistance to abrasion and long-term wear.

Thermal and Chemical Stability

Parylene coatings demonstrate robust thermal stability, enabling their use in environments requiring sustained exposure to elevated temperatures. Standard variants such as Parylene N support continuous operation up to 60°C, C up to 80°C, and D up to 100°C without significant loss of physical properties, as evidenced by long-term performance studies showing no degradation over 10 years at 80°C in air. In contrast, Parylene F and the high-temperature variant Parylene HT extend this capability, with F suitable for continuous use up to approximately 200°C and HT up to 350°C, allowing short-term excursions to 450°C. The temperature (Tg) of Parylene further characterizes its thermal behavior, marking the point where the shifts from a glassy to a rubbery state. For Parylene N, Tg typically ranges from 13°C to 80°C, and for C from 80°C to 100°C, influencing the coating's rigidity and flexibility under , while Parylene F exhibits a Tg around 60°C due to its fluorinated structure, though its overall thermal endurance remains higher. These transitions are determined through techniques like (DSC), highlighting the material's suitability for applications below these thresholds to avoid mechanical weakening. Chemically, Parylene is highly inert, exhibiting insolubility in most organic solvents even at temperatures up to 150°C, which preserves coating integrity during exposure to harsh cleaning agents or processing fluids. It resists degradation from common acids, bases, and salts; for example, immersion in 98% at room temperature results in negligible weight change or structural damage, with swelling limited to less than 1% even at 75°C. This inertness stems from the polymer's non-polar, aromatic backbone, which minimizes interactions with reactive species. Oxidative resistance in Parylene is more limited, particularly without protective additives, with degradation onset between 60°C and 100°C in air due to thermo-oxidative processes. At these temperatures, oxygen attacks the chains, leading to mechanisms such as chain scission—where C-H bonds break, forming carbonyl groups and reducing molecular weight—which ultimately causes embrittlement and loss of mechanical integrity. Studies on Parylene C confirm this pathway through spectroscopic analysis, emphasizing the need for inert atmospheres or antioxidants in high-temperature oxidative environments. Parylene's chemical stability extends to biocompatibility, making it ideal for medical applications where low toxicity is essential. Medical-grade variants, such as Parylene C, comply with USP Class VI standards—the most stringent for plastics—demonstrating no adverse tissue reactions, minimal cytotoxicity, and high biostability in physiological environments. This certification, along with ISO 10993 compliance, underscores its inert nature and lack of leachable components, supporting long-term implantation without inflammation or degradation.

Electrical and Optical Properties

Parylene films exhibit excellent electrical insulation properties, making them suitable for applications requiring high performance. The of parylene N is approximately 7,000 volts per mil, while parylene C and D range from 5,600 to 5,500 volts per mil, enabling effective protection against in thin coatings. The dielectric constant remains low and stable across frequencies, measuring 2.65 at 1 MHz for parylene N, 2.80 for parylene C, and 2.17 for parylene D, which supports minimal signal distortion in electronic devices. Additionally, the is very low, at 0.0006 for parylene N, 0.002 for parylene C, and 0.001 for parylene D at 1 MHz, indicating efficient energy storage with limited losses. Volume resistivity exceeds 10^16 ohm-cm across types, with parylene N reaching 1.4 × 10^17 ohm-cm under standard conditions, ensuring superior resistance to current leakage. Optically, parylene coatings are highly transparent in the visible spectrum, with transmittance exceeding 90% for thin films of parylene C, and they appear colorless due to minimal absorption above 280 nm. The refractive index varies slightly by type, at 1.661 for parylene N, 1.639 for parylene C, and 1.669 for parylene D, influencing light propagation in optical components. Parylene N is semi-crystalline with a crystallinity degree of 50–70%, which contributes to its robust characteristics but introduces minor light scattering that slightly reduces transparency compared to fully amorphous variants. In contrast, parylene C exhibits lower crystallinity around 45%, resulting in smoother films with higher optical clarity.

Barrier and Permeability Characteristics

Parylene coatings provide exceptional barrier protection against moisture, gases, and environmental contaminants due to their uniform, pinhole-free structure achieved through vapor deposition polymerization, which ensures complete encapsulation without defects common in liquid-applied coatings. This conformal nature makes Parylene particularly effective for protecting sensitive and devices from corrosion and degradation. The water vapor transmission rate (WVTR) of Parylene C, widely used for its superior barrier performance among standard types, is characterized by a permeability of 0.08 g·mm/(m²·day) at 37°C and 90% relative . For a typical thickness of 25 µm (0.025 mm), this translates to a WVTR of approximately 3.2 g/m²/day under those conditions; however, measurements at lower temperatures and humidities, such as 23°C and 50% RH, yield values around 0.6 g/m²/day, falling within the 0.1–1 g/m²/day range depending on exact thickness and testing protocol. In comparison, common alternatives like epoxies exhibit WVTR values roughly 10 times higher (e.g., 0.94 g·mm/(m²·day)), underscoring Parylene's superiority from its defect-free morphology. Oxygen permeability for Parylene C is also low at 2.8 (cc·mm)/(m²·day·) at 25°C, significantly lower than the 15.4 for non-chlorinated Parylene N, enabling effective gas barrier applications. For a 25 µm film, this corresponds to an oxygen transmission rate (OTR) of about 112 cc/m²/day/. Chlorinated variants like Parylene C thus offer enhanced protection against oxidation compared to other poly(p-xylylene) types. Permeability in Parylene films is influenced by several factors, including thickness (inversely proportional, with thicker films reducing transmission rates), degree of crystallinity (higher crystallinity from annealing lowers permeability by up to 147% in domain size for Parylene C), and temperature (following Arrhenius behavior, where rates increase exponentially with heat). These attributes align with industry standards such as MIL-I-46058C Type XY for conformal coatings, ensuring reliability in demanding environments like and biomedical devices. The inherent of Parylene further sustains these barrier properties over extended periods.

Deposition Process

System Components and Stages

The Parylene deposition process utilizes a specialized vacuum chamber system designed to facilitate the controlled vaporization, pyrolysis, and polymerization of parylene precursors. Key components include a sublimator (or vaporizer) for heating the solid dimer, a pyrolyzer furnace to convert the dimer gas into reactive monomers, and a separate deposition chamber where substrates are placed for coating. The entire system operates under vacuum conditions, typically at pressures ranging from 10^{-2} to 10^{-1} Torr (10-100 mTorr), to ensure uniform gas flow and prevent contamination. Additional elements, such as a cold trap cooled to subzero temperatures (e.g., -60°C or lower) and a vacuum pump, manage effluent capture and maintain system integrity. The process unfolds in three sequential stages within this integrated setup. First, in the sublimation stage, the solid parylene dimer—typically in granular or powdered form—is loaded into the sublimator and heated to 100-150°C, causing it to sublimate directly into a dimeric gas without passing through a liquid phase. This gas then flows into the pyrolyzer for the second stage, where it is heated to 650-700°C, cleaving the dimer molecules into highly reactive p-xylylene diradical monomers. Finally, in the deposition stage, these monomers enter the room-temperature deposition chamber (around 20-25°C), where they spontaneously polymerize and condense onto the substrates, forming a conformal pinhole-free film. Process parameters are carefully controlled to achieve desired film thicknesses and uniformity. Deposition rates typically range from 0.1 to 1 μm per minute, influenced by factors such as vacuum pressure setpoint (e.g., 5-20 mTorr above base pressure) and the amount of dimer loaded. A full cycle, from loading to venting, generally takes 1-4 hours to produce films of 5-25 μm thickness, depending on the parylene type and system configuration. Safety considerations are paramount due to the involvement of high and operations. The sublimator and pyrolyzer require precise monitoring to prevent overheating, which could generate harmful gases if vented prematurely (e.g., only vent when drop below 90°C). integrity must be maintained to avoid ingress, which can damage pumps; operators use protective gear and follow protocols for handling and adhesion promoters.

Dimer Sublimation and Monomer Generation

The deposition of parylene films begins with the preparation of the precursor, which is typically a cyclic di-p-xylylene dimer in the form of a white solid powder. This dimer, with the molecular formula C16_{16}H16_{16}, serves as the starting material for generating the reactive species. The initial step involves sublimation of the solid dimer, where it is heated to approximately 150°C under reduced pressure (around 1 ) to transition directly into the vapor phase without or . This gaseous dimer then undergoes in a subsequent high-temperature zone, typically at 650–700°C and about 0.5 , where thermal cracking cleaves the central methylene-methylene bonds quantitatively to produce the p-xylylene , a reactive o-quinodimethane biradical with the formula C8_8H8_8. The reaction can be represented as: (C16H16)→2(C8H8)\text{(C}_{16}\text{H}_{16}) \to 2 \text{(C}_8\text{H}_8) This process achieves greater than 95% conversion efficiency, facilitated by an inert atmosphere that minimizes side reactions such as oxidation or recombination. The resulting monomers then proceed to the stage for film formation.

Polymerization and Film Formation

In the and film formation stage of Parylene deposition, the reactive p-xylylene monomers, generated upstream via of the dimer precursor, enter the deposition chamber where the substrate is maintained at near-room temperature. These monomers onto the substrate surface primarily through van der Waals forces, forming a multilayer adsorbate without immediate chemical bonding. The heat of physisorption is approximately 75 kJ/mol, enabling reversible adsorption that contributes to the process's kinetic control. The proceeds as a step-growth mechanism involving diradical between adsorbed . Each p-xylylene possesses diradical character at the methylene groups, allowing spontaneous without the need for initiators, catalysts, or solvents; a single isolated typically does not react, but when a second or third adsorbs adjacent to an existing one or small cluster, they form covalent bonds, initiating via radical -end reactions. This process is self-limiting to roughly a monolayer per adsorption event due to the low sticking coefficient (on the order of 10^{-3} to 10^{-5}, depending on temperature), which prevents excessive buildup in any one area and ensures that growth occurs only where sufficient density allows . The resulting poly(para-xylylene) chains grow linearly, achieving high molecular weights (up to ~500,000 Da) through successive diradical additions, with the reaction ceiling temperature (25–68°C for common variants) limiting at elevated substrate temperatures. The gaseous nature of the monomer phase enables highly conformal deposition, providing uniform coverage over complex geometries, including shadowed regions, undercuts, and high-aspect-ratio features, as the low sticking probability allows monomers to diffuse across the surface before reacting. Film thickness is precisely controlled by adjusting the monomer flux (via ) and exposure time, with growth rates typically ranging from 0.1 to 1 μm/min under standard conditions, allowing layers from nanometers to tens of micrometers. Post-deposition, no curing or annealing is required, as the films form fully and pinhole-free directly upon polymerization completion. The resulting films exhibit densities of 1.1–1.4 g/cm³, depending on the Parylene variant (e.g., 1.11 g/cm³ for Parylene N and 1.29 g/cm³ for Parylene C). to the substrate occurs mechanically through interchain entanglement and penetration into surface pores or roughness, providing robust bonding without chemical primers in many cases.

Advantages and Limitations

Primary Advantages

Parylene coatings provide pinhole-free, uniform conformal coverage due to their vapor-phase deposition , which allows the to penetrate complex geometries and coat all exposed surfaces evenly without the need for solvents, liquid carriers, or masking techniques. This results in a stress-free that adheres intimately to substrates, minimizing defects such as bubbles or shadows that are common in traditional liquid coatings. One of the key strengths of parylene is its , earning it a USP Class VI rating and approval from the (FDA) for use in implantable medical devices, as it exhibits no , , or in biological testing. This hypoallergenic nature stems from its chemical inertness and lack of leachable additives, making it suitable for long-term contact with human tissue without adverse reactions. The room-temperature deposition process of parylene preserves the integrity of heat-sensitive substrates, as occurs spontaneously at ambient conditions under , avoiding thermal stresses or degradation that could affect delicate or biological materials. This solvent-free method also eliminates volatile organic compounds (VOCs), aligning with environmental regulations and reducing health risks during application. Parylene demonstrates exceptional environmental stability, further enhancing its sustainability profile. Its versatility allows application to diverse substrates including metals, plastics, and ceramics, with controllable film thicknesses ranging from nanometers to micrometers, enabling tailored protection levels. Additionally, parylene's low permeability to and gases contributes to its superior barrier performance compared to many conventional coatings.

Challenges and Limitations

Despite its many benefits, parylene deposition involves significant practical challenges that can limit its adoption in certain applications. The process requires specialized vacuum-based equipment, which is capital-intensive, with systems costing hundreds of thousands of dollars and necessitating skilled operators. Additionally, the batch-oriented nature of the deposition—typically taking 8 to 24 hours per run—restricts throughput, making it less suitable for high-volume production where continuous processing is preferred, and leading to higher labor and operational costs. Adhesion of parylene films to substrates remains a key hurdle, particularly on low-surface-energy materials such as (PP) and (PTFE), where the coating's inert nature results in weak without surface . To mitigate this, primers or promoters, like silane-based treatments, are often required, adding extra steps and costs to the process. Poor initial can lead to under stress, compromising the coating's protective function. Achieving uniform thickness across large or complex geometries poses further difficulties due to the vapor-phase deposition mechanism. In structures with significant height variations or intricate features, shadowing effects can cause thinner films or pinholes in recessed areas, reducing barrier performance and necessitating techniques like sample rotation for improved coverage. This non-line-of-sight limitation, while enabling conformality on simple parts, demands careful chamber design and process optimization for demanding substrates. Environmental considerations have increasingly scrutinized halogenated parylene variants, such as types C (chlorinated) and D (dichlorinated), due to their potential to release persistent organic pollutants during disposal or incineration of electronic waste. Global regulations and industry standards, including IEC limits on chlorine and bromine (≤900 ppm each), have driven a shift toward halogen-free alternatives like ParyFree®, introduced in 2019, to address these persistence and toxicity concerns without sacrificing performance. Finally, removing parylene coatings for rework or repair is labor-intensive and not straightforward, as the polymer's chemical inertness resists common solvents at . Effective methods include with oxygen plasma for precise removal or chemical dissolution using hot (THF), but these processes are time-consuming, potentially damaging underlying components, and increase overall manufacturing complexity. Mechanical abrasion offers a faster alternative for spot repairs but risks substrate integrity, highlighting parylene's between durability and reworkability.

Applications

Electronics and Protective Coatings

Parylene serves as a premier for printed circuit boards (PCBs) in , forming a pinhole-free, ultrathin layer that shields components from ingress, dust accumulation, and mechanical vibration. This protection is critical for devices such as smartphones, wearables, and household appliances, where environmental exposure could otherwise lead to or short circuits. The vapor-deposited nature of parylene ensures complete coverage of complex geometries, including undercuts and sharp edges, without adding significant weight or stress to the assembly. In and sectors, parylene coatings meet stringent MIL-I-46058C specifications, providing robust encapsulation for systems and sensors deployed in extreme conditions. These applications demand resistance to rapid temperature fluctuations from -200°C to 150°C, high-altitude , and particles, where parylene's low and high prevent failures in mission-critical electronics. For instance, it safeguards flight control modules and radar interfaces against and chemical contaminants encountered during operations. The leverages parylene for coating electronic control units (ECUs), light-emitting diodes (LEDs) in headlights, and in electric vehicles (EVs), offering superior defense against salt spray from road de-icing, fuel vapors, and vibrational stresses from engine operation. In EVs specifically, parylene protects battery management systems and inverters from thermal cycling and corrosive electrolytes, enhancing reliability under prolonged exposure to automotive fluids and environmental hazards. Its chemical inertness ensures compliance with industry standards like ISO 16750 for environmental testing. Since its commercialization in the for and early , parylene usage has shifted from insulating basic circuit protections to supporting advanced micro-electro-mechanical systems () and in modern devices. This evolution reflects its adaptability, from initial vapor deposition trials in high-reliability circuits to current integration in bendable substrates for wearable tech and sensors, where thin films maintain functionality without compromising flexibility. Parylene's barrier characteristics, including low permeability to and oxygen, underpin its effectiveness in extending the operational life of coated in these sectors. It holds a dominant position in the market for high-reliability applications, widely adopted for its proven performance in demanding environments.

Medical and Biomedical Devices

Parylene coatings, particularly Parylene C, are widely utilized in medical and biomedical devices due to their exceptional and ability to provide a pinhole-free, conformal barrier that protects sensitive components from bodily fluids and . In implantable devices such as pacemakers and neural probes, parylene serves as an encapsulating layer that minimizes tissue irritation and reduces by forming a interface with biological tissues. For stents, the enhances biostability, preventing and ion release while maintaining mechanical integrity during deployment and long-term implantation. In systems, parylene enables the creation of controlled-release membranes with tunable permeability, where coating thickness directly influences drug diffusion rates to achieve sustained release profiles over days to weeks. For instance, parylene-coated nanofibers can extend release from hours to up to 30 days, preventing burst effects and supporting or implantable applications. This tunability arises from parylene's low transmission rate and hydrophobic properties, allowing precise adjustment via vapor deposition parameters. Parylene provides critical electrical insulation for surgical tools, including endoscopes and catheters, by depositing a uniform layer that prevents shorts and leakage while withstanding sterilization cycles. In endoscopes, the coating ensures smooth operation and resistance to bodily fluids, reducing risks during procedures. For catheters, its low coefficient, comparable to Teflon, facilitates navigation through vascular or luminal pathways without particle shedding. Regulatory compliance is a cornerstone of parylene's adoption in biomedical devices, with Parylene C meeting standards for , , and implantation, as well as USP Class VI certification for plastics in prolonged contact with tissue. It is a standard coating for FDA-approved devices, including implants, due to its proven biostability and lack of extractables that could elicit adverse reactions. A notable involves parylene coatings on cochlear implants, where the material's hydrolytic resistance and enable long-term encapsulation of electrodes. This durability, demonstrated through studies, reduces failure rates from fluid ingress and , allowing reliable performance in the cochlear environment. Parylene's mechanical flexibility further aids seamless integration with flexible implant substrates.

Emerging and Specialized Uses

Parylene coatings have found emerging applications in sensors and (IoT) devices, particularly for encapsulation in wearable monitors and environmental sensors. In wearable monitors, Parylene C serves as a biocompatible, flexible piezoelectric layer, enabling highly sensitive pressure sensing for real-time physiological monitoring. For environmental sensors, Parylene provides conformal passivation against moisture, chemicals, and biological agents, enhancing durability in harsh outdoor conditions. Additionally, pyrolyzed Parylene-N transforms into carbon-like electrodes suitable for electrochemical detection of neurotransmitters such as and , offering low impedance and high stability for neural interfaces. Recent reviews highlight pyrolyzed parylene's growing use in advanced sensors for biomedical and as of 2025. These advancements leverage Parylene's inertness and thin-film uniformity to support compact, reliable IoT ecosystems in and . In , Parylene acts as a critical barrier layer in organic light-emitting diodes (OLEDs) and solar cells, mitigating moisture and oxygen permeation to extend device lifespan. For instance, Parylene-C planarization layers enable textile-based OLEDs, facilitating bendable displays with maintained optical performance. In flexible organic solar cells, alternating Parylene-alumina dyads encapsulate modules, achieving water vapor transmission rates (WVTR) as low as 1.7 × 10⁻⁴ g/m²/day under accelerated testing conditions. These barrier properties stem from Parylene's pinhole-free deposition, allowing integration without compromising flexibility. Niche uses extend to cultural heritage conservation, where Parylene conformal coatings protect metallic artifacts from corrosion and environmental degradation through vapor-phase application that penetrates complex surfaces without altering aesthetics. In optics, Parylene AF-4 offers potential for anti-fog treatments on lenses due to its hydrophobic and low-friction properties, though applications remain exploratory. Recent developments include multilayer Parylene-inorganic stacks for OLED encapsulation, such as Parylene-Al₂O₃ configurations yielding WVTR values below 10⁻³ g/m²/day, enhancing device reliability in flexible displays. In smart farming, Parylene C-based humidity sensors integrate sensing and passivation layers for soil moisture monitoring, enabling precise irrigation in IoT-enabled agriculture with resistance to fertilizers and pesticides. Sustainability efforts feature halogen-free Parylene variants like ParyFree®, which eliminate chlorine content while retaining superior barrier and properties, aligning with regulations for eco-friendly . These variants reduce environmental impact in green manufacturing by avoiding emissions during production and disposal. The global Parylene market, driven by such innovations, is projected to reach approximately $1.25 billion by 2032, reflecting growth in specialized sectors like IoT and flexible devices.

References

  1. https://www.physics.rutgers.edu/~podzorov/parylene%20properties.pdf
  2. https://pmc.ncbi.nlm.nih.gov/articles/PMC6187609/
  3. https://pmc.ncbi.nlm.nih.gov/articles/PMC5445890/
  4. https://www.sciencedirect.com/topics/computer-science/parylene
  5. https://royalsocietypublishing.org/doi/pdf/10.1098/rsbm.2006.0025
  6. https://pubs.rsc.org/en/content/articlelanding/1947/df/df9470200046
  7. https://pubs.rsc.org/en/content/articlelanding/1958/qr/qr9581200301
  8. https://scscoatings.com/parylene-coatings/parylene-history/
  9. https://patents.google.com/patent/US3342754A/en
  10. https://www.paryleneengineering.com/history_of_parylene.htm
  11. https://scscoatings.com/parylene-coatings/application-evolution/
  12. https://surfacetechnologies.curtisswright.com/our-services/engineered-coatings/parylene-coating
  13. https://onlinelibrary.wiley.com/doi/10.1002/0471238961.2425122502050103.a01
  14. https://pubs.acs.org/doi/10.1021/acs.chemmater.2c00111
  15. https://scscoatings.com/parylene-coatings/properties/
  16. https://scscoatings.com/newsroom/blog/different-types-of-parylene/
  17. https://nrf.aux.eng.ufl.edu/_files/documents/14393.pdf
  18. https://pubs.acs.org/doi/abs/10.1021/ma202270w
  19. https://onlinelibrary.wiley.com/doi/full/10.1002/admi.201901858
  20. https://www.sciencedirect.com/science/article/abs/pii/S0301010410000285
  21. https://www.mdpi.com/1996-1944/3/3/1803
  22. https://onlinelibrary.wiley.com/doi/10.1002/admi.202300787
  23. https://scscoatings.com/newsroom/specialty-coating-systems-develops-new-halogen-free-parylene-coating/
  24. https://ieeexplore.ieee.org/abstract/document/9795465
  25. https://pmc.ncbi.nlm.nih.gov/articles/PMC10007139/
  26. https://www.researchgate.net/publication/264791564_Enhanced_elasticity_in_parylene_thin_films_by_copolymerization_approach
  27. https://nrf.aux.eng.ufl.edu/_files/documents/176.pdf
  28. https://scscoatings.com/newsroom/blog/elongation-properties-of-parylene/
  29. https://scscoatings.com/newsroom/blog/a-guide-to-parylene-service-temperatures/
  30. https://www.gluespec.com/Materials/SpecSheet/c5d07128-00dd-4e0a-a155-65e90e914062
  31. https://www.paryleneengineering.com/thermal_properties_of_parylene.htm
  32. https://comelec.ch/wp-content/uploads/2020/11/Comelec_parylene_Properties.pdf
  33. https://www.osti.gov/servlets/purl/1158666
  34. https://scscoatings.com/newsroom/blog/solvent-resistance-of-parylene/
  35. https://vsiparylene.com/resources/parylene-properties/
  36. https://scscoatings.com/newsroom/blog/5-key-properties-of-parylene/
  37. https://sensors.myu-group.co.jp/sm_pdf/SM294.pdf
  38. https://www.sciencedirect.com/science/article/abs/pii/S0141391019303520
  39. https://vsiparylene.com/resources/parylene-biocompatibility/
  40. https://pubs.acs.org/doi/10.1021/acsabm.4c00561
  41. https://pmc.ncbi.nlm.nih.gov/articles/PMC11337162/
  42. https://www.utdallas.edu/~rar011300/Parylene/ParyleneDepManual.pdf
  43. https://scscoatings.com/newsroom/blog/parylene-c-conformal-coating-optical-properties-refractive-index-and-application-areas/
  44. https://www.sciencedirect.com/science/article/abs/pii/S0141391021001877
  45. https://scscoatings.com/newsroom/blog/parylene-barrier-properties/
  46. https://scscoatings.com/newsroom/blog/does-parylene-make-my-product-waterproof/
  47. https://pmc.ncbi.nlm.nih.gov/articles/PMC10255625/
  48. https://www.dextermag.com/wp-content/uploads/2023/07/Parylene-Family-7.18.23.pdf
  49. https://pmc.ncbi.nlm.nih.gov/articles/PMC9527014/
  50. https://www.paryleneengineering.com/what_is_parylene.htm
  51. https://scscoatings.com/newsroom/blog/understanding-parylene-deposition/
  52. https://vsiparylene.com/wp-content/uploads/2020/05/VSI-Parylene-Complete-Guide-to-Parylene-Coatings-Digital.pdf
  53. https://www.advancedcoating.com/hubfs/Advanced%20Coating%20Parylene%20Brochure%20Aug2021.pdf
  54. https://www.researchgate.net/publication/231236060_A_Model_for_the_Chemical_Vapor_Deposition_of_Polypara-xylylene_Parylene_Thin_Films
  55. https://www.osti.gov/servlets/purl/1252638
  56. https://www.fda.gov/media/172951/download
  57. https://pmc.ncbi.nlm.nih.gov/articles/PMC12301961/
  58. https://www.azom.com/article.aspx?ArticleID=264
  59. https://scscoatings.com/newsroom/blog/how-to-choose-parylene-thickness-thin-vs-thick-films/
  60. https://www.aculon.com/parylene-coating-comparison/
  61. https://scscoatings.com/newsroom/blog/parylene-disadvantages/
  62. https://hzo.com/blog/ensuring-parylene-adhesion-the-art-of-proper-surface-prep
  63. https://vsiparylene.com/resources/parylene-thickness/
  64. https://hzo.com/coatings/parylene-coating/process
  65. https://imapsource.scholasticahq.com/api/v1/articles/57469-a-new-halogen-free-parylene-for-high-performance-reliability-of-microelectronics-in-harsh-environments.pdf
  66. https://hzo.com/blog/how-do-you-remove-parylene-coating
  67. https://scscoatings.com/newsroom/blog/repairing-parylene-coated-pcbs/
  68. https://scscoatings.com/newsroom/blog/military-conformal-coatings-acrylic-vs-urethane-vs-parylene/
  69. https://tncoating.com/blog/electronic-coating-technologies-applications-comparison-and-guidelines/
  70. https://scscoatings.com/newsroom/blog/4-aerospace-applications-of-parylene/
  71. https://scscoatings.com/newsroom/blog/parylene-for-military-sensors/
  72. https://scscoatings.com/newsroom/blog/conformal-coating-for-automotive-applications/
  73. https://hzo.com/blog/engineered-reliability-automotive-conformal-coatings-for-ecus
  74. https://scscoatings.com/newsroom/blog/protecting-mems-with-parylene-coatings-no-small-feat/
  75. https://scscoatings.com/newsroom/blog/what-is-parylene-used-for/
  76. https://www.hrpub.org/download/20150510/UJBE1-10603768.pdf
  77. https://www.sciencedirect.com/science/article/abs/pii/S0257897221000049
  78. https://www.nature.com/articles/s41598-023-31305-4
  79. https://www.advancedcoating.com/medical-device-coating
  80. https://www.mpo-mag.com/parylene-coatings-for-medical-device-technologies/
  81. https://www.advancedcoating.com/blog/parylene-coating-biocompatibility
  82. https://pmc.ncbi.nlm.nih.gov/articles/PMC6328340/
  83. https://hzo.com/blog/parylene-c-biocompatibility-properties
  84. https://link.springer.com/article/10.1007/s00216-025-06150-1
  85. https://pubs.rsc.org/en/content/articlehtml/2025/tc/d5tc02722e
  86. https://www.marketresearchfuture.com/reports/parylene-market-39916
Add your contribution
Related Hubs
User Avatar
No comments yet.