Detoxification of solid freeform fabrication materials

Details Detoxification of solid freeform fabrication materials Detoxification of solid freeform fabrication materials

2001-03-30

2003-12-09

Tentoni, Leo B. (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Stereolithographic shaping from liquid precursor

C264S232000, C264S234000, C264S236000, C264S308000, C264S442000, C264S497000

Reexamination Certificate

active

06660208

ABSTRACT:

FIELD OF THE INVENTION
The invention relates in general to biocompatible medical structures, and in particular to a process to assure at least long-term dermal contact biocompatibility for three-dimensional objects formed from solid freeform fabrication techniques such as stereolithography. In one application the process is capable of producing a long-term dermal contact hearing aid shell structure custom made by stereolithography.
DESCRIPTION OF THE PRIOR ART
Conventional biocompatible and bioabsorbable materials have been proposed previously for long-term dermal contact applications, such as hearing aids. Generally, biocompatible materials exhibit non-toxic characteristics and do not adversely react with biological matter. Biocompatible testing is often done by placing a material in contact with living cells of an animal for extended periods of time in order to verify that no adverse reaction occurs. Bioabsorbable materials, on the other hand, while also exhibiting non-toxic characteristics, are capable of breaking down into small, non-toxic segments, which can be metabolized or eliminated from the body without harm. Both biocompatible and bioabsorbable materials lack the presence of significant amounts of cytotoxins, that is, substances or particulates that can produce a toxic effect to cells. Identifying specific cytotoxins in a specific material can prove problematic; however, cytotoxicity testing can readily be conducted to essentially determine whether or not a significant level of cytotoxins are present in a particular material. Until recently, the development of biocompatible materials for use in solid freeform fabrication techniques has been limited.
Generally, cytotoxins are constituent species of matter that, when in physical contact with cells, produce a toxic effect such as an allergic reaction. When present in sufficient quantity in an object, cytotoxins render the object unacceptable for biocompatible applications such as long-term dermal contact. Nearly all objects contain some quantity of cytotoxins, however the conventional wisdom to achieve biocompatibility is to start with a material that is inherently non-toxic, i.e. one that contains a de-minimus amount of cytotoxins. For example, several acrylate and methacrylate-type polymers have previously been suggested for a wide variety of applications involving some degree of biocompatibility because many such materials contain a de-minimus amount of cytotoxins. U.S. Pat. No. 5,763,503 to Cowperthwaite, et al. discloses a pourable methacrylate-capped urethane monomer/reactive diluent composition for use in pouring into a mold to form a hearing aid shell structure. Thus, it is generally taught that in order to produce a biocompatible object, one must start by selecting a material that is inherently non-toxic, i.e., one that contains a de-minimus amount of cytotoxins.
Recently, several new technologies have been developed for the rapid creation of models, prototypes, and parts for limited run manufacturing. These new technologies can generally be described as Solid Freeform Fabrication techniques, herein referred to as “SFF”. Some SFF techniques include stereolithography, laminated object manufacturing, selective phase area deposition, multi-phase jet solidification, ballistic particle manufacturing, fused deposition modeling, particle deposition, laser sintering, and the like. Generally in SFF techniques, complex parts are produced from a modeling material in an additive fashion as opposed to traditional fabrication techniques, which are generally subtractive in nature. For example, in traditional fabrication techniques material is removed by machining operations or shaped in a die or mold to near net shape and then trimmed. In contrast, additive fabrication techniques incrementally add portions of a build material to targeted locations, layer by layer, in order to build a complex part. SFF technologies typically utilize a computer graphic representation of a part and a supply of a building material to fabricate the part in successive layers. A wide variety of building materials have been proposed in various SFF techniques; however, they are typically applied in the form of a powder, liquid, gas, paste, or gel. SFF technologies have many advantages over conventional manufacturing methods. For instance, SFF technologies dramatically shorten the time to develop prototype parts and can produce limited numbers of parts in rapid manufacturing processes. They also eliminate the need for complex tooling and machining associated with conventional manufacturing methods, including the need to create molds for custom applications. In addition, customized objects can be directly produced from computer graphic data. However, the use of SFF techniques to produce biocompatible objects has been limited.
It has been envisioned that prosthetic implants could be constructed directly from SFF techniques such as stereolithography. Such biocompatible applications are believed possible because polymerizable acrylates have previously been shown to be biocompatible, and it is assumed that objects formed from these materials in stereolithographic processes will therefore be biocompatible. For example, International Patent Application WO 95/07509 envisions the direct production of implants from a stereolithographic process, however, only the use of a stereolithographic object as an intermediate mold to create biocompatible prostheses is disclosed. This illustrates the recognized inability to directly produce biocompatible objects directly from a stereolithographic process utilizing materials believed to be biocompatible. Thus, there is a need to develop a process in which biocompatible objects can be produced directly by SFF techniques.
Most attempts to achieve biocompatible or bioabsorbable objects formed by SFF techniques have focused primarily on build material formulation. For example, a bioabsorbable stereolithographic resin is disclosed in U.S. Pat. No. 5,674,921 to Regula et al., which comprises a radiation curable, urethane acrylate and a photoinitator. However, it is significant to note that the samples disclosed were completely cured by flood curing with a UV light source, and not by a selectively applying a concentrated beam of UV energy as is done in stereolithography techniques. Furthermore, the formulations were not designed to be biocompatible, but rather bioabsorbable, that is, intended to break down into small non-toxic segments within a biological environment, instead of simply remaining stable and inert within the biological environment.
What appears to be under-appreciated in the prior art is that where the SFF build process produces parts that are typically toxic, additional post processing methods may be deployed to render them non-toxic. In addition, due to the inherently non-homogeneous nature of most SFF build processes, it is theorized that cytotoxins are typically retained within structures formed by such processes. This has been shown to be the case in stereolithography, and even when a part is homogeneously formed by stereolithography, it may still contain cytotoxins. These cytotoxins can undesirably react with biological matter in certain applications, particularly those requiring long-term dermal contact. It is the creation and/or retention of cytotoxins in these structures that presently prevents their use in biocompatible applications.
Generally, in most SFF techniques, structures are formed in a layer by layer manner by solidifying successive layers of a build material that are inherently non-homogeneous. For example, in stereolithography a tightly focused beam of energy, typically in the ultraviolet radiation band, is scanned across a layer of a liquid photopolymer resin to selectively solidify the resin to form a structure. In order to solidify each built up layer of the structure, the focused beam of energy must be driven back and forth across its surface. This build process, or hatching, often does not form a homogeneously cured layer because the focused energy only locally activates the photoinitiator

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