1.  Technical Field
The present invention generally relates to multi-armed macromonomers such as multi-armed polysiloxy macromonomers and their use as biomedical devices. 
2.  Description of Related Art
Biomedical devices such as ophthalmic lenses made from siloxy-containing materials have been investigated for a number of years.  Such materials can generally be sub-divided into two major classes, namely hydrogels and non-hydrogels.  Hydrogels can absorb and retain water in an equilibrium state whereas non-hydrogels do not absorb appreciable amounts of water.  Regardless of their water content, both hydrogel and non-hydrogel siloxy and/or fluorinated contact lenses tend to have relatively hydrophobic, non-wettable surfaces. 
Hydrogels represent a desirable class of materials for many biomedical applications, including contact lenses and intraocular lenses.  Hydrogels are hydrated, crosslinked polymeric systems that contain water in an equilibrium state.  Silicone hydrogels are a known class of hydrogels and are characterized by the inclusion of a siloxy-containing material.  Typically, a siloxy-containing monomer is copolymerized by free radical polymerization with a hydrophilic monomer, with either the siloxy-containing monomer or the hydrophilic monomer functioning as a crosslinking agent (a crosslinker being defined as a monomer having multiple polymerizable functionalities) or a separate crosslinker may be employed.  An advantage of silicone hydrogels over non-silicone hydrogels is that the silicone hydrogels typically have higher oxygen permeability due to the inclusion of the siloxy-containing monomer.  Because such hydrogels are based on free radical polymerization of monomers containing a crosslinking agent, these materials are thermosetting polymers. 
In the field of biomedical devices such as contact lenses, various physical and chemical properties such as, for example, oxygen permeability, wettability, material strength and stability are but a few of the factors that must be carefully balanced in order to provide a useable contact lens.  For example, since the cornea receives its oxygen supply exclusively from contact with the atmosphere, good oxygen permeability is a critical characteristic for any contact lens material.  Wettability also is important in that, if the lens is not sufficiently wettable, it does not remain lubricated and therefore cannot be worn comfortably in the eye.  Accordingly, the optimum contact lens would have at least both excellent oxygen permeability and excellent tear fluid wettability. 
U.S. Pat.  No.  5,019,628 (“the '628 patent”) discloses star polymers containing a core of highly crosslinked segments of difunctional acrylates or copolymers of monofunctional and difunctional acrylates, and attached to the core at least 5 linear copolymeric arms derived from one or more (meth)acrylate monomers, wherein about 5 to 100% by weight of the (meth)acrylate monomers from which the arms are derived are in the form of a block consisting essentially of one or more monofunctional monomeric polysiloxanylalkyl ester units.  The '628 patent further discloses that the star polymers can be used in combination with other polymers to improve the properties of the other polymers such as to impart an improved combination of oxygen permeability and hardness in polysiloxanylalkyl acrylic polymers used for contact lens applications. 
U.S. Pat.  No.  5,244,981 (“the '981 patent”) and U.S. Pat.  No.  5,331,067 (“the '067 patent”) disclose silicone-containing acrylic polymers obtained from the copolymerization of pre-formed macromonomers and/or acrylic star polymers with monomers of a polymer matrix, for example, silicone acrylates, esters of acrylic and/or methacrylic acid ((meth)acrylates) and contact lenses made from these polymers.  The '981 and '067 patents define the term “macromonomer” to describe pre-formed linear silicone-containing acrylic homopolymers, block polymers or random copolymers that preferably have a polymerizable group at one end of the polymer chain. 
U.S. Pat.  No.  5,314,961 (“the '961 patent”) discloses compositions containing macromonomers, graft polymers and acrylic star polymers dispersed or copolymerized throughout a polymer matrix to enhance the characteristics of hard and soft contact lenses.  The '961 patent further discloses the star polymers contain a highly crosslinked core and attached to the core at least 5 linear polymeric arms having at least one substantially hydrophilic block preferably derived from at least about 20 to 25% by weight of hydrophilic acrylic-type monomers and at least one substantially hydrophobic, permeable block derived from at least about 50% by weight of at least one polysiloxanylalkyl ester of an alpha, beta, unsaturated ester (silicone acrylate).  The highly crosslinked core of the star polymer can be highly crosslinked segments of difunctional acrylates, copolymers of monofunctional and difunctional acrylates or a crosslinked polysiloxy core derived from a multifunctional crosslinkable silicone-containing group, such as a polyalkoxysilyl group.  In addition, the '961 patent defines the term “macromonomer” to describe preformed linear silicone-containing acrylic polymers which may be used to produce hydrated oxygen permeable compositions. 
U.S. Pat.  No.  7,297,160 discloses high refractive-index, hydrophilic, arylsiloxy-containing monomers, macromonomers, and polymers, and ophthalmic devices comprising such polymers. 
It would be desirable to provide improved biomedical devices formed from a siloxy macromonomer material that exhibit suitable physical and chemical properties, e.g., oxygen permeability and wettability, for prolonged contact with the body while also being biocompatible.  It would also be desirable to provide improved biomedical devices that are easy to manufacture in a simple, cost effective manner.