Soft Tissue Replacements - Biomaterials - page 220

introduced, which could be folded for insertion through a smaller incision. An IOL includes an optics
part, which is connected to the haptics, which one the appendages or struts that take various forms and
are used to anchor the lens in place. Depending on the implant, the haptics may or may not be made
of the same material as the lens. The shape of the lens plays an important role in determining not only
the refractive index, but also the local tissue response, for example, the migration of lens epithelial cells
onto the posterior capsule. For example, preliminary studies suggest that lenses with sharp corners can
inhibit posterior capsule opacification, caused by cellular migration (Nishi et al., 2000). Foldable IOLs
are more susceptive to discoloration/opacification and slight changes in refractive index, the latter per-
haps attributable to the folding during surgical implantation. Preliminary studies have also focused on
the development of light-adjustable IOLs that can be adjusted to tune refractive index postoperatively
(Packer et al., 2004). IOLs have been developed that allow both distance vision and near vision.
Many blinding diseases of the eye have no implant solution or even therapy. With the progression
of tissue engineering, one approach being investigated toward the treatment of adult onset blindness
is the transplantation of retinal pigmented epithelium, either in sheets or spheroids. A major challenge
in transplanting cellular sheets is the development of readily detachable (e.g., thermoresponsive) or
absorbable materials on which to culture sheets of these or precursor cells. Both polyester-based absorb-
able materials and gelatin scaffolds have been tested as cell carriers in small-animal studies and have
provided platforms on which cells could be delivered in more organized fashion. Nonetheless, the surgi-
cal handling of monolayer cellular sheets is challenging and the clinical feasibility of culturing cells on
scaffolds for extended times preoperatively is low.
9.2.5 Space-Filling Implants
Pectoral, penile, testicular, and vaginal implants are silicone-based; implantation of these devices is
considered elective surgery rather than surgery to improve physical health. Breast implants, the most
common space-filling implants, have been an ongoing topic of regulatory interest. Early attempts to
augment breast tissue were conducted with silicone or paraffin injections; with no means of contain-
ment, these liquids could not maintain shape or form and were more susceptible to infection, causing
patient pain and complications. Subsequently, sponges made of polyvinyl alcohol were applied; however,
these nonabsorbable, porous materials were infiltrated by soft tissue and calcified with time. Silicone
or saline-filled silicone shell implants were developed for total breast reconstruction. Once implanted,
these devices would naturally induce a foreign body response; the formation of collagen around the
implants would in some instances become so tightly packed due to capsular contraction that a “marble”
breast resulted. Additionally, some implants were susceptible to leakage and questions ensued, par-
ticularly with respect to silicone leakage. These implants were in use in the 1960s, before the US FDA
had regulatory oversight of these devices. Accordingly, the US FDA did not initially approve silicone-
filled implants for use; however, they approved a compassionate need exemption policy in 1992, which
allowed silicone-filled implants to remain available, primarily to reconstruction patients and women
who already had breast implants needing replacement. Saline-filled implants, however, were approved
for use in 2000, and in late 2006, the US FDA approved specific silicone gel-filled breast implants with a
number of postapproval conditions, including continued studies and rigorous patient-tracking require-
ments. The silicone breast implant envelope can be smooth or textured, where the debate continues
as to whether the textured implants are more likely to leak and/or induce a greater capsular contrac-
tion. The development of tissue-engineered breast is ongoing, where fat or normal breast tissue may be
derived from the patient and combined with an absorbable scaffold for transplantation (Holder et al.,
1998). Composite injectables, comprised of cells loaded on absorbable beads and delivered in a degrad-
able gel from a syringe, are of high interest as they conform to irregular defects, may be implanted in a
minimally invasive manner, and may be readily modulated to serve a particular patient, or a particular
application (Gomillion and Burg, 2006). A wide array of absorbable and/or degradable materials may be
used in these composite systems, thus facilitating a wide range of application needs.