Biomedical Engineering Reference
In-Depth Information
Another improvement of traditional protein solders relies on the use of syn-
thetic polymers mixed with serum albumin. This provides better flexibility,
as well as improved repair strength over albumin-protein solders alone [19].
Poly(lactic-
co
-glycolic acid) (PLGA) is a class of synthetic biodegradable
polymers that are easily degraded in vivo and are eliminated through normal
metabolic pathways. These materials can be employed also as drug-delivery
systems by adding a range of dopants including antibiotics, anaesthetics, and
various growing factors that may enhance the rate and quality of wound heal-
ing [20].
A more recent development in laser tissue welding is the use of an infrared
temperature feedback control of the laser device. Diagnostic feedback via sur-
face temperature measurements has been used to control laser power delivery
to achieve well-defined welding protocols [21]. Although surface temperature
monitoring may be important for a primary control of the energy delivery,
it does not reveal dynamic changes occurring below the tissue surface, which
is where the weld actually occurs. This problem has been addressed by em-
ploying numerical models of laser-tissue interactions on the basis of directly
measured surface-temperature data [21,22]. The combination of experimental
and simulated data made it possible to characterize surface and bulk tissue
heating, thus allowing for a prediction of the temperature rise within the
irradiated tissues.
Photochemical welding of tissues has also been investigated as an alterna-
tive method for tissue repair without direct use of heat. This technique utilizes
chemical cross-linking agents applied to the cut that, when light-activated,
produce covalent cross-links between collagen fibres of the native tissue struc-
ture. Agents used for photochemical welding include 1,8-naphthalimide [23],
Rose Bengal, riboflavin-5-phosphate, fluorescein, and methylene blue [24].
Studies of photochemical tissue bonding have been conducted for articular
cartilage bonding [23], cornea repair [24], skin graft adhesion [25] and for
repairing severed tendons [26].
As far as the mechanism of tissue welding by means of laser light is con-
cerned, a first distinction must be made between: (1) laser tissue welding with
or without the addition of exogenous chromophores, (2) laser tissue soldering,
and (3) photochemical tissue bonding. For all three cases, the exact under-
lying mechanism is not fully understood. Conversely, several hypotheses do
exist that are based on a few electron and optical microscopy observations
and on some in vitro studies.
It is widely accepted that laser tissue welding is primarily a thermal process
[27]. Thermal modifications of biological components within the connective
tissue have been mainly monitored by means of microscopy. The microscopic
data reported in the literature on laser-welded tissues can be schematically
split into two groups on the basis of different modifications of the collagen
matrix observed upon laser welding.
In certain studies, a full homogenization of the tissue was observed, in
which the loose structure of the collagen fibrils was lost following laser welding
