# Characterization and Optimization of PLA Stereocomplexed Hydrogels for Local Gene Delivery Systems
## 1 Introduction  
Localized gene therapy approaches have found increasing popularity in the fields of tissue engineering, immune therapy, localized disease therapy, long-term depots for drugs, and localized site production \"factories\" for proteins [1; 2; 3]. For localized therapies, hydrogels have been utilized as coating materials for implants, intravascular stents, thin films (membrane), porous scaffolds, and in-situ forming hydrogel depots. Traditionally, hydrogels are produced outside of the body (after incorporating genetic material), and then implanted into the body. The disadvantage of such approach is that the matrix must be implanted through surgical means. Recently, the development of injectable hydrogels, which undergoes solution to gel transformation in the body has attracted considerable attention. Some of the advantages of these injectable delivery systems include minimal invasiveness and surgery related complication, and the materials can be molded to fit specific shapes and crevices [4; 5]. In particular, temperature responsive hydrogels are highly desirable systems because the well-regulated physiological temperature is the only stimulus needed for sol-to-gel transition. The encapsulation and delivery of pDNA and siRNA have been reported using physically crosslinkable injectable hydrogels derived from natural polymers such as alginate, fibrin and gelatin [6; 7; 8; 9; 10; 11; 12]. The slow degradation properties and uncontrolled release profiles for these hydrogels have promoted researchers to develop block copolymer based injectable hydrogels. Some of the commonly utilized physically crosslinkable copolymers include the bioresorbable hydrophilic and hydrophobic blocks such as polyethylene glycol (PEG), polylactides (PLA), poly(lactide-\(co\)-glycolide) (PLGA), and polypropylene glycol (PPG); for example, PLA-PEG-PLA,PLGA-PEG-PLGA, and Pluronic(r) [13; 14; 15]. To improve their low mechanical strength typically associated with physical crosslinking, the chemically crosslinkable systems have been often used. Reactive functional groups such as fumarate, methacrylate, and acrylates are typically conjugated to gelling block copolymer or hydrophilic homopolymer systems (PEG is a commonly utilized hydrophilic homopolymer) [16; 17; 18; 19; 20]. Although chemical functionalization yields robust hydrogels, there are serious safety and biocompatibility concerns. The reactive cross-linkable groups and their degradation byproducts are known to cause localized toxicity and necrosis [21]. For the hydrogel systems incorporate the genetic material (pDNA, siRNA, etc.) in their naked form, the mechanism for loading of the genetic material relies simply on physical entrapment during the sol-to-gel transition process. These systems typically show low therapeutic efficiency due to lack of ability to retain the gene inside and prevent rapid and uncontrolled diffusion. Moreover, the released genetic material has low in-vivo half-life due to degradation by nucleases and inability to successfully cross the cell membrane [22]. To overcome these drawbacks, researchers have used cationic blocks such as polyethyleneimine (PEI) and poly(2-dimethylaminoethyl methacrylate) (PDMAEMA) to condense DNA into a polyplex particle and entrap it within the hydrogel matrix [23; 24; 25; 26; 27]. The main hurdle in the development of injectable hydrogels for the localized gene therapy is the fine balance needed between high mechanical strength and biocompatibility. Crystallization of polymer chains is an attractive alternative to a relatively toxic chemical crosslinking in the production of an injectable hydrogel with biologically relevant mechanical strength [28; 29]. In particular, the stereocomplex crystal is produced through physical association of enantiomeric blocks of a thermo-gelling system (crosslinking junction). This crystal has relatively high thermal/physical stability, which allows it to act as a semi-permanent crosslinker similar to that produced by chemical methods [30].