Víctor H. Pérez-Luna

Study of thermal transport in nanoparticle suspensions using forced Rayleigh scattering

Thermal diffusivity measurements on two nanofluids and their base fluids were made using an optical technique called forced Rayleigh scattering. The nanofluids studied were a citrate-stabilized Au nanoparticle suspension in water and an Al2O3 nanoparticle suspension in a petroleum oil. Thermal diffusivity measurements on the nanofluids and base fluids were made at temperatures in the range of 25–75°C. From these data, it was possible to estimate the thermal conductivity enhancement in the nanofluids as a function of temperature. In contrast to previous reports on similar systems, our experiments are consistent with thermal conductivity enhancement predictions from effective medium theory. In particular, we find that the level of thermal conductivity enhancement is independent of temperature.

Quenched emission of fluorescence by ligand functionalized gold nanoparticles.

A fluorescence-based detection scheme that uses ligand functionalized gold nanoparticles is proposed. The transduction scheme is based on the strong quenching of the fluorescence emission exerted by metallic surfaces on fluorophores positioned in their immediate vicinity (<5 nm). Binding of fluorophore-labeled anti-biotin to biotinylated gold nanoparticles resulted in decreased fluorescence emission intensity. Subsequent competitive dissociation of labeled anti-biotin with D-biotin resulted in increased fluorescence emission intensity. These interactions occurred by means of specific molecular recognition because when the binding sites of anti-biotin were saturated with D-biotin prior to interaction with the gold nanoparticles; changes in the fluorescence emission intensity were not observed.

Three-Dimensional Patterning of Poly(Ethylene Glycol) Hydrogels Through Surface-Initiated Photopolymerization

Photopolymerizable hydrogels have been investigated extensively for biomedical applications, specifically in the area of tissue engineering. While fabrication approaches have shown promise in designing hydrogel scaffolds that guide cell function, the ability to spatially control localization in three-dimensions has been limited. We have developed a method for generating two-dimensional and three-dimensional (3D) patterns within multilayered poly(ethylene glycol) diacrylate (PEG-DA) hydrogels. Covalently attached hydrogel layers are formed using precursor solutions with a 10:1 mole ratio of PEG-DA to PEG-aminoacrylate (Acr-PEG-NH2). Upon illumination of the precursor with visible light (wavelength = 514 nm), a hydrogel layer forms with pendant amine groups induced by the presence of Acr-PEG-NH2 macromer. Pendant amine groups are further functionalized with free carboxyl groups present on the visible light photoinitiator eosin, allowing for the formation of subsequent hydrogel layers. Using noncontact photolithography, the prepolymer solution is polymerized through a photomask, resulting in hydrogel structures with distinct pattern formation in each layer. Unreacted regions immobilized with eosin can be subsequently filled with a different PEG hydrogel. The technique presented shows a great potential for tissue engineering applications, for biosensors, and in the formation of cell and protein patterning for biotechnology.

Role of Thermo-responsiveness and Poly(ethylene glycol) Diacrylate Cross-link Density on Protein Release from Poly(N-isopropylacrylamide) Hydrogels

Thermo-responsive hydrogels have shown promise as injectable materials for local drug delivery. However, the phase-induced changes in polymer properties of N-isopropylacrylamide (NIPAAm) can pose additional challenges for achieving controlled protein release. In this work, thermo-responsive hydrogels derived from NIPAAm and cross-linked with poly(ethylene glycol) diacrylate (PEG-DA) were synthesized via free radical polymerization. The volume phase transition temperature (VPTT) of the hydrogels ranged from 32.9°C to 35.9°C. Below the VPTT, swelling ratios of the hydrogels decreased with cross-linker concentration, and showed a sharp drop (at least 4-fold) upon phase change. Protein encapsulation efficiency was high (84–90%) and decreased with cross-linker concentration. Release of bovine serum albumin, a model protein, at body temperature was significantly higher than at room temperature (67% at 37°C compared to 44% at 23°C after 48 h). The release kinetics of proteins from the hydrogels were initially expected to be a function of cross-link density. However, at the hydrogel compositions explored in this work, protein release did not change significantly with cross-linker mol fraction. The thermo-responsive hydrogels offer a promising platform for the localized delivery of proteins.

Investigation of lysine acrylate containing poly(N‐isopropylacrylamide) hydrogels as wound dressings in normal and infected wounds

The design of materials for cutaneous wound dressings has advanced from passive wound covers to bioactive materials that promote skin regeneration and prevent infection. Crosslinked poly(N-isopropylacrylamide) (PNIPAAm)-based hydrogels have been investigated for a number of biomedical applications. While these materials can be used for drug delivery, limited cell interactions restrict their biological activity. In this article, acryoyl-lysine (A-Lys) was incorporated into poly(ethylene glycol) crosslinked PNIPAAm to enhance biological activity. A-Lys could be incorporated into the hydrogels to improve cellular interaction in vitro, while maintaining swelling properties and thermoresponsive behavior. Polyhexamethylene biguanide, an antimicrobial agent, could be encapsulated and released from the hydrogels and resulted in decreased bacteria counts within 2 hours. Two in vivo animal wound models were used to evaluate the hydrogel wound dressing. First, application of the hydrogels to a rodent cutaneous wound healing model resulted in significant increase in healing rate when compared with controls. Moreover, the hydrogels were also able to decrease bacteria levels in an infected wound model. These results suggest that PNIPAAm hydrogels containing A-Lys are promising wound dressings due to their ability to promote healing and deliver active antimicrobial drugs to inhibit infection.

A study of the intrinsic autofluorescence of poly (ethylene glycol)-co-(L-lactic acid) diacrylate.

Poly (ethylene glycol)-co-(L-Lactic acid) diacrylate (PEG-PLLA-DA) copolymers have been extensively investigated for a number of applications in medicine. PEG-PLLA-DA is biodegradable and the human body can process its degradation products. In this study, we describe the autofluorescence of PEG-PLLA-DA copolymers and compared it to the fluorescence of poly(ethylene glycol) diacrylate (PEG-DA) and the precursor molecules used for their synthesis. In addition, we examined the influence of pH on the fluorescence spectra. We found that PEG-PLLA-DA exhibits higher fluorescence than PEG-DA and all reagents involved in the synthesis of PEG-PLLA-DA. The fluorescence of PEG-PLLA-DA was affected by pH with fluorescence decreasing at high pH values. At high pH, PEG-PLLA-DA could not polymerize into hydrogels and exhibited a dramatic decrease in autofluorescence, suggesting that hydrolysis of the ester bond affected its autofluorescence. At low pH, PEG-PLLA-DA exhibited higher fluorescence and it was able to form crosslinked hydrogels. The autofluorescence of PEG-PLLA-DA could be exploited to monitor polymer degradation and material structure without the need to introduce exogenous fluorescent probes. The origin of fluorescence is not clear at this point in time but it appears to result from a synergetic effect of both lactate units and diacrylate groups in the PEG-PLLA-DA backbone. The observed autofluorescence of PEG-PLLA-DA persists after reaction of the acrylate groups in the polymerization reaction. This autofluorescence is advantageous because it could assist in the study of polymers used for drug delivery and tissue engineering applications.

PEGylation of lysine residues improves the proteolytic stability of fibronectin while retaining biological activity

Excessive proteolysis of fibronectin (FN) impairs tissue repair in chronic wounds. Since FN is essential in wound healing, our goal is to improve its proteolytic stability and at the same time preserve its biological activity. We have previously shown that reduced FN conjugated with polyethylene glycol (PEG) at cysteine residues is more proteolytically stable than native FN. Cysteine‐PEGylated FN supported cell adhesion and migration to the same extent as native FN. However, unlike native FN, cysteine‐PEGylated FN was not assembled into an extracellular matrix (ECM) when immobilized. Here, we present an alternative approach in which FN is preferentially PEGylated at lysine residues using different molecular weight PEGs. We show that lysine PEGylation does not perturb FN secondary structure. PEG molecular weight, from 2 to 10 kDa, positively correlates with FN–PEG proteolytic stability. Cell adhesion, cell spreading, and gelatin binding decrease with increasing molecular weight of PEG. The 2‐kDa FN–PEG conjugate shows comparable cell adhesion to native FN and binds gelatin. Moreover, immobilized FN–PEG is assembled into ECM fibrils. In summary, lysine PEGylation of FN can be used to stabilize FN against proteolytic degradation with minimal perturbation to FN structure and retained biological activity.