China
Africa
Explore chapters and articles related to this topic
Topical and Transdermal Formulation Development
Published in Marc B. Brown, Adrian C. Williams, The Art and Science of Dermal Formulation Development, 2019
Marc B. Brown, Adrian C. Williams
When used for extended times, patches can cause skin irritation. This is partly a result of the permeant, but also because patches tend to be occlusive with consequent hydration of the stratum corneum; whilst hydrating the stratum corneum is valuable for increasing drug delivery, it also poses some difficulties in the longer term. On extended use, the hydrated stratum corneum can encourage microbial growth, and the wet tissue can also adversely affect the adhesive performance. Both the polyisobutylene and polysiloxane adhesives have good water vapour permeability properties and hence reduce problems associated with long-term tissue occlusion. However, the polysiloxanes, whilst offering low toxicity and generally good skin compatibility, can themselves be irritating. One further problem, notable in drug in adhesive formulations, is that the drug can migrate through the adhesive layer on storage. This is usually not a problem with the acrylic-based polymers, but it is evident with polyisobutylene adhesives. Where this phenomenon has occurred, with delivery of scopolamine and clonidine, the migrated drug has formed an “adhesive depot” which provides a drug burst or pulse prior to pseudo steady-state delivery; the manufacturers have shown this to be a valuable method for overcoming initial drug:skin binding.
Retinal Prostheses Current Approaches, Challenges, and Outlook
Published in Iniewski Krzysztof, Integrated Microsystems, 2017
Luke Theogarajan, John Wyatt, Joseph Rizzo
An alternative way of approaching the problem is to design better electrode architectures that can be placed closer to the retinal tissue and thereby lowering the current required for stimulation. An encouraging note is that routinely in the laboratory we use 1–10 A to stimulate retina in vitro [20] where the stimulating electrode is placed very close to the cell. Another approach for enhancing proximity is to induce neurons to grow toward the electrode by using drug-eluting electrodes [21]. The next major challenge is biocompatibility of the retinal implant. Although polymer coatings such as paralyene, polyimide, and polysiloxanes are currently being used in neural prosthesis [22], they degrade over time. Metal encapsulation such as titanium packaging is another possibility although this may make the implant bulky and surgically hard to handle. Apart from general biocompatibility, there is also the issue of tissue encapsulation of the electrodes, which increases the access resistance of the electrodes. The use of special coatings near the vicinity of the electrode array may overcome this [22].
Elements of Polymer Science
Published in E. Desmond Goddard, James V. Gruber, Principles of Polymer Science and Technology in Cosmetics and Personal Care, 1999
E. Desmond Goddard, James V. Gruber
The polycondensation technique can also be applied to prepare organic-inorganic polymers. The most important such polymers are the polysiloxanes, also known as silicone polymers such as dimethicone, based on the —Si—O— linkage present in glass or sand (see Chapter 7). They are made by polymerization of a low-molecular-weight cyclic analog, such as octamethylcyclotetrasiloxane. When this compound is heated above with a trace of an acid or base, it polymerizes to form a highly viscous liquid. Although the dimethylsiloxane structure forms the basis of most silicone polymers, other substituents have also been introduced as cosubstituents. These include vinyl, ethyl, phenyl, and n alkyl groups. Polysiloxanes are among the most flexible macromolecules known. They also repel water. Partly because of this property, they are used in protective hand-and-body lotions and creams. They are incorporated also in hair-care formulations to improve luster and sheen.
Synthesis and characterization of gadolinium-decorated [60]fullerene for tumor imaging and radiation sensitization
Published in International Journal of Radiation Biology, 2021
Joseph Byung-Kyu Kim, Yuri Mackeyev, Subhiksha Raghuram, Sang Hyun Cho, Sunil Krishnan
Other strategies have been employed for tethering multimers of Gd chelates to nanoparticles to simultaneously increase relaxivity and radiosensitivity. In one such formulation, a sub-5nm diameter polysiloxane with 8-12 arms linking DOTA-Gd chelates has been extensively tested in preclinical models as a theranostic agent and is currently in phase II clinical trials in Europe. This formulation is administered intravenously, cleared rapidly by the kidneys (due to their small size that allows evasion of reticuloendothelial uptake and clearance), and therefore able to integrate seamlessly with clinical radiotherapy practice. The reported cellular uptake concentrations with this formulation are ∼1.25 pg/cell after 30 min incubation with 0.5 mM of the nanoparticle (Detappe et al. 2015). This resulted in significant (biological dose enhancement factors of 1.2 when irradiated with 220 kVp x-rays without washing nanoparticles off the media). Our Gd3+ concentrations, achieved with Gd2C60, of ∼5 pg/cell are within this range and yet, no appreciable radiosensitization was observed. The free radical scavenging effect of fullerene may well explain the dichotomy between our findings and those of Gd-decorated siloxanes.
Design and fabrication of a magnetically actuated non-invasive reusable drug delivery device
Published in Drug Development and Industrial Pharmacy, 2018
Joyline Dsa, Manish Goswami, B. R. Singh, Nidhi Bhatt, Pankaj Sharma, Meenakshi K. Chauhan
These drug delivery systems often prefer the use of polymers which are bio-compatible, such as polydimethylsiloxane (PDMS), poly lactic-co-glycolic acid (PLGA), poly acrylic acid (PAA), and poly lactic acid (PLA), etc., and contribute to the drug bio-distribution in response to specific stimuli such as variation in temperature, magnetic field, electric pulses, etc. [10–13]. Amongst various materials, polysiloxanes are a unique class of non-deformable polymers and offer ease of processing, water repelling and remarkable bio-compatible features which are not common with the hydrocarbon polymers [14]. Because of ease of fabrication and high permeability and less cost, the PDMS has become useful for water-soluble drugs and steroids for long-term drug delivery devices for the application in transdermal, subdermal, and implant systems [15–19].
Preparation and characterisation of flame retardant encapsulated with functionalised silica-based shell
Published in Journal of Microencapsulation, 2018
Doan-Trang Hoang, Diane Schorr, Véronic Landry, Pierre Blanchet, Stéphanie Vanslambrouck, Christian Dagenais
In this study, APP was successfully encapsulated in an organic-inorganic hybrid sol prepared initially from a solution of tetraethoxysilane (TEOS) by using methyltriethoxysilane (MTES) as hydrophobic modifier under alkaline condition. The sol-gel method was chosen to prepare the APP microcapsules as it has already proved its great potential in preparing hybrid polysiloxane materials. The sol-gel process involves hydrolysis and condensation of the silicon alkoxide precursors, which is a simple, economical, and ecological technique (Qian et al.2014). Silicon-based systems are relatively new FR additives for wood (Lowden and Hull 2013). In this project, the combination of TEOS/MTES was used to encapsulate APP, which will be added in intumescent formulation for wood substrate.