There are many polymer drug delivery, diagnostic and tissue engineering applications which have not come to practice. The main challenges that biochemical industry is facing is development of materials which are bio-compatible and have adequate association properties with various inherent bio-materials. The infection related issues are common problem in the use of polymers in bio-devices. Many of the smart polymers use acrylamide or acrylic acid type polymers (PNIPAAm and PPAAc) which are potentially toxic and they are not hydrolytically degradable. Further, many of these smart polymer carriers are most effective in delivering drug at their cellular targets when they are of higher molecular weights; such polymers are not readily excreted via the kidneys after delivering the drug, and are not biodegradable, so they would tend to accumulate in the body. The main challenge faced in developing biosensors is very small intensity of bio-stimuli and nanomolar concentration of markers. Although Mathematical modeling of transport processes in various biological barriers of human bio-system is most important in understanding and development of bio-devices, still enough data is not available for evaluations and predictions of various parameters for polymer device development. Further, the stringent approval requirements of FDA also make companies to think twice before investing into new venture.
Current and future trends
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There are a number of examples where smart polymers are used in in -vitro operations but the recent developing interest and demand of advanced medical science has opened new avenues to think in. The world's research trend is shifting towards development of advanced materials for bio-sensing and improving the available polymers for broader implantation. The area of polymers speciï¬c to antigen-antibody interactions, enzymes, and glucose are becoming more and more popular. The polymers which are sensible to light, electric-field, magnetic-field, sonic-field are also being explored. Oral delivery has been most popular way of drug administration for its ease of administration and patient compliance. Hence a lot of research is going on in this area. The other areas where the increasing trend is seen are bio-sensing (due to increasing health concern and expenditure) and improvement of durability of polymer products( due to decrease in average of people getting organ & skin transplant) for organ and bone implantation.
Drug delivery systems target to deliver medicine at specific target without any leak in the way, hence an efficient system is required to identify the right spot and deliver the medicine by means of either a physiological or chemical trigger. More sophisticated delivery systems are required to design an oral delivery system which could tolerate the harsh acidic environment and tight monolayer of endothelial cells present throughout the gastrointestinal tract and deliver the medicine at desired spot. Some specialized systems are required for ophthalmology, cardiovascular and dermatology products. Smart polymers, specific to particular target, offer promising means in these areas but also pose a challenge of meeting biodegradability and non-toxicity requirement.
Among the most deeply explored tactics to stimulate the activation and release of drugs is to exploit the endogenous mechanism within or near the targeted cell. The self-directed nature of such machinery for activation makes this approach particularly attractive for treatment of diseases that are not easily localized, isolated, or tracked in the body. The use of endogenous natural enzymes and acidic environment within the endosomes or lysosomes of cells to trigger the release of drugsÂ is also being explored as potential option. The polymers which are sensitive to redox- microenvironments within the cell are being developed to design an autonomous natural stimulus drugs delivery system. 
Conventional drug delivery system for ophthalmology require frequent installation of drugs, it is seen that only 1-2% of the Pilocarpine hydrochloride, drug used for treatment of glaucoma reaches to target tissues of eye . Hence for ophthalmological drug delivery systems, special polymers are being developed which can offer long retention time even in tear flow environment and form a personalized film that can cover a large surface area in the physiological environment of cornea and sub-conjunctiva after phase transition. These drug delivery systems must not have very large Interference with vision and must have ability to retain medicine even if eyes are rubbed. 
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The skin is considered to be a complex organ for drug delivery because of its structure. Drug delivery systems are developed for the controlled release of drugs through the skin into the systemic circulation, maintaining consistent effectiveness and reducing the dose of the drugs and their related side effects. Hence selection of drug delivery vehicle is the most important. A lot of polymers have been developed till date but still a lot of research is still going on to develop more efficient polymers as current polymers, alone, are not sufficient to meet varied demands of pharma- industry. The current trend is in development of polymers which can regulate release of drug dependent of time elapsed from injection (Eg. the insulin requirement for a diabetes patient varies during whole day cycle). Further the polymers have to identify regional variability in the skin barrier and assess the response of the underlying viable tissues to the absorption. 
Cardiovascular (CV) disease is the most widespread life-threatening clinical problem and is a major cause of disability and economic burden worldwide. CV gene therapy offers the advantage of controlled expression of desired proteins in cell types, which makes it more valuable in providing durable clinical benefits. Success of gene therapy depends on the choice of the vector and the delivery approach. Smart polymers offer a non-viral transport means for these gene material but these have to be adequate enough to overcome multiple extracellular and intracellular barriers. These barriers include binding to the cell surface, escaping lysosomal degradation, traversing the plasma membrane and overcoming the nuclear envelope. 
As the smart polymers are sensible to their environment and their physical and chemical properties can be manipulated over wide ranges of characteristics, the use of these polymers is finding increasing use in development of sophisticated bio-sensors. They are, in current period, extensively being used in measuring gene expression, monitoring metabolic disorder and detect the presence of disease. There is lot of scope of development of new polymers to improve sensitivity, selectivity and decrease reaction scale. The broad utility of polymer stems for their flexibility to incorporate various chemical functional groups into single molecule has triggered the development of macromolecules which will be sensible to nano-molar concentration and sensible to very weak stimuli. The reliability on polymers is increasing in order to move forward from centralized laboratories and develop the diagnostic medicines which will be able to identify the presence of decease. The use of fluorescent particles, semiconductor quantum dots and surface-enhanced Raman spectroscopy has prompted new research toward the development of polymers which can be used as medium/vehicle for these practices. Solid-state polymer sensor devices are being developed which will be based on electrical response to their chemical environment. Â Such a variation in electrical properties of polymer is utilized to detect the disease. 
There is growing need of developing polymers for making the supporting blocks and/or response systems for development artificial organs. These polymers have to be environment sensible, quick responsive, sufficiently sensible to very small stimuli and strong of course. People are trying to develop organs in the laboratory with the help of smart polymers. For example researchers at Wake forest University in North Carolina at trying to develop artificial livers and kidneys while labs in Netherlands and China are involved in development of blood vessels. The future research will be more oriented towards development of artificial heart and brain. Tissue engineers are developing artificial morphological organs with the help of smart polymers which can respond similar to original organs and patient feel like normal. They are involved in developing more responsive and cheaper polymers for plastic surgery. The research in stem-cell friendly polymers is on boom. The new technology of developing the polymers to make scaffold which can support the formation of organs in-vivo is being explored. There is new trend among the athletes and players to replace their critical muscles and organs so as to achieve higher strength. Clinical engineers are investing a lot more time in development of organs which will have higher performance and strength than natural organs. Similarly in a number of cases, athletes have got extra muscles implanted to provide them more strength.
Stimuli responsive Polymers in Sensor applications
Key Challenges in Sensor application:
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Amidst various advantages of polymer sensors over conventional sensors, it has its own set of limitations. The performance of the sensors is greatly affected by characteristics of transducer materials. A very limited size-dependence towards physical properties is shown Conducting-polymer. But very little research has been done in this area to be able to develop sensors readily. During manufacturing, control over the characteristics of polymer is a big challenge to maintain decent sensitivity and to achieve suitable sensing capability. Polymeric materials are susceptible to degradation by harsh environmental conditions such as heat, moisture, and light thus much attention must has to be paid to enriching their long term stability and reliability which is considered to be the most important factor in commercial operations. 
Current and future trends
Due to better usability and wide scale applications smart polymers are getting more and more recognition in recent years. Better measurement standard and high selectivity can be achieved by the use of stimuli responsive polymers. The polymers which are sensible toward the change of ion-concentration, pH, humidity, specific gases or physical stimuli are extensively being explored for manufacturing of sophisticated miniatured chemical, physical and biological sensors
The recent legislations in environment policies, the monitoring of environment and effluent materials has become very important from the point of view of industries as well as monitoring agencies. The amount of nitrogen oxides, sulfur dioxide, and other toxic gases has to be monitored very carefully. Henceforth research in development of polymers which are sensible to these gases is on fire. The readymade sensors of these gases promise an inexpensive and safe solution to monitoring of leakage of hazard gases in environment. 
Polymers for optical fiber sensor transducers are being developed which show reversible changes in optical properties in presence of some solvents, hence they can be deployed to detect volatile organic compounds (VOCs).Â
In the area of control the dielectric elastomers are being developed to generate deformations by transforming electrical energy directly into mechanical work. They are categorized under the name of electro-active polymers (EAP). Many other applicationsÂ of these actuator polymers are visualized such as mini- micropumps, micro air vehicles, microrobots, microvalves, disk drives, flat panel loudspeakers and prosthetic devices.