Parylene C is a promising material for constructing flexible, biocompatible and corrosion-resistant microelectromechanical systems (MEMS) devices. Historically, Parylene C has been employed as an encapsulation material for medical implants, such as stents and pacemakers, due to its strong barrier properties and biocompatibility. In the past few decades, the adaptation of planar microfabrication processes to thin film Parylene C has encouraged its use as an insulator, structural and substrate material for MEMS and other microelectronic devices. However, Parylene C presents unique challenges during microfabrication and during use with liquids, especially for flexible, thin film electronic devices. In particular, the flexibility and low thermal budget of Parylene C require modification of the fabrication techniques inherited from silicon MEMS, and poor adhesion at Parylene-Parylene and Parylene-metal interfaces causes device failure under prolonged use in wet environments. Here, we discuss in detail the promises and challenges inherent to Parylene C and present our experience in developing thin-film Parylene MEMS devices.

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SummaryOxygen saturation was continuously measured using computerised pulse oximetry for 8 h overnight pre-operatively and for the ,first 24 h postoperatively in 40 patients receiving intermittent intramuscular morphine or continuous infusion of morphine following elective upper abdominal surgery. The proportion of time with an oxygen saturation less than 94% was used as an index of desaturation. Patients receiving continuous infusion analgesia received a larger morphine dose and achieved better analgesia than the intramuscular group. Postoperatively, the duration of desaturation increased I0,fold over pre-operative values, 'intramuscular' patients spending 39.0% ( S D , 37.0%) and 'continuous infusion' patients 40.0% (SO, 37.5%) of the time below 94% saturation. Although newer therapies (e.g. epidural analgesia and patient-controlled analgesia) are currently receiving greater attention. the seyuelae of these more traditional analgesic techniques warrant further study. Key wordsMonitoring; oximetry. Complications; hypoxia. Analgesics; morphine. Pain; postoperative.During the past 30 years the occurrence of intermittent and sustained postoperative hypoxia has been reported [ 1-31. The introduction of newer methods of opioid delivery e.g. patient-controlled analgesia (PCA) [4] and spinal opioids [5] has promoted considerable research into adverse effects of these techniques, particularly respiratory depression. In contrast, there has been little work looking at what actually happens following traditional methods of opioid delivery such as intramuscular injection (IM) and continuous intravenous infusion (CI). Catling et al., using the indices of respiratory rate, tidal volume, number of apnoeic episodes and partial pressure of carbon dioxide showed there were differences between IM and CI analgesia following cholecystectomy [2]. However, these measurements were intermittent, and pulse oximetry was not used. Continuous pulse oximetry, which has allowed accurate monitoring of pre-operative hypoxaemia, combined with computerised data collection, has enabled comprehensive analysis of patients' peri-operative oxygen saturation (Spo,). Brose et al. [6] comparing IM pethidine, PCA pethidine and extradural morphine for post-Caesarean section analgesia, reported that patients receiving PCA had long periods of mild desaturation (90-95%); in contrast, more severe desaturation (mean values 83%) occurred for shorter periods of time with extradural morphine and intramuscular pethidine. Continuous pulse oximetry was used by Wheatley et al. who studied patients undergoing lower abdominal procedures in order to assess the respiratory effects of extradural, PCA and IM analgesia [7]. These workers defined the pattern of Spo, readings as stable or unstable depending on the hourly variability of oxyhaemoglobin saturations, and hypoxaemia as a n Spo, < 94% for more than 10% of the time. Patients receiving IM and PCA had unstable patterns of Spo, values with no hypoxaemia, while 50% of patients receiving extradural morphine had unstab...