high-work-function contact. For the same reason, electrons are extracted from C60 at the CalMEH-PPV:C,, interface. The result, then, is that separated carriers are not "wasted": thev are automaticallv collected by the prope; electrode so that external work can be done.The substantial enhancement in n ' C achieved with the bicontinuous D-A network material results from the large increase in the interfacial area over that in a D-A bilayer and from the relatively short distance from any point in the polymer to a charge-separating interface. Moreover, the internal D-A junctions inhibit carrier recombination and thereby improve the lifetime of the photoinduced carriers (6), so that the separated charge carriers can be efficiently collected by the built-in field from the asymmetric electrodes. Similar effects have been observed in MEH-PPV:Cyano-PPV polymer blends (10 , 1 1).The device efficiencies are not yet optimized. Because only -60% of the incident power was absorbed at 430 nm in the thinfilm devices used for obtaining the data in u Fig. 3, the internal carrier collection efficiency and energy conversion efficiency are approximately 1.7 times larger; that is, qc.= 90% e/ph and qr;-5.5% at 10 p,W/cm2. Although nearly 100% absorption can be achieved"by using thicker films, qc is currently limited in thick-film devices by internal resistive losses. Further imorovements in device efficiencies are expected when the blend com~ositionand the network morphology are optimized. SCIENCE VOL. 270 15 DECEMBER 1995

To meet the challenge of regenerating bone lost to disease or trauma, biodegradable scaffolds are being investigated as a way to regenerate bone without the need for an auto- or allograft. Here, we have developed a novel microsphere-based chitosan/nanocrystalline calcium phosphate (CaP) composite scaffold and investigated its potential compared to plain chitosan scaffolds to be used as a bone graft substitute. Composite and chitosan scaffolds were prepared by fusing microspheres of 500-900 microm in diameter, and porosity, degradation, compressive strength, and cell growth were examined. Both scaffolds had porosities of 33-35% and pore sizes between 100 and 800 . However, composite scaffolds were much rougher and, as a result, had 20 times more surface area/unit mass than chitosan scaffolds. The compressive modulus of hydrated composite scaffolds was significantly higher than chitosan scaffolds (9.29 +/- 0.8 MPa vs. 3.26 +/- 2.5 MPa), and composite scaffolds were tougher and more flexible than what has been reported for other chitosan-CaP composites or CaP scaffolds alone. Using X-ray diffraction, scaffolds were shown to contain partially crystalline hydroxyapatite with a crystallinity of 16.7% +/- 6.8% and crystallite size of 128 +/- 55 nm. Fibronection adsorption was increased on composite scaffolds, and cell attachment was higher on composite scaffolds after 30 min, although attachment rates were similar after 1 h. Osteoblast proliferation (based on dsDNA measurements) was significantly increased after 1 week of culture. These studies have demonstrated that composite scaffolds have mechanical properties and porosity sufficient to support ingrowth of new bone tissue, and cell attachment and proliferation data indicate composite scaffolds are promising for bone regeneration.

Methanolysis of {t-Bu2In[μ-P(SiMe3)2]}2 (1) in aromatic solvents gives polycrystalline InP fibers (dimensions 10−100 nm × 50−1000 nm) at 111−203 °C. The chemical pathway consists of a molecular component, in which precursor substituents are eliminated, and a nonmolecular component, in which the InP crystal lattices are assembled. The two components working in concert comprise the solution−liquid−solid (SLS) mechanism. The molecular component proceeds through a sequence of isolated and fully characterized intermediates:  1 → [t-Bu2In(μ-OMe)]2 (2) → [t-Bu2In(μ-PHSiMe3)]2 (3) → 2 → [t-Bu2In(μ-PH2)]3 (4). Complex 4, which is alternatively prepared from t-Bu3In and PH3, undergoes alkane elimination, the last steps of which are catalyzed by the protic reagent MeOH, PhSH, Et2NH, or PhCO2H. In the subsequent nonmolecular component of the pathway, the resulting (InP) n fragments dissolve into a dispersion of molten In droplets, and recrystallize as the InP fibers. Important criteria are identified for crystal growth of covalent nonmolecular solids from (organic) solution. The outcomes of other solution-phase semiconductor syntheses are rationalized according to the functioning of molecular and nonmolecular pathway components of the kind identified here.

Ultrasonic techniques based on measurements of apparent backscatter may provide a useful means for diagnosing bone diseases such as osteoporosis. The term "apparent" means that the backscattered signals are not compensated for the frequency-dependent effects of attenuation and diffraction. We performed in vitro apparent backscatter measurements on 23 specimens of human cancellous bone prepared from the left and right femoral heads of seven donors. A mechanical scanning system was used to obtain backscattered signals from each specimen at several sites. Scans were performed using five different ultrasonic transducers with center frequencies of 1, 2.25, 5, 7.5, and 10 MHz. The -6 dB bandwidths of these transducers covered a frequency range of 0.6-15.0 MHz. The backscattered signals were analyzed to determine three ultrasonic parameters: apparent integrated backscatter (AIB), frequency slope of apparent backscatter (FSAB), and time slope of apparent backscatter (TSAB). Linear regression analysis was used to examine the correlation of these ultrasonic parameters with five measured physical characteristics of the specimens: mass density, X-ray bone mineral density, Young's modulus, yield strength, and ultimate strength. A total of 75 such correlations were examined (3 ultrasonic parameters x 5 specimen characteristics x 5 transducers). Good correlations were observed for AIB using the 5 MHz (r = 0.70 - 0.89) and 7.5 MHz (r = 0.75-0.93) transducers; for FSAB using the 2.25 MHz (r = 0.70 - 0.88), 5 MHz (r = 0.79 - 0.94), and 7.5 MHz (r = 0.80 - 0.92) transducers; and for TSAB using the 5 MHz (r = 0.68 - 0.89), 7.5 MHz (r = 0.75 - 0.89), and 10 MHz (r = 0.75 - 0.92) transducers.

No abstract