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我国现阶段的私营经济是指（）
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Early devices were of the mechanical variety, in which devices like ball-in-a-cage or tilting disc would be used to allow blood to flow under near-normal conditions. Although a few mechanical problems were encountered in the early days, the major difficulty lay with the tendency for any foreign material to initiate a blood clot. So, all valve recipients have to be given anticoagulant therapy. This is not particularly desirable for the patients, who may develop bleeding problems, and in any case is not always successful.
Although good results are achieved with these valves, it was considered necessary to develop alternatives and the direction was that of natural tissues. It's not possible to transplant heart valves untreated because of rejection phenomena, but it became apparent that collagenous tissue could be cross-linked by glutaraldehyde and prepared in the form. of a heart valve. Two sources of tissue were considered for this purpose, bovine pericardium (collagenous tissue derived from the wall of a cow's heart) and porcine valves (heart valves taken from pigs) and the resulting "bioprosthetic valve" appeared to be very promising. It was particularly important that these patients didn't need anticoagulation. Unfortunately, these valves have not proved very durable, the cross-linked collagen suffering from slow calcification and deterioration so most of the replacement valves themselves need to be replaced within a decade.
This would tend to suggest that the mechanical valves give superior performance, notwithstanding the anticoagulation problem, and a move back towards their use might have been expected. However, most of the valves in current use incorporate an alloy'(usually Stellite) for
the housing, and a carbon coated occuluder. The complex shapes of some of the housing have required combinations of casting and welding technologies to be used in their construction and serious problems have arisen with a valve design from one manufacturer, where a small number of catastrophic fractures have occurred within the housing. In patients where this valve has been used to treat aortic valve disease, this fracture is usually fatal and although the risks are small, the problem is important to the industry.
Also, at a time when this dichotomy is exercising the minds of surgeons, scientists and regulatory bodies alike, the emergence of the disease BSE in cattle has placed even further restrictions on the use of animal tissue for this type of application and the whole question of prosthetic heart valves has been turned from a reasonable successful example of reconstructive implant surgery to a very confused area. This serves to highlight some of the very varied problems of facing the use of biomaterials.
This passage is extracted from an article written by an expert on ______.
A．No more than $200.B．Around $200.C．A little more than $200.D．Exactly $200.
与走私罪犯通谋，为其提供货款、资金、账号、发票、证明，或者为其提供运输、保管、邮寄或者其他方便的，以走私罪的共犯论处。()
登记会计账簿要用蓝黑墨水或者碳素墨水书写，除银行的复写账簿外，不得使用圆珠笔或者铅笔书写。()
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