Currently, the paramount problem in the field of biomaterials, the science of replacing diseased tissue with human-made implants, is control over the interface, or surface, between implanted biomaterials and living tissues. The physical properties of most tissues can be matched by careful selection of raw materials: metals, ceramics, or several varieties of polymer materials. Even the requirement that biomaterials processed from these materials be nontoxic to host tissue can be met by techniques derived from studying the reactions of tissue cultures to biomaterials or from short-term implants. But achieving necessary matches in physical properties across interfaces between living and non-living matter requires knowledge of which molecules control the bonding of cells to each other—an area that we have not yet explored thoroughly. Although recent research has allowed us to stabilize the tissue-biomaterial interface by controlling either the chemical reactions or the microstructure of the biomaterial, our fundamental understanding of how implant devices adhere to tissues remains woefully incomplete.
17. According to the passage, the major problem currently facing scientists in the field of biomaterials is
(A) assessing and regulating the bonding between host tissue and implants
(B) controlling the transfer of potentially toxic materials across the interface of tissue and implant
(C) discovering new materials from which to construct implant devices
(D) deciding in what situations implants are needed（A）
(E) determining the importance of short-term implants to long-term stability of tissue-implant interfaces
18. The passage suggests which of the following about the recent research mentioned in lines 19-25?
(A) It has solved one set of problems but has created another.
(B) It has concentrated on secondary concerns but has ignored primary concerns.
(C) It has improved practical applications of biomaterial technology without providing a complete theoretical explanation of that improvement.
(D) It has thoroughly investigated properties of biomaterials but has paid little attention to relevant characteristics of human tissue.（C）
(E) It has provided considerable information on short-term implant technology but little on long-term implant technology.
19. The author’s primary purpose is to
(A) answer a theoretical question in the field of biomaterials
(B) discuss the current state of technology in the field of biomaterials
(C) resolve a research dispute in the field of biomaterial
(D) predict an ethical crisis for biomaterials researchers（B）
(E) suggest some practical benefits of biomaterial implants